Home › Pool Heater BTU Chart
Pool Heater BTU Chart
Quick answer: This chart gives rough pool heater sizes from surface area and the temperature gap you want to hold, using the planning rule of about 12 BTU per hour per square foot per degree in light wind. Gas heaters are labelled by input, so divide the output figures by efficiency. Confirm any size with a licensed professional.
How the chart is built
Holding output ≈ surface area × (target water minus average air) × 12. That factor is a long-used rule of thumb for light wind without a cover. Windy sites may need 15 or more; sheltered sites a little less; a nightly cover can roughly halve the requirement. The chart below shows output in thousands of BTU per hour (kBTU/h).
Output needed (kBTU/h), uncovered, light wind
| Surface area (sq ft) | 10°F gap | 15°F gap | 20°F gap | 25°F gap |
|---|---|---|---|---|
| 300 | 36 | 54 | 72 | 90 |
| 400 | 48 | 72 | 96 | 120 |
| 500 | 60 | 90 | 120 | 150 |
| 600 | 72 | 108 | 144 | 180 |
| 800 | 96 | 144 | 192 | 240 |
| 1,000 | 120 | 180 | 240 | 300 |
Converting to gas heater input
Divide output by efficiency. At 84 percent: 120 kBTU/h output ≈ 143 kBTU/h input; 180 ≈ 214; 240 ≈ 286. Common residential gas heater input sizes range from around 150,000 to 400,000 BTU/h. Choosing the next size up gives faster heat-up, provided your gas supply can feed it.
Converting to heat pump output
Heat pumps are rated by output, often at warm test air. Look up output at your cool-month air temperature and compare with the chart. For a 20°F gap on a 500 sq ft pool, you would look for roughly 120 kBTU/h at that air temperature, or about 60 to 85 kBTU/h with a reliable cover habit.
Converting surface area from pool size
| Pool | Approx. area |
|---|---|
| 12 × 24 ft rectangle | 288 sq ft |
| 15 × 30 ft rectangle | 450 sq ft |
| 16 × 32 ft rectangle | 512 sq ft |
| 20 × 40 ft rectangle | 800 sq ft |
| 18 ft round | 254 sq ft |
| 24 ft round | 452 sq ft |
| 15 × 30 ft oval | 353 sq ft |
Use the pool volume calculator for exact shapes.
Heat-up time on top of holding
The chart covers holding temperature. If you heat from cold for weekend use, you also need enough output to lift the whole volume in an acceptable time: gallons × 8.34 × rise ÷ hours. The pool heater size calculator checks both and uses the larger.
Worked example
Figures below are illustrative examples (2026-10-29), not quotes or guarantees.
Assumptions: 15 × 30 ft pool (450 sq ft), target 82°F, cool-month nights average 62°F, so a 20°F gap; light wind; no cover. Output ≈ 450 × 20 × 12 = 108,000 BTU/h. Gas input at 84% ≈ 129,000 BTU/h, so a pro might consider units from about 150,000 BTU/h up. With a nightly cover, a heat pump with roughly 55,000 to 75,000 BTU/h output at 62°F air may hold temperature.
Limits of a chart
Charts flatten real-world variety. Wind exposure, shade, humidity, water features, nightly cover habits and how often you swim all change the answer. Spas and indoor pools follow different rules. Treat the chart as a starting point and use the calculators with your own numbers.
Why gas supply limits size
Even if the chart suggests a large heater, your gas meter and piping must deliver that input at the right pressure along with every other gas appliance in the home. A licensed gas professional sizes the line; in some cases a larger meter from the utility is needed. That can make a mid-size heater the practical maximum.
Why electrical service limits heat pumps
Large pool heat pumps draw substantial current. A licensed electrician checks panel capacity, breaker and wire size and bonding. Panel upgrades can add significantly to installation cost; see pool heater installation cost.
Reading BTU on older heaters
Older gas heaters often had lower efficiency, so the same input delivered less heat. When replacing, compare output rather than input to understand the change. A new heater with the same input as your old one may heat a little faster thanks to better efficiency.
Picking your temperature gap
Use the average night air temperature for the coolest month you plan to swim, not the hottest summer week. For many northern pools that is late May or September. Local weather service climate normals give monthly averages; subtract from your target water temperature to get the gap. A wider gap moves you right across the chart.
Wind and cover adjustments
To adjust the chart for your site, scale the numbers. For a windy, open yard, multiply by about 1.25 (a factor of 15 instead of 12). For a sheltered courtyard, multiply by about 0.85. For a reliable nightly cover, multiply by roughly 0.5 to 0.7. For example, the 500 sq ft row at a 20°F gap shows 120 kBTU/h; windy and uncovered it becomes about 150; sheltered with a nightly cover it may fall to around 60. The pool heat pump size calculator applies these factors for you.
Metric conversions
For Canadian readers: 1 kW is about 3,412 BTU/h, so 120 kBTU/h is about 35 kW of output. One square metre is about 10.76 sq ft, so a pool of 50 m² is roughly 538 sq ft. A 10°F gap is about 5.6°C.
Spa and pool combinations
If your heater also serves an attached spa, size the heater mainly for the pool's holding need from the chart, then check spa heat-up time separately using its small volume. Most pool-sized gas heaters bring a spa up quickly; heat pumps do so slowly. That spa time is often what pushes owners of combination systems toward a gas heater or a dual setup.
FAQ
Why 12 BTU per square foot per degree?
It is a common industry planning figure for light wind; it is not exact for every pool.
Does depth change the chart?
Not for holding temperature; depth affects heat-up time.
Can I use the chart for a spa?
No. Spas need fast heat-up of small volumes at high temperatures; size from volume and time.
Try the calculator · Back to FAQ & guides