Hot Tub pH Calculator: Grams of pH Minus or pH Plus

The label on a tub of pH minus gives one dose for every hot tub, but the amount that actually moves your pH depends on how much alkalinity is in the water. A tub at 140 ppm alkalinity can need roughly half as much acid again as the same tub at 90 ppm for the same pH drop, which is why the label dose often does nothing or overshoots. Enter your litres, your pH and alkalinity readings and the water temperature, and the calculator works out the grams of pH minus (dry acid) or pH plus (soda ash) from the carbonate chemistry of your water.

Why alkalinity changes the pH dose

pH measures how acidic the water is right now. Total alkalinity measures how much acid the water can soak up before that number moves, and in a hot tub filled from the mains almost all of it is dissolved bicarbonate. When you add pH minus, most of the acid is absorbed by turning bicarbonate into carbon dioxide, and only what is left over shifts the pH. So the more alkalinity in the water, the more acid it takes for the same drop.

The calculator works this out the way a water chemist would. From your pH, alkalinity and temperature it estimates how much dissolved carbonate is in the tub, using the standard equilibrium constants for carbonic acid and water at that temperature, with a small correction for dissolved salts. It then finds how much acid (or soda ash) moves that water from your reading to your target. Sodium bisulphate releases one unit of acid per 120 grams, and sodium carbonate takes up two units per 106 grams, so the result is converted to grams of product and scaled up if your product is less than 100% pure.

As a worked example, a 1,000 litre tub at 38°C with alkalinity of 100 ppm and no stabiliser needs about 13 grams of pure sodium bisulphate to go from pH 8.0 to 7.4. At 150 ppm alkalinity the same drop takes about 20 grams. Going the other way, raising the same 100 ppm tub from 7.0 to 7.4 takes about 20 grams of soda ash, and lifts alkalinity by roughly 19 ppm as it does so.

Fix alkalinity first if it is far out

• Low alkalinity, bouncing pH. Below about 60 ppm the water has little buffer, so small doses swing the pH a long way and it will not hold. Raise alkalinity with bicarbonate of soda first; the hot tub alkalinity calculator gives that dose.
• High alkalinity, pH that keeps creeping up. Hot water and jets drive carbon dioxide out, and with lots of alkalinity in reserve the pH climbs back after every dose. Dry acid lowers alkalinity a little each time; adding it with the jets off, then aerating with the cover open, brings alkalinity down over a few rounds.
• pH plus raises alkalinity too. Soda ash adds alkalinity as well as raising pH, so repeated pH plus doses can push alkalinity above the range your manual gives. The calculator shows where alkalinity should end up.

Dose in stages and retest

• Add half, circulate, test. Most test strips read pH in steps of about 0.2, so one reading can be a step out. Add half the dose with the pump running, wait the time on the product label, then test again before adding the rest.
• Pre-dissolve if your manual says so. Some makers advise mixing granules in a bucket of tub water first so they do not sit on the shell.
• Never mix chemicals. Add pH minus and chlorine or bromine separately, with time to circulate between them, and wear gloves when handling acid.
• Then sanitiser. Chlorine works best once pH is settled; the hot tub chlorine calculator works out that dose.

The calculator does not account for borates, which some hot tub balancers and mineral systems add and which absorb extra acid; if you use them, expect to need a little more pH minus than shown. If readings will not settle after several rounds, a fresh fill may be quicker, and the hot tub water change calculator shows when a drain is due anyway.