How to choose a hot tub water test kit

The best hot tub water testing kit is the one you will use correctly every time. For most owners, that means good test strips for quick checks, backed by a liquid drop kit or a photometer for precise readings. Choose a kit that matches your sanitiser, and replace it before it degrades.

This guide compares the three main types of kit on accuracy, running cost and ease of use. It also explains which readings chlorine, bromine and salt water owners actually need, and how storage and shelf life can quietly change your results.

Which readings does your hot tub test kit need to cover?

Every hot tub needs the same core readings, whatever sanitiser you use: sanitiser level, pH, total alkalinity and calcium hardness. These are the widely used target ranges for domestic hot tubs:

• Free chlorine: 3-5 ppm (parts per million)
• Bromine: 3-5 ppm
• pH: 7.2-7.6
• Total alkalinity: 80-120 ppm
• Calcium hardness: 150-250 ppm

Some manufacturers set slightly different targets, especially for calcium hardness and for certain salt systems. Your hot tub manual takes priority over any general guide, including this one. For the order in which to adjust these readings, see our guide to balancing pH, alkalinity and sanitiser in the right order.

Why alkalinity and calcium hardness matter

Total alkalinity acts as a buffer that holds pH steady. When alkalinity is too low, pH swings up and down with every use and every dose of chemicals. A kit that skips alkalinity cannot tell you why your pH keeps drifting, so you end up correcting the symptom again and again.

Calcium hardness protects your equipment at both ends of the range. Very soft water can attack heater parts and seals, while calcium hardness above range causes scale to form on the heater element. Scale makes the heater work harder and can shorten its life, so the reading matters in hard water areas as much as in soft ones.

Many multi-pad test strips measure total hardness rather than calcium hardness. Total hardness also counts magnesium, so it can read higher than calcium hardness alone. Check which one your strip reports before comparing it with the 150-250 ppm calcium hardness target.

If you use chlorine

Your kit should measure free chlorine, not only total chlorine. Free chlorine is the part that is actively sanitising the water. Total chlorine also includes combined chlorine (chloramines), which is used-up chlorine and the cause of the strong 'swimming pool' smell.

The gap between total and free chlorine tells you when the water needs shocking. A kit that only shows total chlorine cannot tell you this, so it can make dirty water look well sanitised. Our guide to hot tub chlorine levels and how to read them explains free and combined chlorine in more detail.

If you dose with stabilised chlorine granules (dichlor), the stabiliser, called cyanuric acid, builds up in the water with every dose. High levels make chlorine slower to work. If you use stabilised chlorine, a kit that also reads cyanuric acid is worth having.

Cyanuric acid shields chlorine from sunlight. That makes it most relevant for hot tubs left uncovered outdoors, where ultraviolet light breaks chlorine down quickly. If your hot tub stays covered when not in use, sunlight is less of a factor, but the build-up from stabilised granules still needs watching.

If you use bromine

Bromine owners need a kit that reads bromine directly. Many test strips have a dedicated bromine pad. In liquid and tablet kits, the same DPD reagent used for chlorine can measure bromine, read either on a bromine scale or by applying a conversion factor given in the instructions.

Before buying, check that the kit's instructions or colour chart include bromine. Many kits sold for swimming pools only cover chlorine, which leaves bromine owners guessing.

If you have a salt water system

A salt system makes chlorine from dissolved salt, so you still test free chlorine, pH and alkalinity just as a chlorine owner would. You also need to test the salt level, because the chlorine generator only works properly within the range its maker sets.

Target salt levels vary between systems, so check your manual for the exact figure rather than relying on a generic number. Standard multi-pad test strips usually do not read salt. You will need dedicated salt test strips, a digital salt meter or a photometer that offers a salt test.

Useful extra readings

• Cyanuric acid, if you use stabilised chlorine granules or your hot tub is often left uncovered outdoors
• Calcium hardness at every refill, especially in hard water areas, which cover much of south and east England
• Copper and iron, if you fill from a private well or borehole, or notice staining
• Total chlorine as well as free chlorine, so you can work out combined chlorine

Test strips, drop kits and photometers: how do they compare?

There are three main ways to test hot tub water at home. Each one makes a different trade between speed, precision and cost, and many experienced owners use two of them together.

Test strips

You dip a strip in the water, wait for the time stated on the pot, then compare the coloured pads with a chart. A single strip can give several readings within a minute or so, which makes strips the quickest and simplest option.

The weakness is precision. You are judging shades by eye, and the steps between colour blocks are often wide, for example 0, 1, 3, 5 and 10 ppm. That means you are reading a band, not an exact figure. Our guide on how to read hot tub test strips accurately covers the technique in detail.

Liquid drop kits

You fill a small vial with hot tub water and add drops of reagent. In a colour-match kit, you compare the result with a printed scale. In a titration kit, you count the drops needed to make the colour change, and the number of drops gives the reading.

Most chlorine and bromine tests rely on a reagent called DPD, short for N,N-diethyl-p-phenylenediamine. It turns the water sample pink, and the strength of the pink is in proportion to the amount of chlorine or bromine present. That is why a shade on a colour chart, a drop count or a photometer's light reading can be converted into a concentration in ppm.

Titration tests are among the most precise home methods for free chlorine, alkalinity and calcium hardness, because counting drops removes most of the guesswork. The downsides are time, small parts and reagent bottles to handle and store. A full set of tests can take several minutes.

The most precise home test for free chlorine is the FAS-DPD titration, sold under that name. Each drop represents a fixed small step in ppm, set by the sample size the instructions tell you to use. Look for FAS-DPD on the kit's list of tests if you want dependable chlorine readings.

Some liquid kits also include acid-demand and base-demand tests. Rather than just showing that pH is wrong, these tell you how much pH reducer or pH increaser to add, which saves rounds of guessing and retesting.

Photometers

A photometer is a handheld digital meter. You add a reagent, usually a tablet, to a water sample, place the vial in the meter, and it measures the colour with a light beam and shows a number on screen. There is no colour chart to interpret.

Photometers cost more upfront and still need reagents, batteries and clean vials. You usually have to 'zero' the meter with an untreated sample before each test. A scratched or dirty vial can give a false reading, so the result is only as good as your technique.

Single-parameter digital meters, such as pH pens and salt meters, are a related option. They need regular calibration with buffer or reference solutions to stay accurate. Floating units that monitor water continuously are a separate category, covered in our guide to smart hot tub water monitors and how they work.

Digital strip readers

A dedicated digital strip reader is a separate option. You dip an ordinary test strip, place it in a small handheld reader, and it shows each result as a number. This removes colour matching without needing the reagents a photometer uses.

Digital strip readers, and phone apps that read a strip from a photo, are only as accurate as the strip itself and how it was dipped. They cannot correct for a degraded strip or poor technique. Check their readings now and then against a drop kit or photometer.

How accurate does a hot tub test kit need to be?

You do not need laboratory precision for every check. The targets are ranges, so what matters is knowing reliably whether your water is inside, above or below each range.

Hot tubs need more frequent testing than swimming pools. A hot tub holds far less water than a pool and runs much hotter, often up to 40°C (104°F), so sweat, body oils and lotions from even a few bathers have a much bigger effect. Sanitiser and pH can shift within hours of a busy session.

Test strips are fine for routine checks when your water is stable and readings sit comfortably in the middle of the range. They struggle when a reading is close to the edge of a range, when you are troubleshooting cloudy or smelly water, or just after a shock dose.

pH is the reading most worth getting right. Water with pH below range is corrosive, and over time it gradually damages metal heater elements and pump seals. A kit that cannot tell 7.0 from 7.4 reliably can let that damage build up unnoticed.

On multi-pad strips, the total alkalinity pad is often the least precise reading. If alkalinity is a recurring problem, or your pH will not stay steady, confirm alkalinity with a drop test before adjusting it.

Very high chlorine or bromine can bleach the pH indicator in both strips and drop kits, giving a false pH reading. If your sanitiser is well above range, bring it down before trusting the pH result.

A practical approach is to use strips for frequent checks, and a drop kit or photometer once a week or whenever something looks wrong. If your strips and your more precise test disagree consistently, trust the precise one and replace the strips.

Many hot tub dealers will also test a water sample in store. This is a useful cross-check when your own kit gives an odd result. Take the sample from elbow depth in a clean, sealed container and get it to the dealer the same day.

What does each type of test kit cost to run?

The price on the shelf is only part of the cost. Running cost depends on how often you test and how long the consumables last once opened. These are broad guides only, as prices vary between retailers and change over time.

• Test strips: roughly £5 to £20 per pot. Cheap per test, but a large pot may degrade before you use it all.
• Digital strip readers: a one-off cost for the reader, typically more than a pot of strips, then the ongoing cost of compatible strips.
• Liquid drop kits: roughly £15 to £50 for the starter kit, with refill reagents sold separately. Some reagents need replacing every year whether you have used them or not.
• Photometers: roughly £50 to several hundred pounds for the meter, plus ongoing reagent tablets and batteries.
• Salt testing: dedicated salt strips or a salt meter add anywhere from a few pounds to tens of pounds.

The biggest hidden cost is a wrong reading. A degraded or misread kit can lead you to add pH increaser you did not need, shock water that was already fine, or drain and refill early. Each of those costs more in chemicals, water and heating than a fresh test kit.

Wrong readings also cause real harm, not just extra cost. Water that is out of balance can irritate bathers' skin and eyes, turn cloudy, and over months damage the heater, pump and seals. Repairs to those parts cost far more than any test kit.

A cheaper kit that you keep fresh and use carefully will save you more money than an expensive kit used carelessly.

How does shelf life affect your test results?

Every testing chemical breaks down over time, and heat, light and moisture make it happen faster. An expired kit rarely fails in an obvious way. Instead, it gives readings that look believable but are wrong, which is worse than no reading at all.

Test strips

Once a pot is opened, the strips absorb moisture from the air every time the lid comes off. Wet fingers, steam from the open hot tub and leaving the lid off all speed this up. Many manufacturers advise using an opened pot within a set number of months, so check the label on yours.

As an example, some makers set a use-by period of about 90 days after opening. Others allow longer, so the date on your own pot is the one to follow.

An unopened, sealed pot usually lasts much longer, counted from the date of manufacture, sometimes up to about three years. Many labels therefore carry two limits: an unopened expiry date and an after-opening period. Check both, and stop using the strips at whichever limit comes first.

To choose the right pot size, multiply the number of tests you do each week by the number of weeks in the opened-pot shelf life. At three tests a week, a pot with a 90-day use-by period (about 13 weeks) needs only about 40 strips. A 100-strip or 150-strip pot would be partly wasted, because the remaining strips pass their use-by date before you reach them.

Strips packed in several separately sealed pouches or sachets are a good choice. Only the pouch you have opened is exposed to moisture, while the rest of your supply stays sealed and fresh.

Signs that your strips have degraded include:

• Pads that are already discoloured before you dip them
• Readings that drift even though you have not changed the water or added chemicals
• Every strip showing the same result, even after you add chemicals
• Results that disagree with a fresh drop kit or photometer reading

Liquid reagents and tablets

Liquid reagents carry expiry dates, and some, including DPD chlorine reagents, tend to degrade sooner than others. Foil-wrapped reagent tablets usually keep longer than liquids, because each tablet stays sealed until you use it. Manufacturers give different shelf lives, so follow the dates on your own kit.

Store every kit indoors in a cool, dark, dry place with the lids tightly closed. Avoid the hot tub cabinet, a shed that heats up in summer, a car or a sunny windowsill. When you buy, check that the expiry date is well into the future.

Buy a pot of strips small enough to use well within its shelf life, and write the date you opened it on the label. A fresh small pot gives more reliable readings than a large pot that has been open for a year.

Can you read a test kit if you have colour vision deficiency?

Colour vision deficiency is common, particularly in men. According to the NHS, it affects about 1 in 12 men and 1 in 200 women in the UK, and red-green deficiency is the most common type. Many strip and drop kit charts rely on shades of red, pink, orange and yellow, which can be hard to tell apart.

The pH test, which usually changes from yellow to red, and the DPD chlorine and bromine test, which shows shades of pink, are the readings most often affected. Small differences in shade can mean a big difference in the result.

The total alkalinity pad on many strips moves through shades of blue and green. Some people with colour vision deficiency find this range hard to read as well.

If colours are difficult for you, consider these options:

• A photometer or digital meter, which gives a number instead of a colour
• A digital strip reader, which reads ordinary strips and shows each result as a number
• A titration drop kit, where you count drops until a colour change rather than matching a shade
• Asking someone else in the household to read the result
• A phone app that reads strips from a photo, treated as a guide only, because camera and lighting differences affect the result

Good lighting helps everyone. Read results in daylight against a plain white background, not at dusk or under coloured garden lights.

How do you get an accurate reading every time?

Even the most precise kit gives poor results with poor technique. Follow the same routine every time, and always follow your own kit's instructions where they differ from these general steps.

1. Run the pump or jets for a few minutes so the water is well mixed, then switch them off.
2. Take your sample from about elbow depth, around 45 cm (18 in) below the surface, away from the jets and the area where you add chemicals.
3. Dry your hands before touching a strip or a vial, and close the strip pot straight away.
4. For strips, dip for only one to two seconds and do not shake the strip. Hold it flat while it develops so the reagents do not run from one pad to the next.
5. Time the test exactly as the instructions say. Use a timer on your phone rather than counting in your head. Strip development times differ between brands, typically anywhere from about 15 to 60 seconds, so never assume the timing from a previous pot.
6. For drop kits, rinse the vial with hot tub water first, fill it exactly to the line at eye level, and hold the reagent bottle upright so every drop is the same size. Swirl the vial gently after each drop to mix the reagent before adding the next drop or reading the colour.
7. Read the result in daylight against a white background, within the time window stated in the instructions.
8. Pour the used sample away down a drain, not back into the hot tub, then rinse the vial with clean water and replace all caps.
9. Write down the results and the date, so you can spot trends over time.

Test before you add any chemicals, and wait for the time given on the chemical's label before retesting. If the label gives no time, allow at least 30 minutes of circulation as a typical minimum. For a full routine, including how often to test, read our guide on how to test hot tub water and when.

A phone app that logs your results makes record keeping easier. Over a few weeks, a log shows patterns that a single reading cannot, such as pH dropping after weekends of heavy use. That helps you adjust before the water drifts out of range.

Testing reagents are chemicals. Store them out of reach of children and pets, never mix reagents from different kits, and read the safety information on each bottle. If a reagent gets into your eyes or is swallowed, follow the first aid advice on the label and get medical help if needed.

What should you look for in the best hot tub water testing kit?

Once you know which type of kit suits you, use this checklist to compare individual kits:

• It matches your sanitiser: chlorine, bromine or salt
• It measures free chlorine separately from total chlorine, if you use chlorine or salt
• Its scale has fine steps around the hot tub ranges, especially between 1 and 5 ppm for sanitiser
• It covers pH, total alkalinity and calcium hardness, or you have another way to test them
• It has a clear expiry date that is well into the future
• The pot or reagent size suits how often you test
• Refills and replacement reagents are easy to find
• The colour chart is clear, or the meter gives a number, if colour matching is hard for you
• The colour chart is waterproof and fade-resistant, so it stays accurate after splashes and daylight
• It comes with a treatment or dosing guide that explains what to do with each result
• For photometers and meters: reagent availability, calibration needs and whether the unit is splash-resistant
• If it depends on a manufacturer's phone app: whether it still works without the app, because an app can be withdrawn or moved behind a subscription

Words such as 'accurate', 'professional' or 'lab-grade' in a strip's product name are marketing claims, not independently verified specifications. Judge a kit by how its results compare with a drop test or photometer on the same water sample.

Some fuller liquid kits include a water balance calculator, often a printed card or slide chart. It combines your pH, total alkalinity, calcium hardness and water temperature to show whether the water overall tends to corrode surfaces or form scale. This helps in hard water areas, or when scale keeps appearing even though each separate reading looks acceptable.

Mistakes to avoid

• Buying the biggest pot of strips for value, then using it long after it has degraded
• Using a swimming pool kit with no bromine test when your hot tub uses bromine
• Relying on total chlorine alone, which can hide used-up chlorine
• Forgetting to test salt on a salt water system
• Storing the kit inside the hot tub cabinet, where it is warm and damp
• Choosing a strip because it has the most pads, when you only need four or five readings
• Adding chemicals after a single odd reading instead of retesting first

Which test kit should you choose?

For most owners, the best set-up is good-quality test strips matched to your sanitiser for checks two or three times a week, plus a liquid drop kit for a precise weekly check and for troubleshooting. A titration kit for free chlorine and alkalinity gives the most dependable results for the money.

Salt water owners should add a dedicated salt test. If you find colours hard to read, or you simply prefer a number on a screen, a photometer is worth the higher cost.

Whichever kit you choose, keep it fresh, store it indoors and test the same way every time. Regular, reliable testing is one of the main ways to keep hot tub water safe, as the HSE guidance on legionella makes clear.

Frequently asked questions

How often should I test my hot tub water?

Test sanitiser and pH before each use, or at least two or three times a week if the hot tub is used less often. Check total alkalinity once a week and calcium hardness after every refill and roughly once a month. Test more often after heavy use, parties or heavy rain getting into the water. Your manual may recommend a different schedule, so follow it where it does.

Can I use a swimming pool test kit on my hot tub?

Often yes, because the target ranges for chlorine, pH and alkalinity are similar. Check three things first: that the sanitiser scale has fine enough steps between 1 and 5 ppm, that it includes a bromine test if you use bromine, and that it measures calcium hardness. Some pool strips are designed for lower sanitiser levels and give coarse readings at hot tub levels.

Why do my test strips and drop kit give different readings?

Strips read in wide colour bands, while drop kits and photometers give narrower results, so small differences are normal. Large or consistent differences usually point to degraded strips, different timing, poor lighting or very high sanitiser bleaching the pH pad. Check that both kits are in date, retest carefully, and trust the more precise method if they still disagree.

How can I tell if my hot tub water has gone bad?

Warning signs include cloudy or green water, persistent foam, a strong chemical or musty smell, slimy surfaces and sanitiser that disappears within hours. Green water usually means algae growth caused by sanitiser running too low. Stop using the hot tub, test the water and correct it before getting back in. If you develop an itchy rash, sore eyes or feel unwell after using a hot tub, stop using it and speak to a pharmacist or GP.

Why does my hot tub foam or smell strongly of chlorine?

Persistent foam usually comes from lotions, cosmetics and other body care products building up in the water. If foam lasts over a minute after the jets stop, the water needs testing and treating. Foam that keeps returning can also be linked to water that is out of balance, such as high pH or high alkalinity, so test pH and alkalinity as well as sanitiser. A strong chlorine smell usually means chloramines, which form when sanitiser reacts with sweat and body oils. The smell signals too little free chlorine, not too much. Test free and total chlorine, shock if combined chlorine is high, and ask bathers to shower before getting in.

Do I need to test calcium hardness if I live in a soft water area?

Yes. Very soft water can become aggressive, which means it may slowly attack heater parts, seals and fittings. It can also make foaming more likely. Test calcium hardness when you fill the hot tub so you know your starting point, then recheck it monthly. If it is below your manual's target, a calcium increaser will bring it up.

Can a smart water monitor replace a test kit?

Not completely. Smart monitors are useful for spotting trends and reminding you to act, but their sensors need regular calibration and many do not measure every reading, such as calcium hardness. Keep a manual kit, ideally a drop kit or photometer, to confirm the monitor's readings and to check anything it does not cover.