Hot Tub Salt Calculator: How Much Salt to Add

A saltwater hot tub still uses chlorine: the cell makes it from dissolved salt, and it only works properly when the salt sits inside the range your system's maker gives. Too little and the cell cannot make enough sanitiser; too much and you cannot take salt out except by draining. Enter your tub's volume, your test reading and the target from your manual, and the calculator gives the salt to add in grams, kilograms and bags, or the share of water to swap out if you have overshot.

How the sum works

Salt readings are given in parts per million (ppm), which for water is the same as milligrams per litre. Some testers show grams per litre instead; 1 g per litre is 1,000 ppm, and the calculator converts for you. To raise the level, multiply the rise you need by the litres in the tub, which gives milligrams, then divide by 1,000 for grams. A 1,500 litre tub going from 0 to 2,000 ppm needs 1,500 × 2,000 ÷ 1,000 = 3,000 g, or 3 kg.

Lowering the level works the other way. Salt does not evaporate or get used up by the cell in any useful amount, so the only way down is to drain some water and refill with fresh. If you replace a share of the water with tap water that holds almost no salt, the reading falls by roughly that share: swap a quarter of the water and a 2,400 ppm reading drops to about 1,800 ppm. The calculator works out the share you need and turns it into litres.

Before you pour it in

• Use the salt your maker specifies. Salt systems generally want pure, fine salt without anti-caking agents or added iodine. Your manual or the system's supplier will say which product is suitable.
• Add it in stages. Put in most of the dose with the pump running, let it dissolve and circulate, then test again before adding the rest. It is easy to add a little more and a nuisance to take salt out.
• Check the reading after topping up with water. Splash-out and drain-downs take salt with them, while evaporation leaves the salt behind and concentrates it. Top-up water dilutes it again, so test after any big change in level.
• Keep the rest of the water balanced. A salt cell still needs pH and alkalinity in range to keep the chlorine it makes working. Our hot tub pH calculator and hot tub alkalinity calculator do those doses.

If you do not know your tub's volume, start with the hot tub volume calculator. For tubs sanitised with granules rather than a salt cell, the hot tub chlorine calculator gives the dose in grams, and the hot tub water change calculator tells you when a full drain is due.