How the pH math works
Pool water is buffered by carbonate (your alkalinity), and also by cyanurate from stabilizer and by borates if you use them. The calculator models all three, adjusts for temperature, and works out how much acid takes the water from your pH to your target. Simpler calculators use a flat rule of thumb, which can be off by half or more when TA is high or low.
Raising pH works the same way in reverse. Soda ash adds a lot of pH per ounce; borax raises pH with less effect on alkalinity; baking soda barely moves pH and cannot take it above about 8.3. In this 15,000 gallon example, pH 7.0 to 7.5 at TA 80 takes 3.1 lb (1.4 kg) of soda ash.
Adding acid safely
- Wear goggles and gloves, open the jug outdoors and stand upwind.
- Run the pump. Pour slowly in front of a return jet, low over the water.
- Keep each addition to about 1 quart of 31.45% acid per 10,000 gallons. The calculator splits larger doses for you.
- Wait 30 minutes, then retest before adding more.
- Never add acid and chlorine together or in the same spot.
Why pH keeps rising
Aeration (waterfalls, spillovers, jets, rain) drives carbon dioxide out of the water, which pushes pH up. High alkalinity makes this faster. If you add acid every few days, lower your alkalinity to the bottom of the range and pH will hold steadier. The alkalinity calculator explains how.