Sources & formulas

Every calculator on SwimCalcs rests on established math from geometry and water chemistry. Unlike topics with a time dependency, "verification" here is mathematical: each formula is tested against known values and is therefore correct for good. Here are the technical foundations by area.

Pool volume & capacity

• Gallons per cubic foot: 7.48 US gallons per ft³; water weighs 8.34 lb per US gallon.
• Shape multipliers: rectangular 7.5; round/oval 5.9 (π/4 × 7.48 ≈ 5.87 for a circle/ellipse).
• Average depth: (shallow + deep) ÷ 2; kidney/freeform volumes are estimates.

Salt & water chemistry

• ppm dosing: weight of solute (lb) = gallons × Δppm × 8.34 × 10⁻⁶.
• Product strength: product weight = available-chemical weight ÷ strength fraction (e.g. cal-hypo 73%).
• Standard increments (per 10,000 gal): +10 ppm TA ≈ 1.5 lb baking soda; +13 ppm CYA ≈ 1 lb stabilizer; +10 ppm CH ≈ 1.25 lb calcium chloride; +1 ppm FC ≈ 10.7 fl oz of 12.5% liquid chlorine.
• Dilution: to lower salinity/CYA/CH, drain fraction = 1 − target/current.

Heating

• Specific heat of water: 1 BTU per pound per °F; energy = gallons × 8.34 × ΔT.
• Fuel units: 1 therm = 100,000 BTU; 1 kWh = 3,412 BTU; heat-pump output = electrical input × COP.
• Surface rule of thumb: heater BTU/hr ≈ surface ft² × ΔT × 12.

Costs & operations

• Fill time: hours = gallons ÷ (GPM × 60). Turnover: GPM = gallons ÷ (hours × 60).
• Pump power: kW = HP × 0.746 ÷ motor efficiency.
• Filter sizing: area = flow ÷ design rate (sand ≈ 15, cartridge ≈ 0.375, D.E. ≈ 2 GPM/ft²).
• Cost tools use your entered rate ($/gal, $/1,000 gal, $/kWh, $/therm) — no tariff is stored, so the site needs no maintenance.