Active Ingredient % — Start Here: Get this right first and everything else falls into place

Not all chemicals are created equal

Here's the thing — two tubs of "Chlorine Granules" sitting side by side on a shelf can have completely different strengths. One might be 55% available chlorine, the other only 30%. The same goes for bromine, pH adjusters and everything else you use. If you get the percentage wrong here, your doses could end up double what you need or less than half.

Have a quick look at your product label — the active ingredient percentage is usually on the back. If it's not there, check the safety data sheet (SDS) or product spec. It only takes a minute and it makes all the difference.

A quick example of why this matters

Say you need to top up the chlorine in your 1,500 litre hot tub. With a 55% dichlor product, we'd recommend around 5.5g.

But if your product is actually only 30% available chlorine and the percentage isn't updated here, you'd add 5.5g thinking you're sorted — when you really needed 10g. That leaves your water under-sanitised, which is no good for anyone.

It works the other way too — putting in a lower percentage than your product actually has could mean you're adding too much, which can irritate skin and eyes.

Stick with granules — keep it fuss free

We've built this calculator for use with granules (chlorine or bromine). Tablets dissolve at different rates depending on water flow, temperature and tablet size — which makes getting your dose spot-on a bit of a guessing game. Granules are the way to go: pre-mix them in warm water, pop the solution in your tub, and you'll get consistent results every time. Fast, reliable and hassle free.

Measuring your dose

We'd always recommend digital scales. A basic kitchen scale accurate to 1g costs under £10 and takes the guesswork out completely. Worth every penny for peace of mind.

Don't have scales handy? No worries — a standard 5ml kitchen teaspoon works as a rough guide. Here's what a level teaspoon of each chemical typically weighs:

Chemical Level tsp Heaped tsp
Chlorine granules≈ 5g (0.18 oz)≈ 8g (0.28 oz)
Bromine granules≈ 5g (0.18 oz)≈ 8g (0.28 oz)
pH Plus (soda ash)≈ 5g (0.18 oz)≈ 8g (0.28 oz)
pH Minus (dry acid)≈ 7g (0.25 oz)≈ 10g (0.35 oz)
Alkalinity up (bicarb)≈ 5g (0.18 oz)≈ 7g (0.25 oz)
Non-chlorine shock≈ 5g (0.18 oz)≈ 8g (0.28 oz)

Know your spoons — it makes a big difference!

This catches people out more than you'd think. A dessert spoon holds twice as much as a teaspoon — grab the wrong one from the drawer and you've just doubled your dose without realising.

Teaspoon Teaspoon 5 ml ≈ 5g (0.18 oz)
Dessert spoon Dessert spoon 10 ml ≈ 10g (0.35 oz) — 2x!
Tablespoon Tablespoon 15 ml ≈ 15g (0.53 oz) — 3x!

Always use a standard 5ml teaspoon. If you're not sure, pick up a set of measuring spoons — they're cheap as chips.

All teaspoon weights are approximate and will vary by granule size. For spot-on accuracy, digital scales are your best friend.

Can't find the percentage on your label?

Give your chemical supplier or manufacturer a quick call or drop them an email — they're required to provide this information and most will happily confirm it straight away. Ask for the Safety Data Sheet (SDS) and it'll have everything you need.

Your Water Setup

Most hot tubs hold 1,000 to 1,500 litres · Not sure? Pop over to the Volume Calculator tab

Your Test Readings

Pop in the numbers from your test strips or kit and we'll do the rest

Total Alkalinity (ppm)
Spot on ✓
0
300
Ideal: 80–120 ppm
pH Level
Spot on ✓
6
9
Ideal: 7.2–7.6
Free Chlorine (ppm)
Spot on ✓
0
20
Ideal: 3–5 ppm
Bromine (ppm)
Spot on ✓
0
20
Ideal: 3–5 ppm

Your Water Care Guide

Follow these 6 steps and you'll keep your water crystal clear, safe and ready for a soak.
A little regular TLC prevents most water problems before they start.

Remember — "Tonight's A Perfect Hot tub Session"

Total Alkalinity first, then pH, then your Hot tub Sanitiser

Every two to three days (daily if you're using it a lot)

Here's How

  1. 1 Dip a test strip about 15cm below the surface for two to three seconds
  2. 2 Take it out and hold it flat — don't shake the water off
  3. 3 Wait 15–30 seconds and match the colours to the chart on the bottle
  4. 4 Make a note of your pH, chlorine or bromine, and alkalinity readings

What You'll Need

Test strips

Top Tips

  • Always test before adding anything — guessing is never a good idea
  • Wait at least 30 minutes after your last dose before testing again
  • Swap out your test strips every 12 months — old ones can give you dodgy readings
Weekly, or whenever your readings are off

Here's How

  1. 1 You're aiming for 80–120 ppm
  2. 2 Too low? Add Alkalinity Increaser (sodium bicarbonate)
  3. 3 Too high? Add pH Minus (sodium bisulphate) with the jets on high
  4. 4 Wait 20–30 minutes with jets running, then test again

What You'll Need

Alkalinity Increaser pH Minus

Top Tips

  • Low alkalinity makes your pH bounce around all over the place — not ideal
  • High alkalinity makes it really tricky to bring pH down
  • Little and often is the way to go — don't try to fix it all in one hit
Every two to three days, or when it's out of range

Here's How

  1. 1 You're looking for 7.2–7.6
  2. 2 Too low (acidic)? Add pH Plus (soda ash)
  3. 3 Too high (alkaline)? Add pH Minus (sodium bisulphate)
  4. 4 Always dissolve your chemicals in a bucket of warm water before adding them

What You'll Need

pH Plus pH Minus

Top Tips

  • Below 7.0 can be harsh on your skin and tough on your equipment
  • Above 7.8 and your chlorine basically stops working properly
  • Hot water naturally pushes pH up, so keep an eye on it
After every soak, or at least every two to three days

Here's How

  1. 1 Chlorine: keep it at 3–5 ppm for hot tubs, 1–3 ppm for pools
  2. 2 Bromine: 3–5 ppm — stays more stable in warm water, which is why it's great for hot tubs
  3. 3 Dissolve your granules in warm water first, then add with the circulation running
  4. 4 Pick either chlorine or bromine and stick with it — never mix the two

What You'll Need

Chlorine Granules (Dichlor) Bromine Granules

Top Tips

  • Bromine is a popular choice for hot tubs — less smell and it handles the heat well
  • Chlorine works fast but doesn't last as long in hot water
  • Always give your tub a dose after a bathing session — it's a good habit to get into
Once a week, or after a busy session

Here's How

  1. 1 For your regular weekly shock, non-chlorine shock (MPS) does the job nicely
  2. 2 For anything heavier (cloudy water, algae), reach for a chlorine shock instead
  3. 3 Pop the cover off, get the jets running, and add your shock
  4. 4 Give it at least 20 minutes before putting the cover back on or getting in

What You'll Need

Non-Chlorine Shock (MPS) Chlorine Shock

Top Tips

  • Shocking is about oxidising — it's not a replacement for your regular sanitiser
  • Water looking a bit cloudy? Give it a shock first, then check your sanitiser
  • Best done in the evening so your tub's ready for a morning soak

Here's How

Cleaning paper based filters (actually polypropylene in many cases) opens and disrupts the fibres which allows larger particles of solids to pass through (you can’t see them but it is happening)! Don’t let ‘looking clean’ fool you, looking clean and performing as they should does not match - after 7-10 days of regular use, as the manufacturer specifies, they should be changed to perform effectively and not just look like they are!

Top Tips

  • Never use household cleaners on your tub — they'll cause all sorts of foaming
  • Nine times out of ten, cloudy water comes down to dirty filters
Upgrade to a SUPA Filter

Fresh Water, Happy Soaking

We recommend changing your water at 50% of the industry-standard TDS threshold — that means when your Total Dissolved Solids hit around 750 ppm above your fill water, it's time for a fresh start. Why? Because nobody wants to soak in someone else's old water. Fresh water means a cleaner, healthier and more enjoyable experience every time you hop in. Please note: TDS monitors are available to purchase online for around £10.

When your tub is SUPA Heated, you don’t need to fear the hassle of a water change.

With a SUPA Heater, you can drain, refill and reheat your hot tub in a fraction of the time that traditional electric heaters take. No more waiting hours (or even days) to get back up to temperature. That means changing your water more often is finally fast and hassle-free, giving you the freshest hot tub water on the planet without the wait.

Hot Tubs: We'd suggest a full drain and refill every 6 to 8 weeks for the best experience, or whenever your TDS hits that 750 ppm mark.

Work Out Your Water Volume

Hot tubs are usually 0.7 to 1.0m deep · Pools are usually 1.0 to 2.0m

Your Estimated Volume

0 litres
0 US gallons | 0 UK gallons

1 UK gallon = 4.546 litres · 1 US gallon = 3.785 litres

Common Sizes — Hot Tubs