How the estimate works
pH resists change because of buffers. Carbonate alkalinity is the main one; cyanuric acid and borates add more. The calculator converts your total alkalinity (minus the part contributed by CYA and borates) into total carbonate, then works out how much acid or base it takes to move the whole buffer system from your current pH to the target. It assumes no carbon dioxide escapes while you adjust, which is close to true for the first few hours.
Lowering pH
- Muriatic acid (hydrochloric acid, 31.45% is standard; 14.5% “safer” versions need about twice as much). Pour slowly in front of a return jet with the pump on.
- Dry acid (sodium bisulfate) is easier to handle but adds sulfates over time.
Raising pH
- Soda ash (sodium carbonate) raises pH and also raises TA.
- Borax raises pH with little change in TA and adds a little borate buffer.
- Aeration (fountains, jets, spillovers) raises pH for free by driving off CO2, without changing TA.
Add about two-thirds of a large dose, circulate for an hour and retest. Test-kit pH readings can be off when FC is above about 10 ppm.
Not professional advice. Results are estimates for residential pools and hot tubs. Read and follow product labels: add chemicals separately, to water (never water to chemicals), with the pump running, and retest before adding more. Commercial and public pools must follow local health codes.