Pool Heater Calculator
Estimate how many BTUs it takes to warm your pool and how long your heater will need. Enter the pool size, current and target water temperatures, and your heater rating.
How to use the Pool Heater Calculator
- Enter the pool length, width and average depth in feet.
- Enter the current and target water temperatures in °F.
- Enter the heater output (BTU/h) and efficiency.
How it works
Pool volume = length × width × average depth × 7.48 gallons. Heating one gallon of water by 1 °F takes 8.34 BTU, so heat required = gallons × 8.34 × temperature rise. Dividing by the heater’s effective output (rated BTU/h × efficiency) gives the heating time.
Real pools also lose heat through evaporation and the surface — often more than the heater adds on cool, windy nights — so use a pool cover to hold the heat.
Heat needed to raise water temperature
| Pool size | Gallons | BTU per 1 °F | BTU for +10 °F |
|---|---|---|---|
| 12×24 ft, 4 ft deep | 8,618 | 71,870 | 718,704 |
| 15×30 ft, 4.5 ft deep | 15,148 | 126,335 | 1,263,348 |
| 16×32 ft, 5 ft deep | 19,150 | 159,712 | 1,597,121 |
| 20×40 ft, 5 ft deep | 29,922 | 249,550 | 2,495,501 |
Frequently asked questions
How long does it take to heat a pool?
A 400,000 BTU gas heater can raise a typical 20,000-gallon pool about 2 °F per hour; heat pumps take longer, often 24–72 hours.
What size pool heater do I need?
A common rule is about 50,000 BTU/h of heater output for every 10,000 gallons for a reasonable heat-up time.
Does a pool cover help?
Yes — covers can reduce heating costs by 50–70% by preventing evaporation.