Pool Heater BTU Sizing Calculator

The base formula. Energy to heat water = volume (gal) × 8.34 (lb/gal) × ΔT (°F) × 1 BTU/(lb·°F). The 1 BTU/lb/°F is the specific heat of water. Divide by hours to heat = BTU/hr capacity needed.

For a closed system (no heat loss), that is all you need. Pools are not closed systems.

Heat loss is the killer. An outdoor pool loses heat constantly to evaporation (the biggest single source), conduction to the cooler ground, and radiation to the night sky. On a warm day with no cover the pool can lose 5°F overnight. With a solar cover, more like 1-2°F.

Standard adjustment factors:

• Indoor pool: 1.25× (small loss, mostly through walls)
• Outdoor pool with cover: 1.5× (significant evaporative loss when uncovered)
• Outdoor pool no cover: 2.0× (major evaporative loss)

Common heater sizes for residential pools:

• 100,000 BTU/hr: small spas, hot tubs
• 200,000 BTU/hr: small pools (<15,000 gal) with cover
• 300,000 BTU/hr: medium pools (15-25,000 gal)
• 400,000 BTU/hr: large pools (25-40,000 gal)
• 500,000+ BTU/hr: very large pools or fast-heat applications

Worked example. 20,000 gal outdoor pool, no cover, raise 65°F to 82°F (17°F rise) in 24 hours.

• Base BTU = 20,000 × 8.34 × 17 = 2,835,600 BTU total
• ÷ 24 hours = 118,150 BTU/hr base
• × 2.0 (outdoor no cover) = 236,300 BTU/hr

So a 250,000 BTU heater is correct for this pool, and a 200,000 would never quite catch up.

A bigger heater does not burn more gas for the same job. This one gets stated backwards constantly. Warming a given body of water by a given number of degrees takes a fixed amount of energy, and a 400,000 BTU unit simply delivers it in half the time. If anything it uses slightly less total gas, because the pool spends fewer hours sitting warm and evaporating into the night air while the heater plays catch-up.

What a bigger heater actually costs you is the purchase price, and a gas supply that can feed it. A 400,000 BTU heater wants a meter and a gas line sized for it, and retrofitting that is often more expensive than the heater. Size for the job, then check what your gas line can actually deliver before ordering.

Heat pump vs gas vs solar:

• Gas heater: heats fast (24 hours typical), works any temperature, expensive to run.
• Heat pump: heats slow (3-5 days for big rises), only works above 50°F air temp, much cheaper to run (~5x efficiency).
• Solar: free fuel, slow, only works in the sun, oversized panel area can match gas heating speed.

The cover question. A solar cover (the bubbly plastic kind) cuts evaporation by more than 90% and roughly halves heater run time. A pool with no cover but an active heater is essentially heating the sky. If you adopt one habit from this page, make it covering the pool overnight. Nothing else you can buy saves as much energy for as little money.