Pool Heater BTU Sizing Calculator
Work out the energy to raise your pool's temperature and the heater output (BTU/hr) needed to reach your target temperature in a chosen number of hours. Free.
Pool Heater BTU Sizing Calculator
Sizing a pool heater comes down to physics: how much energy it takes to raise the water temperature, and how fast you want it done. Enter your pool volume, the temperature change you want, and a target heat-up time to see the heater output you need.
How to Use the Pool Heater Sizing Calculator
Enter your pool volume
In gallons. If you don't know it, a rough estimate for a rectangular pool is length × width × average depth × 7.5; for a round pool, diameter² × average depth × 5.9. Most residential pools hold 10,000–30,000 gallons.
Set the temperature change
Your current water temperature and the target. Most swimmers are comfortable at 78–82°F. The energy scales directly with this difference, so heating from 65 to 82 (a 17° rise) takes nearly twice the energy of a 9° bump.
Choose a heat-up time
How many hours you'll allow to reach temperature. A faster heat-up needs a bigger (more powerful) heater. Gas heaters can be sized to warm a pool in hours; heat pumps are gentler and slower but far cheaper to run.
Read the heater output
The result is the BTU/hr output you need before losses, rounded up to a standard size. Add margin for surface heat loss, and use a cover — it's the cheapest "heater" you can buy.
The Physics of Heating a Pool
Energy In Equals Water Times Degrees
Heating a pool obeys one of the most reliable equations in physics. It takes exactly one British Thermal Unit (BTU) to raise one pound of water by one degree Fahrenheit, and a gallon of water weighs 8.34 pounds. So the energy to warm your whole pool is simply its volume in gallons, times 8.34, times the temperature rise you want. A 20,000-gallon pool warmed by 17 degrees needs about 2.8 million BTU — a big number, but a one-time cost to reach temperature. To turn that into a heater size, you divide by the number of hours you're willing to wait: spread that 2.8 million BTU over 24 hours and you need roughly 118,000 BTU per hour of heater output; demand it in 12 hours and you need double. This is why "how big a heater?" always has two answers — it depends entirely on how fast you want the water warm. The heat-up energy itself is exact and unchanging; only your patience sets the required output.
What the clean equation leaves out is heat loss, which is why a real pool always takes longer than the ideal calculation suggests. A pool is a large, warm, open surface constantly shedding heat to the air through evaporation (by far the biggest loss), convection from wind, and radiation to the night sky. The warmer you make the water relative to the air, the faster it loses heat, so a heater is partly fighting a moving target. On a calm warm day the losses are modest; on a cool, breezy night they can be substantial, sometimes consuming a large fraction of the heater's output just to hold temperature. This is the single most important practical lesson in pool heating: a cover is the cheapest and most effective upgrade you can make. A solar or thermal cover cuts evaporative and convective losses by 50–70%, dramatically reducing both heat-up time and running cost — often saving more energy than any heater choice.
"The heat-up math is exact: gallons times 8.34 times degrees. The real cost is the heat your pool sheds every hour to the sky and the wind — which is why a cover, not a bigger heater, is the smartest purchase."
Gas vs Heat Pump, and Right-Sizing
The two common heater types make very different trade-offs. Gas heaters (natural gas or propane) deliver high output — often 150,000 to 400,000+ BTU/hr — so they heat a pool fast and work in any weather, but they're expensive to run because you're burning fuel. Electric heat pumps move heat from the air rather than creating it, so they're three to five times more energy-efficient and far cheaper to operate, but their output is lower and they slow down (and eventually stop working) as air temperature drops, making them best for maintaining temperature in mild climates rather than fast heat-ups. Your heat-up-time input effectively chooses between these: a few-hour target points to a large gas heater, while a day-long, gentle heat-up suits a heat pump. Whichever you choose, size with some margin above the calculator's figure to overcome surface losses, and lean on a cover to keep the running cost sane. For readers using metric or in other climates, the same physics applies — convert volume to litres (1 gallon ≈ 3.785 L) and use the equivalent energy in kWh shown here (one BTU is about 0.000293 kWh), and the warmer your nights and the more you cover the pool, the less heater you need.
10 Facts About Pool Heating
Energy = gallons × 8.34 × the temperature rise (in BTU).
Heater size = that energy ÷ how many hours you'll wait.
One BTU raises one pound of water by 1°F; a gallon is 8.34 lb.
Evaporation is the biggest source of pool heat loss.
A cover cuts heat loss by 50–70% — the best value upgrade.
Gas heaters are fast and weather-proof but costly to run.
Heat pumps are 3–5× more efficient but slower and weather-dependent.
Most swimmers are comfortable at 78–82°F.
Real heat-up takes longer than the ideal because of surface losses.
Always size with margin above the ideal figure for losses.
Frequently Asked Questions
- Work out the energy to heat the water — gallons × 8.34 × the temperature rise, in BTU — then divide by how many hours you'll wait to get the heater output in BTU per hour. A 20,000-gallon pool raised 17°F over 24 hours needs about 118,000 BTU/hr. Add margin for surface heat loss, which makes real heat-up slower. This calculator does the math and rounds up to a standard heater size.
- 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 you size with margin above the ideal figure, and why a cover, which cuts those losses by 50–70%, makes such a big difference.
- 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. For an oval, use length × width × average depth × 6.7. Average depth is roughly (shallow end + deep end) ÷ 2. Most residential in-ground pools hold 10,000–30,000 gallons. Enter your best estimate; the energy scales directly with volume, so a rough figure gives a useful heater size.
- Gas heaters deliver high output (150,000–400,000+ BTU/hr), heat fast, and work in any weather, but cost more to run because you burn fuel. Heat pumps are three to five times more efficient and much cheaper to operate, but heat slowly and lose effectiveness as the air cools, so they suit mild climates and maintaining 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.
- 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.
- Yes — it's the single most cost-effective pool upgrade. Most of a pool's heat loss is evaporation from the surface, and a solar or thermal cover dramatically reduces it, cutting total heat loss by 50–70%. That means faster heat-up, much lower running costs, and less water and chemical loss too. 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.
- Most swimmers find 78–82°F comfortable; competitive swimmers prefer the cooler end and casual or young/elderly bathers the warmer end. 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.
- 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.
- 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.
- 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 for the heat-up energy plus a margin for losses.
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