Pool Heater BTU Sizing Calculator

How to Use the Pool Heater Sizing Calculator

The Physics of Heating a Pool

DOE sizes a pool heater by its surface, not its volume

Frequently Asked Questions

• How do I size a pool heater?+
  The U.S. Department of Energy's approximate method: take the temperature you want, subtract the average air temperature for the coldest month you'll use the pool, and multiply that rise by the pool's surface area in square feet and by 12. An 18 × 36 ft pool (648 ft²) held at 82°F against a 60°F month needs about 171,000 BTU/hr. DOE bases the formula on a 1–1¼°F per hour rise and a 3½ mph wind, and recommends a professional sizing analysis before you buy. The calculator also shows the separate heat-up energy, gallons × 8.34 × the rise.
• Why does the real heat-up take longer than calculated?+
  Because the calculator gives the ideal energy to warm the water, but a pool constantly loses heat from its surface through evaporation, wind (convection), and radiation to the sky. The heater has to overcome those losses as well as warm the water, so the real time is longer — sometimes much longer on a cool, windy night. That's why DOE sizes heaters from the surface rather than the volume, and why covering the pool, which DOE says can cut heating costs by 50–70%, makes such a big difference.
• How do I find my pool's volume in gallons?+
  For a rectangular pool: length × width × average depth (all in feet) × 7.5 gives gallons. For a round pool: diameter × diameter × average depth × 5.9. Average depth is roughly (shallow end + deep end) ÷ 2. For the heater size you also need the surface area: length × width, or 0.785 × diameter² for a round pool. Enter your best estimate; the energy scales directly with volume, so a rough figure gives a useful heater size.
• Gas heater or heat pump — which should I get?+
  Gas heaters come in outputs DOE puts at 75,000–450,000 BTU/hr, heat fast, and can hold any temperature regardless of the weather, but cost more to run because you burn fuel. Heat pumps usually have a coefficient of performance of 3.0 to 7.0, according to DOE, and work efficiently while the outside air stays above about 45–50°F, so they suit mild climates and holding temperature rather than rapid heat-ups. If you want a pool warm in hours or live somewhere cool, choose gas; if you'll keep it warm steadily in a mild climate and care about running cost, choose a heat pump.
• How much does it cost to heat a pool?+
  It depends on the energy needed (shown here in BTU and kWh), your fuel or electricity price, and your heater's efficiency. The initial heat-up is a fixed energy cost, but the ongoing cost of holding temperature is driven almost entirely by surface losses — which is where a cover saves the most. As a rough guide, gas heating is fast but pricey per session, while a heat pump's high efficiency makes steady-state heating far cheaper. Multiply the kWh figure by your rate (and divide by efficiency) for an electric estimate.
• Does a pool cover really make that much difference?+
  Yes — it's the single most cost-effective pool upgrade. DOE calls evaporation by far the largest source of a pool's energy loss and says covering the pool when it is not in use is the single most effective way to cut heating costs, with savings of 50–70% possible. DOE adds that a cover reduces make-up water by 30–50% and chemical use by 35–60%. Many owners find a cover saves more energy than any heater upgrade. Use it whenever the pool isn't in use, especially overnight when the air is cool and losses are highest.
• What temperature should I heat my pool to?+
  There is no single right answer; DOE's own heating-cost tables compare 78, 80 and 82°F, and in every city they list, each step up costs more. Every degree higher increases both heat-up energy and ongoing loss (because the gap to air temperature grows), so heating to 84–86°F costs noticeably more than 80°F. Pick the lowest temperature that's comfortable for your swimmers, and let a cover hold it. The calculator shows how the energy scales — try a couple of target temperatures to see the difference.
• Can I make the heater smaller and just wait longer?+
  Up to a point, yes — a smaller heater uses the same total energy to reach temperature, just over more hours, which is exactly what a longer heat-up time in the calculator models. But there's a floor: the heater must out-pace the pool's heat loss, or it can never reach the target on a cool night. A heat pump that's too small will run constantly and still fall short in bad weather. Size so the output comfortably exceeds the losses, and use the longer-heat-up approach only with a cover and in mild conditions.
• Is solar heating an option?+
  Yes, and it pairs well with the rest. Solar pool heating circulates water through roof-mounted collectors, using free sunlight to add heat — running cost is essentially just the pump. It's slower and weather-dependent (no sun, no heat), so it's usually used to extend the season or supplement a gas heater or heat pump rather than as the sole fast heater. Combined with a cover to hold the gained heat overnight, solar can carry much of the load in sunny climates at very low operating cost.
• Does this work in metric / other climates?+
  The physics is universal. To work in metric, convert volume to litres (1 gallon ≈ 3.785 L) and note the energy is shown here in kWh as well as BTU (1 BTU ≈ 0.000293 kWh). Heating 1 litre of water by 1°C takes about 1.16 watt-hours. Warmer climates need far less heating and benefit most from heat pumps and solar; cooler or windier locations need more output and gain the most from a cover. Whatever your units and weather, size the heater from the surface with DOE's rule, and treat the heat-up energy as a separate, one-off cost.

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