Pool heater size calculator
Raising this pool 15°F in 24 hours takes roughly 78k BTU/hr of delivered heat — before heat losses. In practice, size up and use a cover; an uncovered pool sends your heating budget into the sky as steam.
Treat this as a starting dose: add less than the full amount when unsure, circulate for a few hours, retest, repeat. Always follow your product's label.
An undersized heater never quite warms the pool; an oversized one wastes money. The calculator above sizes a heater to your pool and your climate, and helps you weigh gas, electric-resistance, and heat-pump options.
What it calculates
Enter your pool's surface area or volume, your desired water temperature, and your climate or the temperature rise you need, and the tool estimates the heater capacity required. Surface area matters because most heat is lost from the water's surface — a bigger surface loses heat faster and needs more heating power, which is why two pools of equal volume but different shapes can need different heaters.
Gas vs heat pump vs electric
The type shapes both speed and running cost. Gas heaters heat fast and work in any weather, ideal for quick, on-demand warming, but cost more to run per hour. Heat-pump heaters are far more energy-efficient to run and great for maintaining temperature, but heat slowly and lose effectiveness in cold air. Electric-resistance heating suits small volumes like spas but is costly to run for a full pool. Match the type to whether you want fast heat (gas) or cheap steady heat (heat pump).
Don't skip a cover
Whatever you size and choose, a cover is the biggest lever on heating cost. Most of a pool's heat escapes from the surface through evaporation, so a cover dramatically cuts the energy any heater needs — pairing a right-sized heater with a cover is how you get warm water without a brutal bill. Confirm the final heater choice against the manufacturer's sizing guidance for your climate.
Frequently asked questions
What size pool heater do I need?
A quick estimate uses pool surface area, the temperature rise you want, weather exposure, and heater type. DOE's gas-heater sizing guidance uses pool area, temperature rise, and a multiplier, but a pro should size expensive heater installations before purchase.
Is a heat pump or gas pool heater better?
A heat pump is usually better for steady heating in mild or warm weather, while a gas heater is better for faster heating, spas, cold snaps, and pools used occasionally. Local utility costs can change the answer.
Can a pool heater be too big?
Yes. Oversizing can raise equipment cost and may create short-cycling or inefficient operation depending on the system. Undersizing can leave the heater running hard without reaching the temperature you want.
Does a pool cover reduce the heater size I need?
A cover can reduce heat loss, especially evaporative heat loss. DOE says pool covers can significantly reduce pool heating costs, so cover use should be part of the heater sizing conversation.
Should I size a pool heater by gallons?
Gallons matter, but surface area and temperature rise often matter more for outdoor heater sizing because the pool loses heat through the water surface. Use gallons for context and surface area for the main estimate.
Embed this calculator (free)
Run a pool site or blog? Drop this tool into any page — it stays free and links back here. Paste this HTML where you want it to appear.
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