Hot Tub Chemical Levels: The Complete Guide to Balancing Your Water

Last updated: September 2026

Balanced water is the difference between a hot tub you look forward to and one that smells, foams, goes cloudy and quietly eats your heater. The good news is that it’s not rocket science. There are only a handful of numbers to keep an eye on, and once you understand what each one does and which to fix first, it takes a few minutes a week.

A quick disclaimer: I’m not chemically trained, and I’m not affiliated with BISHTA (the British and Irish hot tub trade association) or anyone else. This is what I’ve learned from running my own tub and helping more than 1,500 people build theirs, checked against the official guidance, which I’ve linked so you can read it yourself. If your hot tub’s manual says something different, follow the manual.

Why a hot tub needs chemicals at all

Warm water, skin, sweat and a bit of organic muck from the garden is a perfect breeding ground for bacteria. The two that matter most are Pseudomonas, which causes “hot tub rash”, and Legionella, which causes Legionnaires’ disease. The CDC has a whole Legionella control toolkit for hot tubs for a reason. Hot tubs are warmer and have far more people per litre of water than a swimming pool, so the sanitiser gets used up much faster.

Chemistry does two jobs:

• Sanitising (chlorine or bromine) kills the bugs.
• Balancing (pH, alkalinity and calcium hardness) keeps the sanitiser working, keeps bathers comfortable and stops the water attacking or scaling up your tub and equipment.

Get the balance wrong and your sanitiser doesn’t work properly, even if the chlorine reading looks fine. That’s why the order you fix things in matters.

Hot tub chemical levels at a glance

Here are the target ranges I’d work to, with where each number comes from. Official figures vary a little between sources, so where they differ I’ve gone with the more cautious one.

Test	Target	Where the number comes from
pH	7.2–7.6	CDC: 7.0–7.8. UK HSE (HSG282): 7.0–7.6. 7.2–7.6 sits inside both.
Total alkalinity (TA)	80–120 ppm	Taylor Technologies test kit guidance; widely used by the spa industry.
Calcium hardness (CH)	150–250 ppm (acrylic) 200–400 ppm (tiled or concrete)	Taylor: 150–250 ppm for spas, 200–400 ppm for pools. Some acrylic tub makers go lower, so check your manual.
Free chlorine	3–5 ppm	CDC: at least 3 ppm in hot tubs. HSE: 3–5 mg/l.
Bromine (if you use it instead)	4–6 ppm	HSE and Taylor: 4–6. CDC’s public hot tub range is 4–8.
Combined chlorine	0, and under 1 ppm	HSE: ideally 0, under 1 mg/l acceptable. Total minus free chlorine.
Cyanuric acid (CYA)	As low as possible	CDC: don’t use it in hot tubs. HSE: keep it below 100 mg/l by diluting.

ppm (parts per million) and mg/l are the same thing for our purposes, so UK and US test kits are talking the same language.

Sources: CDC home pool and hot tub water treatment, HSE HSG282 (spa-pool systems), Taylor K-1004 kit instructions and Taylor on calcium hardness.

I used to see 1–3 ppm quoted as the “acceptable” free chlorine level and always ran my tub a bit higher, but current guidance is at least 3 ppm in a hot tub (CDC), and 3–5 from the HSE, so that’s what I’d aim for. On hardness, don’t fill from a water softener. Fully softened water has almost no calcium and is the water most likely to attack grout, heater elements and pump seals. More on that below.

The order to adjust things in

This is the bit most people get wrong. Chemicals affect each other, so if you chase pH before alkalinity is right, you’ll go round in circles. Taylor’s own instructions put it plainly: always bring total alkalinity into range before adjusting pH.

1. Test everything first. Write the numbers down. On a fresh fill, test the tap water too, so you know what you’re starting with.
2. Total alkalinity into 80–120 ppm.
3. pH into 7.2–7.6.
4. Calcium hardness into range (usually only needed on a fresh fill).
5. Sanitiser: bring chlorine to 3–5 ppm or bromine to 4–6 ppm.
6. Shock if the water has had heavy use, smells of “chlorine” or looks dull.

Add one chemical at a time. Run the jets with the lid off, give it at least 20–30 minutes (or whatever the product label says), then retest before you add the next one. Hot tubs hold very little water, so it’s easy to overshoot. Add about half to three-quarters of the calculated dose, then top up.

Total alkalinity: the buffer

Total alkalinity (TA) is a measure of the dissolved carbonates and bicarbonates in the water. Think of it as a shock absorber for pH. With TA in range, pH stays where you put it. With it too low, pH swings all over the place. With it too high, pH creeps up and won’t come down.

TA too low (under 80 ppm)	TA too high (over 120 ppm)
pH “bounces” and is hard to hold	pH keeps drifting up and is hard to lower
Corrosion of metal parts and heater	Scale and cloudy water
Etching of grout and plaster	Sanitiser works less well
Eye and skin irritation	Eye and skin irritation

To raise TA: add alkalinity increaser (sodium bicarbonate, sold as “TA Plus” or “Alkalinity Up”). It nudges pH up a little too, which is usually what you want.

To lower TA: there’s no “TA Down” product, which confused me when I started. You use pH decreaser (sodium bisulfate, sold as pH Minus or dry acid) or, in the USA, diluted muriatic acid. You could bring the pH back up afterwards with pH increaser, but that (sodium carbonate) adds alkalinity back, so you undo some of your work. Instead, lower it with acid, then run the jets and air blower with the lid off to bring the pH back up. Aeration drives off carbon dioxide, which raises pH without raising TA. AQUA Magazine explains why jetted water pushes pH up. It may take a couple of rounds. There’s more on acid in my muriatic acid guide.

pH: how well everything works

pH measures how acidic or alkaline the water is, on a scale of 0–14 where 7 is neutral. Below 7 is acidic, and above 7 is alkaline. Your eyes are comfortable at around 7.4–7.6, and chlorine does its job far better at the lower end of the range than the higher end.

pH too low (under 7.2)	pH too high (over 7.6)
Corrodes heater elements, pump seals and metal fittings	Chlorine works much less well
Etches grout, render and plaster	Scale on surfaces, heater and jets
Stinging eyes and itchy skin	Cloudy water and shorter filter life
Eats away at total alkalinity	Eye and skin irritation

In a hot tub, pH tends to rise on its own. The HSE’s spa guidance notes that heating the water drives off carbon dioxide, and the jets and blower do the same. So you’ll use far more pH decreaser than increaser. Bromine tablets and most chlorine tablets are acidic, so if you use those you may see the opposite.

Corroding a heater element or pump seal is an expensive way to learn this. Low pH is the usual culprit, so don’t ignore it. If you’re chasing heater problems, see hot tub not heating.

Calcium hardness: don’t use softened water

Calcium hardness (CH) is the amount of dissolved calcium in the water. You want some. Water that’s too soft goes looking for calcium elsewhere, and it finds it in your grout, render and plaster. It’s also more aggressive towards metal parts like heater elements. Water that’s too hard drops scale on the surfaces, heater and jets, and goes cloudy, especially when it’s hot.

• Acrylic shells (most shop-bought tubs): Taylor recommends 150–250 ppm for spas. Some manufacturers go lower. Hot Spring, for example, specifies 75–150 ppm, and lower still for its salt system. If you’ve got a branded tub, the manual wins.
• Tiled concrete, block or ICF tubs (most DIY builds): you have cement-based grout and render to protect, so I’d work to the upper end of the spa range or the pool range of 200–400 ppm.

To raise CH: add calcium hardness increaser (calcium chloride). To lower CH: there’s no chemical for it. Partly drain and refill with softer water, or dilute hard tap water with some softened water. Just don’t fill entirely from a softener.

If you live in a hard-water area (much of the south and east of England, and plenty of US states), your tap water may already be over 250 ppm. Test it before you fill. Stain and scale remover (a sequestrant) helps hold minerals in solution. My guide to filling a hot tub the correct way covers pre-filters and the first dose.

Sanitiser: chlorine or bromine?

Your sanitiser is what actually kills the bugs. There are two main choices. I use chlorine in my own tub, and it’s the more popular choice, but bromine has real advantages in a hot tub. I’ve compared them in detail in should I use bromine or chlorine in my hot tub?, so here’s the short version.

	Chlorine	Bromine
Target	3–5 ppm free chlorine	4–6 ppm
Speed	Fast acting, a strong oxidiser	Slower, but steadier
In hot water and at higher pH	Loses effectiveness as pH rises, so keep pH tight	More stable in hot water and holds up better at higher pH
Smell and feel	The “chlorine smell” is really chloramines, a sign it needs shocking	Less smell, and many people with sensitive skin find it gentler
Sunlight	Burnt off by UV, so keep the lid on	Also burnt off by UV, and CYA doesn’t protect it
Cost	Cheaper	Dearer per week, and harder to bring down if you overdose

If chlorine doesn’t agree with your skin, bromine is often the option people switch to.

Test free chlorine, not just total

Free chlorine is the chlorine that’s still available to sanitise. Combined chlorine (chloramines) is chlorine that’s already been used up on sweat and other organics, and it’s what smells and stings. Total chlorine is the two added together. If total is more than about 1 ppm above free, it’s time to shock. The HSE’s guidance is that combined chlorine should ideally be zero and not above 1 mg/l.

My own routine (and a tweak I’d make)

My tub holds about 2,700 litres (roughly 710 US gallons). My routine is 5 x 20g chlorine tablets once a week. I drop them in on a Wednesday, knowing we’ll be using the tub on the Friday. If it’s been a big weekend with lots of guests, I add a few extra on the Sunday night. The water is crystal clear most days, and none of my family has ever had a problem.

One thing to watch: most 20g “multifunctional” chlorine tablets are trichlor, which is stabilised. That means every tablet adds cyanuric acid, and it also pulls pH and TA down over time. So keep an eye on both, put tablets in a floating dispenser or the skimmer basket rather than inside the filter cartridge, and read the next section.

Cyanuric acid (CYA): why less is more in a hot tub

Cyanuric acid, also called stabiliser or conditioner, protects chlorine from sunlight in outdoor swimming pools. In a covered hot tub it does more harm than good, because it also slows chlorine down. The CDC recommends not using cyanuric acid or chlorine products containing it in hot tubs, and its Legionella toolkit says the same, because it slows disinfection.

The catch is that the two most common hot tub chlorines both contain it:

• Dichlor granules (the usual “chlorine granules” in the UK) are about half CYA. For every 10 ppm of chlorine you add from dichlor, CYA rises by about 9 ppm.
• Trichlor tablets (most 20g tablets and sticks) add it too.

CYA doesn’t get used up. It only leaves when you drain water out. So in a hot tub dosed with dichlor, it climbs every week. The HSE’s spa guidance says to keep it below 100 mg/l by diluting with fresh water, but the lower the better. Watch for chlorine that reads fine but the water still goes dull, or starts needing more and more sanitiser.

What to do about it:

• Test for CYA every month or two if you use dichlor or tablets. Test strips with a CYA pad are fine for this.
• Change the water regularly (see below), or do a partial drain and refill when it creeps up.
• Consider an unstabilised chlorine: liquid chlorine (sodium hypochlorite) or calcium hypochlorite, which adds a little calcium. Or switch to bromine, which doesn’t use CYA at all.

I’ve gone deeper on this in cyanuric acid in a hot tub.

Shock: the weekly reset

Shock is a big dose of oxidiser that burns off the sweat, oils and chloramines your normal sanitiser level can’t keep up with. It also “reactivates” bromide back into bromine. Do it after a busy session, when the water smells or looks flat, and as a weekly routine if the tub gets regular use.

• Chlorine shock (dichlor or cal-hypo) oxidises and sanitises. You’ll need to wait for chlorine to fall back into range before getting in.
• Non-chlorine shock (MPS, potassium monopersulfate) oxidises without adding chlorine, so you can usually get back in quickly. It doesn’t sanitise on its own, and it can make a total chlorine reading look higher than it is.

Get pH into range first, shock in the evening with the lid off and the jets running, and follow the dose on the tub. Products vary too much for one dose rate, so go by the label. My full guide is hot tub shock: what is it all about?

The optional extras

Anti-foam

I use it, but it’s a sticking plaster. Foam comes from detergent on swimwear, body products and water that’s out of balance or overdue a change. Anti-foam knocks the bubbles down, a bit like dropping a bar of soap into a bubble bath, but it doesn’t fix the cause. See foam in a hot tub.

Clarifier

Clarifier clumps tiny particles together so your filter can catch them. It’s useful for a slight haze. If the water is properly cloudy, check your sanitiser, pH and filter first. See cloudy water in a hot tub.

Stain and scale remover

This is worth having in hard-water areas, or if your fill water has iron or copper in it.

Alternatives and add-ons

Ozone, UV-C, mineral cartridges (silver and copper) and salt systems can all cut how much chlorine or bromine you need. But apart from salt, which makes chlorine itself, none of them replaces a measurable sanitiser level. I cover them in ozone in a hot tub, copper silver ionization and saltwater in a hot tub.

Testing: strips, drop kits or digital?

• Test strips: quick and cheap. They’re fine for a daily check, but not very precise. Keep the tub sealed and dry.
• Liquid drop kits (DPD): more accurate. The CDC recommends a DPD kit for chlorine and pH. Taylor kits are the usual choice in the USA, and Palintest in the UK.
• Digital photometers and testers: this is what I use. They’re more money up front but take the guesswork out of reading colours.

How often? The CDC’s advice for home hot tubs is to test chlorine and pH at least twice a day when the tub is in use. Realistically, most home owners test before each soak when it’s busy, and at least two or three times a week otherwise, with TA and hardness weekly. And yes, I still find testing oddly satisfying. Crystal clear water means I’m doing something right.

When to change the water

Every chemical you add leaves something behind, as do bathers. Eventually the water gets “tired”: sanitiser demand goes up, foam appears, and it won’t stay balanced however hard you try. The HSE’s spa guidance uses total dissolved solids (TDS) as the marker. It shouldn’t be more than 1,000 mg/l above your fill water. For a home tub, a drain, clean and refill every three to four months is a sensible rhythm, and sooner if it’s heavily used or CYA has crept up. Here’s how to drain and clean a hot tub, and don’t forget the filter cartridge.

Troubleshooting common problems

Problem	Likely cause	What to do
Strong “chlorine” smell, red eyes	Chloramines (combined chlorine), not too much chlorine	Check pH, then shock
Cloudy water	Low sanitiser, high pH/TA/CH or a dirty filter	Balance, shock, clean the filter, then use clarifier
pH won’t stay put	TA too low	Raise TA to 80–120 first
pH keeps climbing	TA too high, plus aeration	Lower TA with acid, then aerate
Foam	Detergents, body products, tired water	Shock, rinse swimwear, change the water if it keeps coming back
Chlorine reads OK but water still dull	CYA build-up	Test CYA, partially drain or change water
White crust on tiles or jets	Scale from high CH, pH or TA	Lower pH/TA, use scale remover, dilute hard water

There’s more in how to fix murky or cloudy water and my beginner’s guide to hot tub maintenance.

Chemical safety

• Never mix chemicals, even two different chlorines. Dichlor and cal-hypo together can react violently, and chlorine plus acid gives off chlorine gas.
• Add chemicals to water, never water to chemicals. Pre-dissolve granules in a bucket of tub water if the label says to.
• Store them separately, dry and cool (the CDC says below 35°C / 95°F), out of reach of kids and pets, and away from each other, especially chlorine and acid.
• Wear gloves and eye protection, and don’t add chemicals on a windy day or with people in the tub.

Need parts or supplies?

If you’re chasing a corroded heater, a tired filter or a pump that’s lost its seal to bad water, you’ll find replacement parts at parts4tubs.co.uk in the UK and parts4tubs.com in the USA. And if you’re still at the planning stage of your own tub, good water chemistry starts with good design: proper filtration, a skimmer and enough circulation. That’s all built into my DIY hot tub plans.

FAQs

What order should I add hot tub chemicals in?

Total alkalinity first, then pH, then calcium hardness, then sanitiser, and shock last. Add one at a time with the jets running, and retest between each.

What should the chlorine level be in a hot tub?

3–5 ppm free chlorine. The CDC recommends at least 3 ppm, and the UK HSE says 3–5 mg/l. The older figure of 1–3 ppm is a swimming pool number and too low for a hot tub.

What should the bromine level be in a hot tub?

4–6 ppm is the usual target (HSE and Taylor). The CDC’s range for public hot tubs is 4–8 ppm.

How do I lower total alkalinity without raising pH?

You can’t lower TA without lowering pH. Add pH decreaser or acid to lower both, then aerate with the jets and blower, lid off, to bring pH back up without adding alkalinity. Repeat until TA is in range.

Can I fill my hot tub with softened water?

Not entirely. Fully softened water has almost no calcium and can attack grout, heater elements and seals. Blend it with tap water or add calcium increaser so hardness ends up in range.

Do I need cyanuric acid (stabiliser) in a hot tub?

No. The CDC recommends against it in hot tubs because it slows chlorine down. It builds up from dichlor and trichlor anyway, so test for it and dilute when it climbs.

Got a chemistry question, or a reading that won’t behave? Drop it in the comments below with your numbers. I read them all.

Cheers,
Andi

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Hi, Andi here. I own Buildahottub.com and also write all of the articles and info pages on the site. Some years back now, I built my own hot tub but struggled to find the information I needed. So, once my tub was complete, I started this website to help others in their own pursuit of hot tub and plunge pools DIY building information.

Fast forward to 2025, I've helped over 1400+ DIY customers just like you all over the world build hot tubs and pools. Have a good look around the site, there are lots of resources here. Please do get in touch if I can help you. - Cheers, Andi