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How many gallons is my pool?
Measure the pool. The gallons come from plain geometry. The pump flow and heater size that follow come from published building codes and energy.gov's sizing method, with real pumps and heaters to match.
Pumps
Every number comes from the maker's own page or spec sheet, or ENERGY STAR's list of certified pumps. If a maker doesn't publish a number, it says so instead of guessing.
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Pumps pull hard at the drain. From the CPSC's Pool Safely campaign: "Do not play or swim near drains or suction outlets, especially in spas and shallow pools, and never enter a pool or spa that has a loose, broken or missing drain cover." Pool Safely: safety tips
Heaters
Gas heaters are sold by fuel input, so each card works out the heat that reaches the water: input × the maker's efficiency. Heat pumps are matched on the maker's rating at 50°F air, the cooler of the two standard test points.
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Gas heaters burn fuel, and burning fuel makes carbon monoxide. The CDC says: "Have your heating system, water heater, and any other gas, oil, or coal burning appliances serviced by a qualified technician every year." CDC: carbon monoxide basics
Where the numbers come from
Gallons
Plain geometry. Surface area × average depth gives cubic feet, and a cubic foot holds 7.48 US gallons (a gallon is 231 cubic inches; a cubic foot is 1,728). When the floor slopes evenly from the shallow end to the deep end, the average depth is (shallow + deep) ÷ 2. A kidney or freeform pool is split into pieces you can measure, so there's no rule-of-thumb factor anywhere.
Pump flow
Flow in gallons per minute = gallons ÷ (turnover hours × 60). No federal rule sets a turnover time for home pools, so the tool shows the two published limits and lets you pick:
- The International Swimming Pool and Spa Code (ISPSC Section 810), as its publisher ICC summarizes it: equipment "sized to provide a turnover of the pool water not less than once every 12 hours". ICC
- California's energy code, Title 24 Part 6, Section 150.0(p)1C: the filtration flow rate may be "not greater than the rate needed to turn over the pool water volume in 6 hours or 36 gpm, whichever is greater". California Energy Commission, 2025 standards
Horsepower isn't on the plate because it can't be read from gallons alone. energy.gov's advice was to "use the smallest size pump possible for your swimming pool" and to match the pump to your piping with a pool supplier's design chart.
Federal rules now push every new pump toward variable speed. Dedicated-purpose pool pump motors of 1.15 to 5 total horsepower made from September 29, 2025 "must be a variable speed control dedicated-purpose pool pump motor", and the same applies to 0.5 to 1.15 horsepower motors from September 28, 2027 (10 CFR 431.485). Pool pumps themselves have had minimum efficiency (WEF) ratings since July 19, 2021 (10 CFR 431.465). ENERGY STAR explains why: "reducing pump speed by one-half allows the pump to use just one-eighth as much energy." ENERGY STAR pool pumps
Heater size
energy.gov's published method for an outdoor pool, word for word:
Determine your desired pool temperature. Determine the average temperature for the coldest month of pool use. Subtract the average temperature for the coldest month from the desired pool temperature. This will give you the temperature rise needed. Calculate the pool surface area in square feet. Use the following formula to determine the Btu/hour output requirement of the heater:
Pool Area x Temperature Rise x 12
This formula is based on 1º to 1-1/4ºF temperature rise per hour and a 3-1/2 mile per hour average wind at the pool surface. For a 1-1/2ºF rise multiply by 1.5. For a 2ºF rise multiply by 2.0.
energy.gov also says: "You should have a trained pool professional perform a proper sizing analysis for your specific pool to determine pool heater size," and that windier spots, lower humidity and cool nights "will require a larger heater". It gives the same formula for heat pump pool heaters.
The "physics check" line on the plate is separate from energy.gov: warming water takes 1 BTU per pound per °F, and a gallon of water weighs 8.34 pounds. It counts no heat loss at all, so a real heater takes longer.
Sources: U.S. Department of Energy, Energy Saver, Gas Pool Heaters and Heat Pump Swimming Pool Heaters, and Choosing, Installing, and Operating an Efficient Swimming Pool Pump. energy.gov took these pages down in July 2026; the links go to the Internet Archive's copies. All sources retrieved October 30, 2026.
Questions
How do I work out how many gallons my pool holds?
Multiply the surface area by the average depth to get cubic feet, then by 7.48, the gallons in a cubic foot. A rectangle's area is length × width; a round pool's is 3.1416 × the radius squared. If the floor slopes evenly, the average depth is (shallow + deep) ÷ 2.
How do I measure a kidney-shaped or freeform pool?
Split it into rectangles, circles and half circles that don't overlap, measure each one and its depth, and add them up. Each piece is exact geometry, so there's no rule-of-thumb factor.
What size pool pump do I need?
Size it by flow: the pool's gallons ÷ the turnover time in minutes. The pool code (ISPSC) asks for equipment that turns the water over at least once every 12 hours, and California's energy code caps filtration flow at the 6-hour rate or 36 gpm, whichever is greater. Which pump delivers that flow depends on your pipes and filter.
Do I have to buy a variable-speed pool pump?
Federal rules (10 CFR 431.485) require dedicated-purpose pool pump motors of 1.15 to 5 total horsepower made from September 29, 2025 to be variable speed, and 0.5 to 1.15 horsepower motors from September 28, 2027. ENERGY STAR notes that halving the pump's speed lets it use one-eighth as much energy.
What size pool heater do I need?
energy.gov's method: surface area × temperature rise × 12 gives the output needed in BTU per hour. The rise is the water temperature you want minus the average air temperature of the coldest month you swim. For a faster 1½°F per hour, multiply by 1.5; for 2°F per hour, by 2.
Does a 400,000 BTU gas heater put 400,000 BTU into the water?
Gas heaters are sold by fuel input. energy.gov defines efficiency as "the ratio of usable output to energy input", so the heat that reaches the water is input × efficiency. Hayward rates its W3HS400FDN at 399,900 BTU/h input and 84% thermal efficiency: 335,916 BTU/h of usable heat.
| Shape | Area (sq ft) |
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| One turnover every | Flow |
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| Heat-up speed | Multiply by |
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