Hot Tub Alkalinity Calculator: Bicarb Grams for Your Litres

If your hot tub's pH will not stay put, the number to fix first is usually total alkalinity, the buffer that stops pH swinging every time you add chlorine or get in. Plain bicarbonate of soda raises it, and a tub of "alkalinity increaser" is often the same chemical at a higher price. Enter your litres and your test strip reading below and the calculator gives the dose in grams to raise alkalinity, or the amount of dry acid to bring it down, worked out from the chemistry rather than a rule of thumb.

How the dose is worked out

Total alkalinity is reported as ppm of calcium carbonate, and 1 ppm of it is worth 0.02 millimoles of acid-neutralising capacity in every litre. Sodium bicarbonate supplies one unit of that capacity per molecule and weighs 84 grams per mole, so it takes 1.68 grams to raise 1,000 litres by 1 ppm. Soda ash supplies two units per molecule at 106 grams per mole, so it takes 1.06 grams. Going the other way, sodium bisulphate neutralises one unit per molecule at about 120 grams per mole, so it takes about 2.4 grams of the pure chemical to lower 1,000 litres by 1 ppm, and proportionally more if the product is less than 100% sodium bisulphate.

So to take an 800 litre tub from 60 to 100 ppm with bicarbonate of soda, the sum is 40 × 0.8 × 1.68, which is about 54 grams. The calculator runs that sum with your figures 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).

Bicarbonate of soda or pH plus?

• Bicarbonate of soda raises alkalinity a lot and pH only a little. It is the right choice when alkalinity is low and pH is roughly where it should be, or low too. Kitchen bicarbonate of soda and branded alkalinity increaser are usually the same chemical; check the ingredients.
• Soda ash (pH plus) raises pH sharply as well as alkalinity. Use it when pH is low but alkalinity is already in range. Using it to fix alkalinity alone tends to push pH too high, which can turn the water cloudy.
• Baking powder is not the same thing. It contains acid and starch as well as bicarbonate. Do not put it in a hot tub.

Lowering alkalinity that is too high

High alkalinity makes pH drift upwards and resist correction. Dry acid brings both alkalinity and pH down. A common method is to add the dose with the jets off, let the pump circulate it, then run the jets and air controls with the cover open: bubbling drives carbon dioxide out of the water, which raises pH again without raising alkalinity. Repeat in stages until both readings are where you want them. If alkalinity keeps creeping up after every refill, your tap water is probably hard and high in alkalinity. Your water company publishes hardness figures for your postcode.

Dose in stages and retest

• Add half, wait, test. Test strips are only accurate to a band, so a big correction worked out from one reading can overshoot. Add half the dose with the pump running, let it circulate for the time your product label gives, then test again.
• Fix alkalinity before pH. pH will keep wandering until alkalinity is in range, so sort alkalinity first.
• Pre-dissolve if your manual says so. Some makers advise mixing the powder in a bucket of tub water first so it does not sit on the shell.
• Then chlorine. Chlorine works best once pH is settled; the hot tub chlorine calculator works out that dose.

If the readings will not come right after several rounds, a fresh fill may be quicker than chasing them; the hot tub water change calculator shows when a drain is due anyway.