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Hot tub & pool chemicals, without the guesswork.

Tell it how much water you've got and what your test strip says. It'll tell you what to add, how much of it, and in what order — in grams, not "a capful-ish".

Start calculating How it works

1 Your water

What are you treating?
How do you want to enter the size?
Water volume 1,000 litres (220 UK gal)

2 Your test readings

Leave anything you didn't test blank and it'll be skipped.

How it works

Most chemical tubs give you a dose for a "standard" pool that probably isn't the size of yours, and leave you to do the maths. This does the maths for you.

01

Work out your volume

Type in the litres if you know them (it's usually in the manual), or measure it. Use the average depth with people out of the water.

02

Test the water

Dip a test strip or use a drop kit, then enter free chlorine, pH and total alkalinity. Anything you skip is ignored.

03

Add, wait, retest

You'll get a dose in grams for each product, in the right order. Add one thing at a time, let it circulate, then test again.

The levels you're aiming for

These are the ranges the calculator works to. Hot tubs run warmer and with more people per litre, so they need a bit more chlorine than a pool.

ReadingHot tubPoolWhy it matters
Free chlorine3 – 5 ppm1 – 4 ppmKills bacteria. Too low isn't safe; too high irritates skin and eyes.
pH7.2 – 7.67.2 – 7.6Chlorine works badly outside this range, and the water gets harsh.
Total alkalinity80 – 120 ppm80 – 120 ppmActs as a buffer so your pH doesn't bounce around.

Why the order matters

Alkalinity first, then pH, then chlorine. It sounds fussy but there's a good reason for it. Alkalinity holds your pH in place, so if you fix pH while alkalinity is off, it'll just drift back. And chlorine is far less effective when the pH is wrong, so there's no point topping it up until the pH is sorted.

If more than one thing needs adjusting, do them one at a time. Add the first product, run the jets or pump, wait, then retest before moving on. You'll often find the second adjustment needs less than you thought.

Questions people ask

How accurate are the doses?

They're a good starting point, not gospel. Products vary in strength from brand to brand, test strips are only so precise, and things like water temperature and how many people have been in all have an effect. That's why it always tells you to retest. If you're unsure, add a bit less than suggested — it's much easier to add more than to take it back out.

Which products does it assume I'm using?

Sodium bicarbonate (sold as alkalinity increaser or plain baking soda), soda ash for raising pH, dry acid (sodium bisulphate) for lowering pH, and chlorine granules — dichlor if you've ticked the stabilised box, calcium hypochlorite if not. If the label on your tub gives a different dose rate, trust the label.

My alkalinity is high. Why doesn't it give me a dose?

There isn't really a product that only lowers alkalinity. The usual approach is to use pH decreaser, which brings both down, then aerate the water (jets on, air valves open) to push the pH back up while alkalinity stays lower. It can take a couple of rounds.

What's the deal with stabilised chlorine?

Stabilised chlorine contains cyanuric acid (CYA), which protects chlorine from sunlight. The catch is it doesn't go away — it builds up every time you dose. Too much CYA makes your chlorine sluggish even when the test strip looks fine. The only real fix is to drain and replace some of the water now and then.

How do I measure the volume of my hot tub?

The easiest way is to check the manual or the manufacturer's website — most hot tubs hold somewhere between 800 and 1,500 litres. If you can't find it, measure the inside length and width at water level and the average depth, then use the "Measure it" option. It'll be a bit over because of the seats, which is fine.

Do you store my readings?

No. Everything is worked out in your browser. Nothing you type is sent anywhere, and there are no accounts, adverts or tracking.

Please read: this calculator gives estimates to help you along. Always confirm your levels with a reliable test kit before anyone gets in. Never mix chemicals together, add them to the water (not water to them), and keep everyone out until they've dissolved and circulated. Follow the instructions on your product labels.