How to shock a hot tub, step by step

To learn how to shock a hot tub, start by testing and balancing the water, then switch on the jets, take the cover off and add a measured dose of chlorine or non-chlorine shock based on your tub's litres. Leave the cover open for at least 20 minutes, then test again before anyone gets in.

That is the whole job, and it takes about half an hour. The details matter, though: which type of shock to use, how much your tub really needs, how often to do it and how long to wait afterwards. This guide covers each of those in turn.

What does shocking a hot tub actually do?

Every time someone uses your hot tub, they leave behind sweat, skin oils, body lotion, deodorant and other organic waste. Your daily sanitiser (chlorine or bromine) reacts with this waste and gets used up. Some of the chlorine combines with it to form chloramines, which smell strongly of chlorine and do very little cleaning.

Urine is another bather waste that reacts with chlorine to form chloramines, along with sweat, skin oils and cosmetics. Asking everyone to shower and use the toilet before a soak cuts down this waste, so your sanitiser lasts longer and the tub needs shocking less often.

That sharp chlorine smell, and the sore eyes and itchy skin that often come with it, are usually caused by chloramines, not by too much chlorine. Adding less chlorine will not fix the problem. The answer is to shock the water, which breaks the chloramines down.

Shocking means adding a large, one-off dose of an oxidiser to burn off that waste. It breaks down chloramines, frees up your sanitiser so it can work properly again, and helps keep the water clear and fresh. It is sometimes called oxidising or super-chlorinating.

Shock is not a replacement for your normal sanitiser. You still need to keep free chlorine or bromine in the usual range of 3-5 ppm between shocks. Shock is the periodic deep clean on top of that daily protection.

A simple sign that your tub needs shocking is a strong chlorine smell. Clean, well-balanced water has very little smell. If you test for both free and total chlorine, the difference between them is your combined chlorine. A combined chlorine reading above about 0.5 ppm is a common signal to shock. Our guide to reading hot tub chlorine levels explains these numbers in more detail.

Chlorine or non-chlorine shock: which should you use?

Both types oxidise waste, but they behave differently. Many owners keep both in the shed and use each one for a different job.

Non-chlorine shock (MPS)

Non-chlorine shock is usually potassium monopersulfate, often shortened to MPS. It oxidises organic waste and breaks down chloramines without adding more chlorine to the water. It suits regular weekly maintenance in both chlorine and bromine tubs.

Its main advantage is a short wait before bathing, often 15 minutes to an hour according to many product labels. It also has a milder smell. In a bromine tub, MPS has a second job: it reactivates used bromide in the water and turns it back into active bromine.

The limitation is that MPS does not kill bacteria on its own. It is not a sanitiser. If you have a contamination problem, such as cloudy water, a slimy surface or water that has been left untreated, you need a chlorine shock instead.

MPS is also a known skin sensitiser. It has been linked to contact dermatitis, hives and asthma in sensitive people. If you have a persulfate allergy or very sensitive skin, speak to your GP or a pharmacist before using it regularly. The NHS has general information on contact dermatitis.

If you or anyone in your household reacts to MPS, you do not need to stop shocking. Use a chlorine shock (dichlor) for your routine weekly oxidising instead, and allow the longer wait for chlorine to fall back to 3-5 ppm before bathing.

Chlorine shock (dichlor)

Most chlorine shock sold for hot tubs is sodium dichloroisocyanurate, known as dichlor, in granule form. It is the same product many owners use as their daily chlorine, simply added in a larger dose. It both oxidises waste and kills bacteria and algae.

Use chlorine shock after a fresh refill, after a heavy session with lots of bathers, when the water is cloudy or smells musty, or after the tub has been left for a while. The downside is a longer wait before bathing, because the chlorine level has to fall back to the normal range first.

If the water has turned green or you can see algae, use chlorine shock at the higher heavy-contamination dose given on the label or in your manual. Non-chlorine shock (MPS) is not strong enough to clear an algae or bacterial bloom, so it is the wrong choice for this job.

Dichlor contains a stabiliser called cyanuric acid, which builds up over time with repeated use. This is one of the reasons hot tub water needs changing every few months. Calcium hypochlorite, a common pool shock, is generally not recommended for hot tubs, because it raises calcium hardness and pH in a small volume of water.

Cyanuric acid does more than accumulate. As its level rises, it holds on to more of the chlorine and weakens its sanitising power, so the same free chlorine reading kills germs more slowly. Regular draining and refilling keeps this under control.

Never mix shock products

Different chemicals can react dangerously with each other, even in small amounts. Never mix two products in the same scoop, bucket or container, and never add them to the water at the same time. Leave at least 20 minutes and let the jets run between adding different chemicals.

Use a bucket and measuring scoop that have never held any other pool or spa chemical, or keep a separate, labelled set for each product. Even leftover residue from a different chemical can react violently with shock, so a quick rinse of a shared scoop is not enough.

If your tub uses a hydrogen peroxide sanitiser system, do not shock it with chlorine or standard MPS. Mixing these oxidisers can react dangerously. Use only the shock made for peroxide systems, as described in your system's instructions.

How much shock does your hot tub need?

Every dose depends on your water volume, so start by knowing how many litres your tub holds. A typical four to six seat hot tub holds somewhere between 1,000 litres (about 260 US gallons) and 1,600 litres. Check your manual, or see our guide on how to work out your hot tub's litres.

If the manual is missing, you can estimate the volume of a rectangular tub. Multiply the inside length by the width and by the average water depth, all in metres, then multiply by 1,000 to get litres. For example, 1.8 m × 1.8 m × 0.45 m × 1,000 is about 1,460 litres. Seats make the depth uneven, so treat the result as an estimate.

Dosing non-chlorine shock

MPS products vary in strength from brand to brand, so there is no single correct figure. Find the dose on the label, which is usually given in grams per 1,000 litres, then scale it to your tub. Divide your litres by 1,000 and multiply by the label dose.

For example, if the label says 20 grams per 1,000 litres and your tub holds 1,300 litres, you need 1.3 × 20 = 26 grams. Many labels give a lower dose for routine weekly use and a higher dose after heavy use, so check which one applies.

If your label does not give a clear dose, a typical starting point is about 30 g per 1,000 litres for routine weekly use. For cloudy water or after heavy use, about double that, around 60 g per 1,000 litres, is common. Treat these as rough figures and always check them against the label.

Dosing chlorine shock

With dichlor, you can work out the dose yourself. Dichlor granules are typically about 55-56% available chlorine. As a rule of thumb, about 1.8 grams per 1,000 litres raises free chlorine by roughly 1 ppm. Always check your product's label, because concentrations differ.

Advice on how high to raise the chlorine for a shock varies between manufacturers. A common approach is to bring free chlorine up to around 10 ppm for a normal shock. For a fresh fill or a problem tub, follow the higher dose on your product label or in your hot tub manual.

To use the table, subtract your current free chlorine reading from your target, then multiply by the 1 ppm figure for your tub. For example, a 1,200 litre tub reading 1 ppm, with a target of 10 ppm, needs a rise of 9 ppm. That is 9 × 2.2 = about 20 grams (roughly 0.7 oz).

Use a proper measuring scoop or a set of kitchen scales kept only for hot tub chemicals. Guessing with a household spoon is the most common cause of over-shocking, especially in small tubs.

How to shock a hot tub, step by step

Always add chemicals to water, never water to chemicals. Wear gloves and eye protection, add shock with the cover fully open, and keep children and pets away from the tub and the product while you work.

Before you start, gather everything you need in one place next to the tub, so you never leave open chemicals unattended:

• Test strips or a liquid test kit
• pH increaser and pH decreaser
• Alkalinity increaser
• Your chosen shock
• A dedicated measuring scoop or kitchen scales
• A clean plastic bucket
• Chemical-resistant gloves and eye protection

Total alkalinity acts as a buffer that holds pH steady. If alkalinity is too low, pH swings easily, and if it is too high, pH is hard to move. Correct alkalinity first, then pH, so the pH does not drift while the shock is working.

pH has a bigger effect on chlorine shock than many owners expect. Once pH rises above about 7.8, chlorine shock loses most of its oxidising power before it can act on the waste. Shocking high-pH water wastes much of the dose, which is why pH must be corrected first.

1. Test the water with fresh test strips or a liquid test kit. Note your pH, total alkalinity and free chlorine or bromine.
2. Balance the water first if needed. Aim for total alkalinity 80-120 ppm, then pH 7.2-7.6. Shock works much less effectively when pH is high, so correct this before you shock and wait about 20 minutes for it to mix.
3. Test again after that 20 minutes to confirm pH and alkalinity are now in range. If they are not, adjust again and wait before retesting. Only move on once the readings are correct.
4. Make sure nobody is in the tub, then take the cover fully off. Leave it open somewhere it will not be splashed.
5. Take out removable pillows and headrests, floating toys, floating thermometers and any tennis ball. High sanitiser levels can fade or degrade them. Rinse them and put them back once the water has settled.
6. Switch on the jets and circulation pump so the water is moving. If your tub has air controls, many manufacturers suggest closing them so the shock is not lost too quickly into the air. Check your manual. Turn off the air blower as well, and run the jets on a low setting if you can, because air injection makes foam and works against even mixing.
7. Measure the correct dose for your litres and the product you are using, as described above.
8. Add the shock to the water. Some labels say to sprinkle granules slowly across the surface near the jets. Others say to dissolve the dose first in a clean plastic bucket of tub water. Follow your product's instructions. If you pre-dissolve, use roughly 10 litres of warm tub water and stir until every granule has dissolved, because undissolved granules settling on the shell can bleach or mark the surface.
9. Pour the shock around the edge of the tub into the main body of water. Never pour it directly into the filter housing, the skimmer or a jet, because concentrated oxidiser can damage the filter cartridge.
10. Leave the jets running and the cover open for at least 20 minutes. This mixes the shock and lets gases escape instead of collecting under the cover, where they can damage the cover's underside and the pillows.
11. Test the water again before anyone gets in. Free chlorine or bromine should be back in the 3-5 ppm range.
12. Put the cover back on once the water has settled and you are not planning to use the tub straight away.

Evening is often the best time to shock. Strong sunlight breaks down chlorine quickly, so a dose added in the middle of a bright day works less efficiently.

If your hot tub is indoors or in an enclosed room, open doors or windows or run an extractor fan while you shock. Keep the room well ventilated for the whole off-gassing period, not just while you add the product.

Keep shock in its original sealed container. If shock gets on your skin or in your eyes, rinse straight away with plenty of clean water for 15-20 minutes, then contact NHS 111 for advice.

How often should you shock a hot tub?

For most home hot tubs, once a week is the standard routine. Many owners use non-chlorine shock for the weekly dose and switch to chlorine shock when the water needs a stronger clean. Your manual may suggest a different schedule, so check it.

Pick a fixed day and time for the weekly shock, such as every Sunday evening. A set slot makes the routine hard to forget, and an evening dose goes in after dark, when sunlight cannot weaken it.

Treat once a week as a minimum, not a limit. Shocking more than once in the same week after heavy use is fine, as long as you dose correctly and test before bathing.

You should also shock outside your normal routine whenever the water has had extra stress. Common reasons include:

• A party or a session with more bathers than usual
• A session with four or more people, or a soak lasting more than about two hours
• A strong chlorine smell or a combined chlorine reading above about 0.5 ppm
• Cloudy, dull or foamy water
• Heavy rain getting into the tub, or leaves and debris falling in
• The tub being left uncovered or unused for a week or more
• Free chlorine or bromine dropping to zero
• After draining and refilling with fresh water

If anyone who has been unwell has used the tub, shock straight away. Keep everyone out until the sanitiser is back in the normal range.

If the tub has not been shocked for more than two weeks, use the higher heavy-use dose on the label rather than the routine dose. Waste will have built up, and a routine dose may not be enough to clear it.

A new fill is a special case, because tap water contains no sanitiser and may carry minerals and organic matter. Most manufacturers recommend a chlorine shock after filling. Our guide on whether to shock a hot tub after refilling covers that first dose in detail.

How long should you wait before getting back in?

The safe waiting time depends on the type of shock and the dose. The rule that always applies is simple: test the water before anyone gets in, and only bathe when the sanitiser level is back in the normal range.

• Non-chlorine shock: many labels allow bathing after 15 minutes to an hour, once the water has mixed. Check the label for your product.
• Chlorine shock at a normal dose: wait until free chlorine falls back to 3-5 ppm. This can take several hours.
• Chlorine shock at a high dose, such as after a refill: it is often best to shock in the evening and test again the next day.

As a rough guide, a normal-dose chlorine shock usually comes back down to 3-5 ppm in about 2-4 hours with the jets running and the cover off. A heavy dose can take overnight.

If a test after a chlorine shock still reads above 5 ppm, wait another 30-60 minutes and test again. Do not rely on time alone, because water temperature, sunlight and the number of bathers all change how quickly chlorine falls.

Your pre-bathing test should check pH as well as sanitiser. Shock products can shift pH, so confirm it is still 7.2-7.6 and correct it, letting the water mix, before anyone gets in.

If you use test strips, dip each strip for exactly the time given on the packaging, usually a few seconds. Then compare it with the colour chart within the time stated. Reading a strip too early or too late gives false results.

Do not add your routine chlorine or bromine dose at the same time as the shock. Wait until the shock has worked through and a test shows where your sanitiser level has settled, then top up only if it is needed.

If the test shows free chlorine or bromine has dropped below the normal range, add a routine dose of your usual sanitiser. Let it mix with the jets running, then test again before anyone gets in.

MPS can affect some chlorine test kits. It can show up as combined chlorine on a total chlorine test for a while after you add it. If your readings look odd straight after a non-chlorine shock, wait an hour and test again.

If the chlorine is still too high when you want to use the tub, leave the cover off with the jets running and the water will usually come down on its own. Our guide on how to lower chlorine in a hot tub safely explains the faster options.

Getting in while chlorine is too high can irritate skin, eyes and airways. If you notice a rash, sore eyes or a cough after using a hot tub that does not settle within a day or two, speak to a pharmacist or your GP. If anyone has difficulty breathing after breathing in chemical fumes, get medical help straight away through NHS 111 or call 999 in an emergency.

Over-chlorinated water can also fade and weaken swimwear. This is one more reason to wait until the reading is back in range.

What about inflatable and saltwater hot tubs?

Inflatable hot tubs

Inflatable hot tubs are shocked in the same way, but they usually hold less water, often around 600 to 1,000 litres. A smaller volume means a smaller dose, so it is easy to overdo it. Measure carefully and work out the dose from the actual litres your tub holds when filled to its line.

Keep the filter pump running while you add the shock, because many inflatables do not have strong jets to mix the water. Most inflatable manufacturers recommend chlorine as the sanitiser, and some advise against certain products, so read the manual before you start.

Pre-dissolving the shock is especially important in an inflatable tub. Undissolved granules resting against the PVC liner can discolour or weaken it.

Heat the water to at least about 20°C (68°F) before you shock. Cold water slows mixing and makes the treatment less effective.

Saltwater hot tubs

A saltwater system makes its own chlorine from dissolved salt, but the water still builds up waste and still benefits from shocking. Many systems have a boost or super-chlorinate setting that raises output for a few hours. Some manufacturers also allow MPS or dichlor.

MPS is often the preferred routine shock in a saltwater tub. It oxidises waste without pushing chlorine above what the salt cell is already producing, so levels stay easier to control.

Because the cell makes chlorine continuously, a saltwater tub may need shocking less often. It still needs a shock after a fresh fill (typically with dichlor, before the cell takes over), after heavy use, or when the water turns cloudy or smells.

Saltwater systems vary more than most, and the wrong product can affect the cell or your warranty. Check your manual and use only the shock methods the manufacturer approves.

What goes wrong when shocking, and how do you fix it?

The water is cloudy after shocking

Some cloudiness straight after shocking is normal, as the shock breaks down waste and the filter catches it. It should clear within a few hours with the pump running. If it does not, the usual causes are high pH, high alkalinity, a dirty filter or not enough shock for the amount of waste in the water.

If the water is still cloudy 24 hours after a correctly dosed shock, do not add more shock yet. Retest pH, total alkalinity and calcium hardness, and check the filter first. One of these, not too little shock, is the usual cause, and extra shock will not fix it.

High calcium hardness is another cause, especially in hard water areas. When the water balance shifts, for example during a shock, excess calcium can make the water hazy. If your tub tends to go cloudy, aim for around 150-200 ppm.

Clean or replace the filter, rebalance the water and shock again if needed. Our guide on why a hot tub goes cloudy after shocking walks through each cause and the fix.

If the filter is heavily soiled, rinse the cartridge thoroughly with a hose. Then soak it overnight in a cartridge filter cleaner and rinse it again before you refit it.

After rebalancing, run the filtration continuously for 6-24 hours before you decide the cloudiness has not cleared. Fine particles take time to reach the filter.

If the water is still hazy after about 24 hours of filtering with balanced chemistry, a spa clarifier can help. It clumps very fine particles together so the filter can catch them.

If cloudiness keeps returning despite correct chemistry and repeated shocks, the cause is often high total dissolved solids. These build up from chemicals, bathers and top-up water, and no product removes them. A partial or full drain and refill is the only way to dilute them.

The chlorine reading is far too high

Over-shocking usually happens when the dose was guessed or calculated for the wrong volume. Very high chlorine can bleach test strips so they read as zero, which fools some owners into adding even more. If a strip turns very pale or white straight away, assume the level is high, not low.

Other signs that the tub has been over-shocked include:

• A sharp, strong chemical smell above the water
• Stinging eyes or skin, even after a short time near or in the water
• Accessories left in the water looking bleached or faded

Leave the cover off, run the jets and keep everyone out until the reading is back to 3-5 ppm. Repeated heavy over-shocking can fade the cover and harden pillows and seals, so measure carefully next time.

If you can, leave the cover off during daylight hours. The ultraviolet light in sunlight breaks down excess free chlorine much faster than it falls at night.

Test every 30-60 minutes while you wait for the level to fall back to 3-5 ppm. This shows you when the tub is ready without guessing.

A chlorine neutraliser such as sodium thiosulfate can bring very high chlorine down quickly. Use it sparingly and only when you need the tub urgently, because too much can strip out the sanitiser entirely and leave the water unprotected. Add a small amount, let it mix and test before adding more.

The smell will not go away

A strong chlorine smell that returns soon after shocking often means the water is overloaded with waste or stabiliser. If you have been shocking weekly for several months without draining, the water may simply be worn out. Draining, cleaning and refilling is often the quickest fix.

Shocking has stopped working

If the water keeps going cloudy or smelly within days of a correct shock, biofilm may be building up inside the plumbing. This slimy layer protects bacteria from sanitiser, so shocking the surface water does not reach it. A purpose-made line flush (sometimes called a purge) added before you drain, or a visit from a technician, can clear it.

Holiday lets and shared hot tubs

The routine in this guide is for private home use. Commercial or shared hot tubs, such as those at holiday lets, fall under the Health and Safety Executive's guidance on spa pools, HSG282. If you run one, follow that guidance and your local authority's advice rather than a home-use routine.

When to call a professional

Ask a hot tub technician to look at the tub if the water stays cloudy or green after two correct shocks, if the pump or heater stops working, or if you see slime inside the filter housing or pipes. Bacteria such as Legionella can grow in poorly maintained hot tubs, and the NHS has information on Legionnaires' disease and its symptoms. Before any work near the equipment, switch the tub off at the isolator.

The short version

• Shock once a week, and again after heavy use, rain, a refill or a strong chlorine smell.
• Shock straight away if someone who has been unwell has used the tub.
• Use non-chlorine shock (MPS) for routine weekly care and a quick return to bathing.
• Use chlorine shock (dichlor) after a refill or when the water is cloudy, smelly or neglected.
• Use chlorine shock at the heavy-contamination dose for green water or visible algae, never MPS alone.
• Balance pH to 7.2-7.6 first, then dose by your tub's actual litres.
• Pre-dissolve granules and pour them into the main body of water, never into the filter.
• Keep the cover off and the jets running for at least 20 minutes.
• Test before anyone gets in, and only bathe at 3-5 ppm free chlorine or bromine.

Frequently asked questions

Why do people put a tennis ball in a hot tub?

A tennis ball floating in the water soaks up some of the oils from skin, hair products and lotions, which can reduce scum lines, cut down surface foam and help the filter. It is a cheap extra, but it does not sanitise the water or replace shocking. Swap it for a fresh one when it looks grey or greasy, and remove it before anyone uses the tub. Take it out and rinse it before you shock, too.

Can I use swimming pool shock in a hot tub?

Only if the product label says it is suitable for hot tubs or spas. Many pool shocks are calcium hypochlorite, which raises calcium hardness and pH quickly in a small volume of water and can lead to scale and cloudiness. Pool products are also dosed for tens of thousands of litres, which makes accurate measuring for a hot tub much harder.

Can I shock a bromine hot tub?

Yes. Non-chlorine shock is the usual choice in a bromine tub, because it oxidises waste and also reactivates used bromide back into active bromine. Some bromine users also shock with chlorine occasionally after heavy use. Check your manual first, as manufacturers differ, and never mix the products. Bromine works best with pH in the same 7.2-7.6 range.

Can I put the cover back on straight after shocking?

It is best not to. Shock releases gases as it works, and a closed cover traps them against the underside of the lid and the pillows. Over time this can fade, crack or weaken the vinyl and foam. Leave the cover open with the jets running for at least 20 minutes, and longer after a heavy chlorine dose.

Does hot tub shock go off over time?

Yes. Both chlorine and non-chlorine shock slowly lose strength, and faster in warm or damp conditions. Granules that have formed hard lumps or smell different may have absorbed moisture. Store shock sealed, upright, in a cool, dry place out of direct sunlight, away from other chemicals and out of reach of children and pets. Check the label for any use-by advice.

Should I shock if my water already looks clear?

Yes, clear water can still hold dissolved waste and chloramines that you cannot see. Regular weekly shocking keeps that build-up under control, so problems such as cloudiness, foam and strong smells are less likely to start. If your test shows combined chlorine above about 0.5 ppm, shock even if the water looks perfect.