Ice Bath Chlorine Calculator: Grams to Add for Your Tub

A cold plunge that stays full for weeks needs a sanitiser, and the dose that keeps it clear is far smaller than most people expect. Tell the calculator how much water your tub holds (or its measurements), what your test strip reads now and the level you want, and pick your product. It gives you the dose in grams or millilitres. One part per million is one milligram per litre, so a 400 litre tub needs only 0.4 g of actual chlorine to rise by 1 ppm. The rest of what you weigh out is the carrier in the product.

How the dose is worked out

Free chlorine is measured in parts per million, and in water one ppm is one milligram per litre. So the chlorine you need is the rise in ppm multiplied by the litres, divided by 1,000 to give grams. That figure is pure chlorine, and no product is pure chlorine: dichlor granules are sold as around 56% available chlorine, calcium hypochlorite around 65%, and liquid pool chlorine usually 10 to 15%. The calculator divides by the strength on the label to give the amount of product to weigh out. For liquids it reads the label percentage as grams of chlorine per 100 ml, which is how liquid pool chlorine is normally sold.

Worked example: a 400 litre tub reading 0 ppm, aiming for 3 ppm, needs 3 x 400 / 1,000 = 1.2 g of chlorine. With 56% granules that is 1.2 / 0.56 = 2.1 g of product.

What level to aim for

Hot tub guidance usually sits at 3 to 5 ppm free chlorine, and pools lower than that. A cold plunge sits between the two: the water is too cold for bacteria to multiply quickly and chlorine breaks down more slowly in the cold, so many owners hold 1 to 3 ppm. Your tub or chiller maker's manual has the final word, and some chillers set a maximum because chlorine attacks certain heat exchangers and seals.

Getting the dose in safely

• Weigh it. For a small tub the dose is a couple of grams, which is too little for kitchen scales to read reliably. A 0.1 g jewellery scale costs very little and removes the guesswork.
• Dissolve granules first in a bucket of tub water, then pour it in with the pump or filter running. Undissolved granules sitting on a liner can bleach it.
• Add chemicals to water, never water to chemicals, and never mix two chlorine products together or with any other chemical.
• Retest after circulating for 20 to 30 minutes before anyone gets in, and add a smaller top-up if the reading is still short rather than doubling up.
• Watch stabiliser build-up with dichlor. Dichlor adds cyanuric acid each time you dose, which slowly makes the chlorine less effective. If your strips read high stabiliser and the water still clouds, a partial drain and refill is the fix.
• Avoid household bleach unless the bottle says it is plain sodium hypochlorite with no scents or thickeners, and check its strength, which is usually far lower than pool chlorine.

Chlorine is not the only option: bromine and hydrogen peroxide systems are also used in plunge tubs, and this calculator does not cover their doses. If you are still sizing the tub, our ice bath chiller size calculator and ice calculator cover the cooling side.