Pool Heater Size Calculator
Enter the pool's gallons and surface area, how many degrees you want to raise it in how many hours, your desired water temperature and the coldest air you'll swim in. You get the heat-up and maintenance loads, the gas heater and heat pump sizes that cover them, and the gas a heat-up burns.
Heat is lost from the surface; the volume page gives it.
Heat-up load
139K BTU/h
20,000 gal × 8.34 × 20°F ÷ 24 h
Maintenance load
166K BTU/h
512 sq ft × 27°F × 12
Gas heater
200K BTU
input rating, next standard size
Heat pump
140K BTU
over the largest single unit; use two or gas
Heating up by 20°F burns about 40 therms of gas ($60 at $1.5/therm) at 84% efficiency. A heat pump does the same for roughly a fifth of the energy cost but takes two to three times longer and stops working well below about 50°F air.
Heater size by pool, for a 20°F rise in 24 hours and a 25°F air gap
| Pool | Heat-up BTU/h | Maintain, uncovered | Maintain, covered | Gas heater (input) | Heat pump |
|---|---|---|---|---|---|
| 10,000 gal · 300 sq ft | 70K | 90K | 45K | 150K | 100K |
| 15,000 gal · 400 sq ft | 104K | 120K | 60K | 150K | 120K |
| 20,000 gal · 512 sq ft | 139K | 154K | 77K | 200K | 140K+ |
| 25,000 gal · 600 sq ft | 174K | 180K | 90K | 250K | 140K+ |
| 30,000 gal · 720 sq ft | 209K | 216K | 108K | 300K | 140K+ |
| 40,000 gal · 900 sq ft | 278K | 270K | 135K | 400K | 140K+ |
A "+" means the load exceeds the largest common single heat pump; use two units or gas.
The two formulas
- Heat-up: BTU/h = gallons × 8.34 lb/gal × °F rise ÷ hours. One BTU warms one pound of water one degree; the rest is arithmetic.
- Maintenance: BTU/h = surface area × (water temp − coldest air temp) × 12. The 12 is the Hayward/Raypak rule for an uncovered pool in light wind; a cover roughly halves it, a windy site can double it.
- Gas input vs output: a 400K BTU heater delivers about 336K at 84% efficiency; sizes quoted by makers are input.
- Heat pumps are rated by output at 80°F air and 80% humidity; at 60°F air the same unit delivers 25-35% less.
Cutting the load before buying the heater
Evaporation is 70% or more of a pool's heat loss, which is why a $200 bubble cover changes the heater size and the gas bill more than any other decision. A windbreak, a smaller heated season, and accepting 80°F instead of 86°F each shave the maintenance load. Run the calculator with the cover option on and off; for most pools it moves a size or two.
The same BTU arithmetic sizes the equipment inside the house. If you are comparing a pool heat pump with the one that heats your rooms, the BTU calculator at Watts Math sizes air conditioning and heating by square footage and climate zone with the same units.
Frequently Asked Questions
What size pool heater do I need?
Two numbers: the heat-up load (gallons × 8.34 × degrees of rise ÷ hours) and the maintenance load (surface area × the gap between water and coldest air × 12 BTU/h per sq ft per °F). Size to the larger. A 20,000-gallon, 512 sq ft pool warmed 20°F in 24 hours needs 139K BTU/h; holding 82°F in 55°F air uncovered needs 166K. A 200K-input gas heater or a large heat pump covers it.
Gas heater or heat pump?
Gas heats fast and works in any weather; it costs the most to run. A heat pump moves heat from the air, delivers 5-6 units of heat per unit of electricity, and costs a fraction to operate, but it heats slowly and loses capacity below about 50°F air. Weekend-use pools in cool climates favour gas; daily-use pools in the Sun Belt favour a heat pump. Solar panels are the third option where roof space and sun allow.
Why size a heater bigger than the maths says?
Gas heater ratings are input BTU; output is about 84% of that for a standard unit and 95% for a condensing one. Wind over the surface multiplies losses, and the 12 BTU rule assumes a sheltered pool. Undersized heaters run all day and still fall short; oversized ones just cycle. When in doubt, go up a size, and buy a cover.
How much does it cost to heat a pool with gas?
Raising 20,000 gallons by 20°F takes about 40 therms at 84% efficiency, roughly $60 at $1.50 per therm, before the cost of holding it there. A cover cuts the holding cost by half or more; most of a pool's heat leaves by evaporation from the surface.
These calculators use published seeding rates, extension-service fertilizer guidance, and standard pool-chemistry dosing figures, and are for planning. Read the product label; it governs the dose. Pool chemicals are hazardous when mixed or mishandled, and a pool store's water test beats any estimate here.