Pool Heating Time Calculator — How Long to Heat a Pool

Quick answer

Heating time follows straight physics: hours ≈ gallons × 8.34 × temperature rise ÷ heater output BTU/hr. A 20,000-gallon pool rising 10°F needs about 1.67 million BTU of heat; a 400,000 BTU/hr input gas heater delivers ≈ 328,000 BTU/hr after efficiency, so figure 5 hours of pure physics and realistically 6–7 once wind and evaporation take their cut. Run your exact pool through the calculator above — it converts input vs output ratings for you.

Where the numbers come from

• Gallons — the pool's volume, from dimensions: length × width × average depth × 7.48 for rectangles (the calculator accepts round and oval shapes too).
• 8.34 — pounds in one gallon of water. Raising one pound of water 1°F takes one BTU, so the pool's weight in pounds × the rise in degrees = total BTU needed.
• Output, not input — gas burners lose heat up the flue; federal law (10 CFR 430.32) only requires 82% of the fuel's energy to reach the water. Heat pumps and electric resistance deliver what their rating says.

The real-world margin

The pure-physics result is the fastest possible heat-up: every BTU the heater makes goes into the water and stays there. In an open pool, two losses run concurrently:

1. Evaporation at the surface — the same loss that drives sizing, active the whole time you're heating. A solar cover is the single best fix; the DOE credits covers with 30–50% less evaporation.
2. Wind-driven loss — an exposed site stretches the estimate further; a fenced yard helps.

Expect 20–25% longer than the physics figure without a cover, and treat a heat-up that beats the estimate as a pleasant surprise.

Heat pumps and cold air

The calculator holds the heat pump rating constant, which is optimistic below about 55°F air. Manufacturers rate COP (heat moved per unit of electricity) at warm-air test conditions — typically 5–6 at 80°F — and output falls as the air cools, with the IECC low-air floor at COP 4.0. If your heat-up is happening on a cold spring morning, add generous margin or lean on the heat pump vs gas comparison to see whether a gas backup pays for itself.

Reading the result honestly

Treat the physics figure as the floor, not the promise. Pools with heavy wind exposure, no cover, or heat pumps running on cold mornings routinely land 20–25% over it — and that is normal, not a broken heater. If the honest estimate still doesn't fit your schedule, the fix is capacity, not optimism: re-run the BTU calculator with a shorter heat-up window and shop for the rating it returns.

From here

If the heat-up time came back longer than your patience, the answer isn't a bigger heater by guesswork — re-run the BTU calculator with a shorter heat-up window and it will tell you the rating to shop for. Then check what that rating costs to run at your fuel prices in the running cost calculator.