Hot tub chemical dosing: working out the right amount

Step 1: know your water volume

Most inflatable tubs list their water capacity in litres, such as 669 L or 916 L. That is usually to the fill line. If you do not know it, use our water volume calculator with the inner diameter and water depth.

Using the real volume matters. Many overdoses come from guessing high, especially with a compact four person tub. See inflatable hot tub sizes for typical figures.

Step 2: read the label dose

Labels give a dose per 1,000 litres, per 100 litres or per cubic metre (1 cubic metre is 1,000 litres). Some give separate doses for start up, routine and shock. Note which one you need and the measure used: grams, scoops or millilitres.

If the label uses US gallons, 1 US gallon is about 3.785 litres. If it uses UK gallons, 1 gallon is about 4.546 litres.

Step 3: scale the dose

Divide your litres by the label's volume and multiply by the dose. For example, if a product says 10 g per 1,000 L and your tub holds 800 L, the dose is 800 / 1,000 x 10 = 8 g.

Start with slightly less than the full calculated dose, let it circulate and retest. It is easy to add more and harder to take it out.

Target ranges to aim for

For more on balancing, read pH and alkalinity.

• pH: about 7.2 to 7.8.
• Total alkalinity: often around 80 to 120 ppm (check your kit).
• Free chlorine: about 1 to 3 ppm, or as the product suggests.
• Bromine, if used: often around 3 to 5 ppm.

Dosing safely

1. 1

   Test first and decide what to adjust.

2. 2

   Measure with a clean, dry scoop or measuring cup kept for that product.

3. 3

   Add to the water with the filter or bubbles running and the cover open.

4. 4

   Add one product at a time and wait as the label says.

5. 5

   Retest before adding more.

Note:Never mix chemicals together outside the tub. Add chemicals to water, not water to chemicals, and store them in original containers away from children and pets.

Making it easier

Write your tub's volume and your usual doses on a card kept with the chemicals. Use our guide to adding chemicals for the order, and see what chemicals a hot tub needs if you are starting out. If chlorine overshoots, see how to lower chlorine.

Choosing a tub? Our 2 to 4 person guide lists capacities so you can see the water volume before you buy.

See compact tubs and their capacities →

Why the same tub needs different doses

A dose that worked last week may be too much or too little this week. More bathers, longer soaks, hot sunny days with the cover off and leaves blowing in all use up sanitiser faster. A quiet, cool week with the lid on uses much less. That is why testing before you dose matters more than any fixed chart.

Water that has been in the tub for a while also behaves differently from fresh water. Over weeks, dissolved solids build up and chemicals become less effective, so doses creep up. When that happens, it is usually time for a water change rather than more chemicals.

Frequently asked questions

How much chlorine do I put in an inflatable hot tub? It depends on the product and your litres. Scale the label dose to your water volume and aim for the free chlorine range on your test kit.

How much chlorine for a Lay-Z-Spa? Use the tub's listed capacity, such as 669 L or 916 L, and scale the dose on your chlorine product's label.

Is there a chemical calculator I can use? Our water volume calculator gives your litres. Then multiply by the label dose per litre.

Why does the same dose give different results? Bather load, temperature, sunlight and how long the cover is off all change how fast chemicals are used.