Can I size a heater from gallons alone?
Not for an outdoor pool. Heat loss, desired temperature and time, cover and weather conditions also matter.
Pool Heater Size Calculator
Estimate a heating-output requirement for a discussion with your installer. Outdoor heater sizing needs heat loss as well as water volume; a lossless heat-up figure alone is not a heater recommendation.
Published by Pool Cost Planner. Updated October 19, 2026.
Outdoor sizing needs a heat-loss assessment. Heat-up energy alone cannot select a heater.
What belongs in the heat-loss assessment?
Ask the designer to document water surface area, target water temperature, design air temperature, wind exposure, humidity, cover use and the months you plan to swim. The average loss entered here must represent those conditions over the heat-up period. Cover and climate are not automatic percentage discounts. A covered overnight pool and an uncovered windy pool cannot safely share a guessed loss allowance.
Thermal output is not electrical or gas input
Useful thermal output reaches the water. Electrical input or gas input is purchased energy. The relationship depends on efficiency and operating conditions; a heat pump's output at mild air temperature is not necessarily its output on the coldest planned swimming day. Compare manufacturer performance tables at your design conditions, not just the largest number on a product nameplate.
Independent heat-up example
Using the Pentair training model, 20,000 US gallons warmed by 20 degrees F requires approximately 3,332,000 BTU of sensible heat. Over 24 hours, that is about 138,833 BTU/h before outdoor losses. Adding a professional average loss estimate increases useful output; it is not a second energy-input charge.
| Quantity | Example |
|---|---|
| Water volume | 20,000 US gal |
| Temperature rise | 20 degrees F |
| Desired time | 24 hours |
| Lossless useful output | About 138,833 BTU/h |
| Outdoor design losses | Must be assessed separately |
Methodology and sizing limits
Heat-up BTU = US gallons x 8.33 lb per gallon x temperature rise in degrees F. Divide by hours, then add the entered average heat-loss rate in matching power units. The explicit lossless option excludes this loss and reports only a lower bound. The model does not select a heater, predict changing weather, estimate COP or replace manufacturer sizing. A qualified installer must check capacity, minimum operating conditions and compatibility.
Sources and review scope
Sources checked October 19, 2026. Model-specific instructions are not universal guidance. No independent professional review is claimed.
- Pentair Sta-Rite training manual: heat-up energy and sizing. Historical training manual; publication date not stated.
Related calculators and guides
- Start with measured water dimensions: Pool Volume Calculator.
- Total equipment and installation quotes: Pool Heater Cost Calculator.
- Estimate electricity or gas consumption separately: Pool Heating Cost Calculator.
- Keep equipment records and service scope together: Pool Equipment Maintenance Guide.
FAQ
What if I do not know the heat loss?
Use the explicit lossless mode only to see heat-up energy and a lower bound. Ask a qualified installer for a design-loss assessment before choosing equipment.
Is the kW output the amount of electricity I will buy?
No. It is useful thermal output. Operating cost needs actual electrical or fuel input and the operating schedule.