Pool pH and alkalinity calculator

Why alkalinity and CYA change the answer

Most pH charts say something like "6 fl oz of acid per 0.1 pH per 10,000 gallons." That's a rule of thumb, and it can be off by a factor of two, because the same dose moves pH by very different amounts in different water.

pH is held in place by buffers. The main one is your total alkalinity: dissolved bicarbonate soaks up acid before pH can move. Stabilizer (CYA) is a second, smaller buffer. The higher your alkalinity and CYA, the more acid it takes to drop pH the same amount. This calculator works that out with the carbonate chemistry pool chemists use, so it gives a dose for your water.

Example, 10,000 gallons, pH 7.8 down to 7.4: with alkalinity at 80 and CYA at 30 it takes about 12 fl oz of 31.45% muriatic acid. At alkalinity 120 and CYA 50, it takes about 18.

Getting pH right

• Aim for 7.4 to 7.6. Chlorine works well, swimmers are comfortable, and plaster is protected. 7.2 to 7.8 is acceptable.
• pH drifts up in most pools, especially salt pools and pools with lots of aeration (waterfalls, spillovers). Higher alkalinity makes it drift faster. If you're adding acid every few days, lowering alkalinity to 60 to 80 often slows it down.
• Add acid in stages. Pool operator guidance is no more than about a quart per 10,000 gallons at a time; circulate, retest, then add more.
• Safety: always add acid to water, never water to acid. Pour slowly in front of a return with the pump running, and keep it off skin, eyes and the pool deck.

Raising and lowering alkalinity

To raise it, use baking soda (sodium bicarbonate): 1.4 lb raises 10,000 gallons by 10 ppm and barely moves pH. Soda ash raises alkalinity too, but it pushes pH up a lot, so use it when pH is also low.

To lower it, add acid, which lowers pH and alkalinity together, then aerate the water to bring pH back up. Aeration drives off carbon dioxide, which raises pH without raising alkalinity. Repeat until alkalinity is where you want it.

Next: how much chlorine to add.

Sources

Alkalinity and chlorine dose rates match the Water Chemistry Adjustment Guide adapted from the National Swimming Pool Foundation Pool & Spa Operator Handbook, as published by the Indiana State Department of Health. pH doses use standard carbonate equilibrium chemistry with the usual correction for CYA's share of measured alkalinity; borates are not included. These are estimates for planning. Always follow the product label and retest.