How we calculate
Formulas, constants and assumptions behind every calculator.
Chlorine
Grams of available chlorine needed = ΔFC (mg/L) × volume (L) ÷ 1000. Liquid pool chlorine uses trade percent (12.5% = 125 g/L available chlorine). Household bleach uses % sodium hypochlorite by weight, converted with an estimated density (1 + 0.0118 × %) and the ratio of Cl2 to NaOCl molar mass (70.906/74.442). Solids use their labelled available-chlorine percentage. Side effects: cal-hypo adds 100.09/(2×70.906) ≈ 0.71 ppm CH, dichlor 129.07/(2×70.906) ≈ 0.91 ppm CYA and trichlor 129.07/(3×70.906) ≈ 0.61 ppm CYA per ppm FC.
pH, alkalinity and acid
Total alkalinity is modelled as carbonate + cyanurate + borate + hydroxide − hydrogen ion, using conditional constants for typical pool ionic strength at 25°C: pK1 6.29, pK2 10.17, cyanuric acid 6.83, boric acid 9.17. From your pH and TA we derive total carbonate; acid (or base) needed is the change in modelled alkalinity between the two pH values, with carbonate held constant (no CO2 loss). Muriatic acid 31.45% is taken as 10.0 mol/L HCl (density 1.16), 14.5% as 4.26 mol/L, dry acid as 93.2% sodium bisulfate. Soda ash adds 1 mol carbonate and 2 equivalents of alkalinity per mole; borax adds 4 mol borate and 2 equivalents.
Calcium, CYA and salt
Calcium chloride: 110.98 g per mol Ca, with purity 94% (anhydrous pellets), 77% (flake) or dihydrate (147.01 g/mol). Hardness is expressed as CaCO3 (100.09 g/mol). Stabilizer and salt are dosed by mass at 99% purity. Partial drain fraction = (current − target) ÷ (current − fill water).
Saturation index
Langelier: LSI = pH − [(9.3 + A + B) − (C + D)], A = (log10TDS − 1)/10, B = −13.12 log10(T°C + 273.15) + 34.55, C = log10(CH) − 0.4, D = log10(carbonate alkalinity).
Testing
Every formula is covered by automated tests, including published reference doses such as 10.2 fl oz of 12.5% liquid chlorine and 1.4 lb of baking soda per 10 ppm in 10,000 gallons.
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.