Pool Heater Calculator
Last updated 15 October 2026
Enter water volume, temperatures and heat-up time to get the required heater output.
Your result will appear here
Enter your measurements, then choose Calculate.
Use useful output and purchased input to compare heater capacity.
Still unknown or unconfirmed
Formula, sources and blank printable worksheet
HEAT-1: Btu = US gal × 8.33 × max(Δ°F,0). Useful Btu/h = Btu/hours + stated loss. Input depends on efficiency/COP.
Calculation version 1.0 uses historical engineering references for arithmetic, not current residential law.
- Sta-Rite Engineering and Design Manual, second edition, 2003, printed pp. 2, 27, 29, 36-40, 44 https://www.pentair.com/content/dam/extranet/nam/pentair-pool/residential/manuals/training/Sta-Rite%20Engineering%20and%20Design%20Manual.pdf
- NIST Handbook 44, Appendix C: US gallon and length units https://www.nist.gov/system/files/documents/2019/11/19/00-20-hb44-final11152019pdf.pdf
- NIST SP811: IT Btu and SI energy conversions https://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication811e2008.pdf
| Quantity / unit | Reading | Source and date |
|---|---|---|
| Water volume | ________________ | ________________ |
| Volume unit | ________________ | ________________ |
| Temperature unit | ________________ | ________________ |
| Starting water temperature | ________________ | ________________ |
| Target water temperature | ________________ | ________________ |
| Heat-up hours | ________________ | ________________ |
| Energy conversion | ________________ | ________________ |
| COP (heat pump) or efficiency fraction (gas/resistance) | ________________ | ________________ |
| Source model and air/water/humidity rating conditions | ________________ | ________________ |
Mark missing measurements Unknown. Keep exact model, source revision and assumptions with this worksheet. Report a calculation or source correction with reproducible inputs.
This pool heater calculator separates heat delivered to water from energy bought as fuel or power. Enter water volume. Add its starting and target temperatures. Choose a heat-up time.
State what you assume about heat loss. These inputs define the task before you compare heaters.
Water-only heat is US gallons × 8.33 × temperature rise in °F, in Btu. Divide by heat-up hours for useful output before losses. Add a stated constant loss for that scenario. If losses are unknown, the water-only result is a lower bound. It cannot prove fit.
Gas efficiency and heat-pump COP turn useful heat into input energy on different bases.
On this page7
Pool heater calculator heat-up inputs
Use the water-volume tool if volume is unknown. Enter the water temperatures in °F or °C. Changing temperature units converts the existing temperatures. If the target is at or below the current water temperature, the water-only heat-up demand is zero.
The 8.33 factor approximates water heat capacity per US gallon per °F. The Sta-Rite reference, printed page 27 uses that convention. It suits a labelled planning estimate. It is not a model of precise water properties across all temperatures.
Heater calculator loss choices
Choose a zero-loss lower bound, a supplied measured/source loss, or the historical outdoor reference. The lower-bound mode does not assert that a real pool loses no heat. It simply isolates the energy needed to warm the water.
The old reference on printed page 27 describes an uncovered outdoor pool at 3.5 mph wind. Its table covers 200-1,000 ft² and a water-air rise of 10-30°F. Within that range, loss is 10.5 × area × water-air rise in Btu/h. For 400 ft² and 30°F, that is 126,000 Btu/h. This old scenario cannot forecast local weather or savings from a cover.
For other conditions, supply a sourced loss or keep the lower-bound label. Note whether the pool is covered. Record wind and source conditions too. The tool holds the chosen loss constant during heat-up. It does not model evaporation as the weather changes.
Worked useful-output example
At 15,000 US gallons and a 10°F rise, water-only energy is 1,249,500 Btu. Over 12 hours that requires 104,125 Btu/h, about 30.52 kW of useful heat. The NIST SI guide supplies the IT Btu conversion used here: about 3,412.14 Btu per kWh.
At an entered gas efficiency of 0.80, input would be 130,156.25 Btu/h. At COP 5, the corresponding heat-pump electrical input would be about 6.10 kW. Those are artificial efficiency scenarios, not ratings for an unnamed heater.
Known heater output and heating time
If useful heater output is known, estimated time equals water heat divided by useful output minus constant loss. Output must exceed loss for a positive temperature rise. Otherwise the target is not reached under that constant-loss scenario.
Read useful heater output at its rating conditions. Gas input is not useful output. A heat pump's output and COP have stated air and water temperatures. Humidity matters too.
The Raypak residential catalogue shows those separate fields. Use a current manual for the exact model before a purchase.
Limits and next steps
Version 1.0 follows the named heat and unit sources. This tool estimates heat for home pools. It does not design spas, gas pipes, wiring or public pools. Check heater flow with the pool pump size calculator and filter tool.
Compare gas, resistance and heat-pump heating. Use the heating-cost calculator for fuel expense, or read what drives heating cost.
FAQ
Can volume alone choose a heater?
No. Volume and temperature rise give water-only energy. Heat-up time sets the output target. Losses add to it. Rating conditions can limit what a heater supplies. Check flow and controls for the exact model too. Keep its allowed running conditions with any claim that it fits the task.
Does the outdoor table predict my weather?
No. The old table describes an uncovered pool at a stated wind speed. It has a defined range. The tool does not guess weather from an address. It does not apply a made-up cover factor. If your conditions differ, supply a documented loss or keep the lower-bound result.
Verdict on pool heater sizing
The pool heater calculator shows the heat needed and what remains unknown. Save water inputs with the loss mode. Keep rating conditions with the result too. A lower bound helps with planning. It does not establish the final heater size for a real pool.