Pool Heater Size Calculator
Calculate the BTU heating capacity needed for your pool. Accounts for surface area, temperature rise, wind exposure, and pool cover insulation.
Heater Sizing Inputs
Pool heating with heater unit and pool cover insulation
Heating a Pool: What Really Matters
BTU: The Boring But Important Part
One BTU heats one pound of water 1°F. Pool formula: surface area × temp rise × 12 = BTU/hour. A 400 sq ft pool heated 10°F needs 48,000 BTU/hour. Not complicated, just a formula.
A 400,000 BTU Raypak or Hayward will heat a 15×30 pool about 1 degree per hour. Factor in wind and an uncovered pool, and cut that in half.
Wind and Evaporation Kill Your Budget
Exposed pools lose heat fast. Sheltered pools hold it longer. A windy day at 70°F can cost you 2–3 degrees overnight. That's why wind exposure matters. High wind? Add 20–30% to your BTU calculation.
Pool Covers: The Easiest ROI
A $200 cover cuts heating costs by 50–70%. That's a $1000/year heater bill dropping to $300–500. The cover pays for itself in weeks, not years. Basic bubble covers do 40–50% savings. Premium solar covers do 60–70%.
Gas vs Heat Pump: The Real Choice
Gas (the quick option): Heats fast. 15–30 minutes to raise 10 degrees. Works when it's freezing outside. Costs $1500–3000. But electricity runs $500–1500/month during season. Not eco.
Heat pump (the smart option): Slow warm-up (4–6 hours for 10 degrees) but uses 4–5x less energy than gas. Costs $3000–6000 upfront but runs $100–300/month. Works down to 45°F outside. Breaks even in 2–3 seasons, then free heating.
Solar (the free option): Free energy from sun. Silent. Lasts forever. But slow and needs roof space. Usually paired with gas backup.
Running the Numbers
A 20,000-gallon pool, 15°F rise, 6 months heating: Gas costs about $16,900/season. Heat pump costs about $930/season. Do the math. Even at $3000 more upfront, heat pump pays back in two heating seasons and saves $4000/year after that.
Real-World Talk
Quick warm-ups? Go gas. Long heating season? Heat pump wins. Want free? Solar, but it's slow. Most people pick gas for peace of mind or heat pump for dollars and sense.
Pool Heater Sizing Reference
| Pool Size | For 10°F Rise | For 15°F Rise | Gas Cost/Yr | Heat Pump Cost/Yr |
|---|---|---|---|---|
| 15×30 ft (400 sq ft) | 48,000 BTU | 72,000 BTU | $4,500–6,000 | $800–1200 |
| 20×40 ft (800 sq ft) | 96,000 BTU | 144,000 BTU | $9,000–12,000 | $1600–2400 |
| Round 20 ft (314 sq ft) | 37,680 BTU | 56,520 BTU | $3,500–4,500 | $600–900 |
| Small 12×24 (288 sq ft) | 34,560 BTU | 51,840 BTU | $3,200–4,200 | $550–800 |
Costs assume 180 days heating/year, no pool cover. With cover, reduce by 50–70%. Heat pump assumes COP 4.0 and $0.14/kWh. Gas assumes $1.50/therm.
