Pool Heater BTU Calculator

There are two separate questions here, and we keep them apart so heat loss is never counted twice. Sizing asks what heater you need; heat-up time asks how long a given heater takes.

SIZING
recommended BTU/hr = surfaceArea(sq ft) × rise(°F) × M
                     M = 12 (average) | 15 (windy/shaded)
cover on ⇒ × 0.70   (−30% load)
kW = BTU/hr ÷ 3412.142

HEAT-UP (needs a pool volume)
total energy (BTU) = volume(gal) × 8.33 × rise(°F)
heat-up hours      = total energy ÷ (heater output × efficiency)
                     output = entered, else the recommended size

dimensions mode: surface area = L × W (rectangle), π(d/2)² (round),
                 L × W × π/4 (oval); volume = pool-volume engine
                 (shape × avg depth × 7.48 gal/ft³)

Sizing uses surface area × rise × 12, which already folds in steady-state surface loss. Heat-up time uses volume × 8.33 × rise, which is the pure energy to warm the water. They're deliberately separate so loss isn't counted twice. We do not stack a hidden 25% buffer on top of the surface-area formula — that would double-count the loss the ×12 already captures.

The recommended size is the raw computed BTU/hr — we don't snap it to a brand's rating lineup or steer you toward a specific model. Heater type doesn't change the size: gas and heat pump get the same surface-area-derived number. The difference is that a heat pump derates in cold air and runs longer, which we flag as a caveat rather than a second blind number.

The 8.33 BTU/(gal·°F) is the specific heat of water (1 BTU/lb·°F × 8.33 lb/gal). In From dimensions mode the gallons come from the same engine the Pool Volume Calculator uses (7.48 gal/ft³, primary-source-validated shape constants). In Volume mode we back-derive the surface area from your gallons (assuming a 5 ft average depth), so that sizing figure is looser than a measured area.

Sources: AquaDoc — What Size Pool Heater Do I Need? (surface-area ×12 method, exposure band, heat-pump derating); Calculator Academy — Pool Heat Time Calculator (volume × 8.33 ÷ output); MBTEK — Pool Calculator (~30% cover reduction); CalcSmart — Pool Heater Calculator (heat pumps ~2–3× longer than gas).