Hot Tub Chlorine Calculator: Grams to Add for Your Litres

The question "how much chlorine do I put in my hot tub?" has no single answer, because it depends on three numbers: how much water the tub holds, what your test strip says now, and how strong the granules in your tub are. A scoop sized for a 1,500 litre hard-shell tub is two and a half times the right dose for a 600 litre inflatable. Enter your own figures below and the calculator gives you the dose in grams, worked out from the percentage of available chlorine printed on the tub.

How the dose is worked out

A reading of 1 ppm means 1 milligram of chlorine in every litre of water. So to lift 800 litres from 1 ppm to 3 ppm you need 2 × 800 = 1,600 milligrams of actual chlorine. Granules are not pure chlorine, though: a product labelled 56% available chlorine delivers 0.56 grams of chlorine for every gram you add. Divide 1.6 grams by 0.56 and the answer is about 2.9 grams of granules. The calculator does exactly that sum with your numbers, and converts gallons to litres first if you enter the volume that way (one UK gallon is 4.546 litres, one US gallon is 3.785).

The answer is the amount needed to reach the target in the water as it stands. It does not allow for chlorine that is used up as it works, which is why a busy tub, warm weather or a lot of bathers can leave the reading lower than expected a few hours later. Test again before you add more rather than doubling the dose up front.

Top-ups and shock doses

• Top-up: bring the free chlorine back into the range in your manual. Test before each use and top up when it drops.
• Shock: a deliberately higher dose to clear cloudy water, a smell of chlorine (which usually means used-up chlorine, not too much) or a heavy session. Manufacturers and chemical labels give different shock levels, so the calculator lets you set your own figure. Leave the cover open while it gasses off and do not get in until the reading is back in your normal range.
• Non-chlorine shock (potassium monopersulphate) is a different chemical with its own label dose. This calculator does not cover it, or bromine.

Things that will change your reading

• Stabiliser builds up. Chlorine granules sold for hot tubs are usually dichlor, which carries cyanuric acid (stabiliser) with it. As a rule of thumb, every 1 ppm of chlorine from dichlor adds roughly 0.9 ppm of stabiliser, and trichlor adds about 0.6 ppm. It never gasses off, so after weeks of dosing the level climbs and chlorine works less well. Draining and refilling is the usual fix.
• pH matters. Chlorine works best when the pH is in the range your manual gives. Correct the pH first if it is well out, then dose.
• Trichlor is acidic. It lowers pH as it dissolves, which is why many hot tub makers advise against it in small volumes of hot water. Check your manual before using it.
• Pre-dissolve if your manual says so. Some makers advise mixing granules in a bucket of water first, with the jets running, so undissolved grains do not sit on the shell.

Running a tub at home has other costs too: the hot tub running cost calculator adds up the electricity and refills. If you would rather someone else handled the water chemistry, our guide to private spa hire across the UK lists places to book a hot tub by the hour.