What size pool heater do you need?
Multiply the pool's surface area in square feet by the temperature rise in degrees F, then by 12. That is the heat output you need in BTU an hour. Gas heaters are sold by input, so divide by 0.82 and buy the next size up.
A 16 x 32 ft pool is 512 square feet. Holding 80F water when the coldest month you swim averages 60F is a 20F rise. 512 x 20 x 12 is 122,880 BTU an hour of output and 149,854 of input, so the next size sold is 150,000 BTU. The calculator on this page runs the same steps with the same constants.
The seven steps, worked on one pool
- Surface area. Length x width for a rectangle. 16 x 32 ft is 512 square feet.
- Temperature rise. The water temperature you want, less the average air temperature of the coldest month you swim. 80F less 60F is 20F.
- Heat output. Area x rise x 12. 512 x 20 x 12 = 122,880 BTU an hour. The formula is the Department of Energy's, and it assumes the water warms 1 to 1 1/4F an hour under a 3 1/2 mph wind.
- Wind. Multiply by 1.25 in about 5 mph of wind and by 2 in about 10 mph. Hayward's international calculator and Coates, an electric heater maker, both print those factors. Our pool is sheltered, so 122,880 stands. At 5 mph it becomes 153,600, and at 10 mph 245,760.
- Input. The formula gives output, and a gas heater is sold by input. Divide by the efficiency. The calculator uses 82 percent, the figure Raypak states for its Digital heaters and the federal minimum for gas pool heaters built before May 30, 2028. 122,880 divided by 0.82 is 149,854. Raypak's own 399,000 BTU heater is the proof: 399,000 BTU an hour goes in and 327,180 comes out.
- Next size up. Gas heaters are sold at 105,000, 150,000, 200,000, 266,000, 333,000 to 336,000 and 399,000 to 400,000 BTU. 149,854 lands on a 150,000 BTU heater with 146 BTU in hand. In a 5 mph wind the input is 187,317 and the answer is a 200,000 BTU heater.
- Heat-up time. Pentair's volume method: hours = gallons x 8.34 x rise, divided by (input x efficiency). Gallons are length x width x average depth x 7.5, so 19,200 at 5 ft deep. 19,200 x 8.34 x 20 is 3,202,560. Divide by 150,000 x 0.82, which is 123,000, and you get 26 hours, about 0.8F an hour. The 200,000 BTU heater takes 19.5 hours.
That is the best case: Pentair says its chart leaves losses out. If 26 hours is too long, go up a size.
Pool heater sizing chart
Each cell is the largest pool in its row put through steps 1 to 6: a 3 1/2 mph wind, 82 percent efficiency, nothing taken off for a cover, and the next size sold.
| Pool surface area, sq ft | 15F rise | 20F rise | 25F rise |
|---|---|---|---|
| Up to 288 (12 x 24 ft) | 105,000 | 105,000 | 150,000 |
| 289 to 392 (14 x 28 ft) | 105,000 | 150,000 | 150,000 |
| 393 to 512 (16 x 32 ft) | 150,000 | 150,000 | 200,000 |
| 513 to 648 (18 x 36 ft) | 150,000 | 200,000 | 266,000 |
| 649 to 800 (20 x 40 ft) | 200,000 | 266,000 | 333,000 |
| 801 to 1,000 (25 x 40 ft) | 266,000 | 333,000 | 399,000 |
Two cells sit within 400 BTU of a boundary: 288 square feet at 25F asks for 105,366, and 512 at 20F for 149,854. Any breeze settles them upward. Every size is under pool heaters.
The 10 BTU a gallon rule, and what it leaves out
The heater finder on our pool heaters page asks for gallons and uses a quicker rule: about 10 BTU of input rating for each gallon raises the water about 1F an hour. It is step 7 turned around: 8.34 divided by 0.82 is 10.2. Our 19,200 gallon pool comes to 192,000 BTU, so the finder says 200,000 where the area method said 150,000. Step 7 agrees: 150,000 BTU manages 0.8F an hour here.
What size pool heater for 10,000, 20,000 or 30,000 gallons?
By the gallon rule a 10,000 gallon pool takes a 105,000 BTU gas heater, a 20,000 gallon pool takes 200,000 and a 30,000 gallon pool takes 333,000. Each raises its pool about 1F an hour, so a 20F rise is about 20 hours of firing.
| Pool size | Gas heater, input rating | Degrees an hour | Hours for a 20F rise |
|---|---|---|---|
| 10,000 gallons | 105,000 BTU | 1.0F | 19.4 |
| 15,000 gallons | 150,000 BTU | 1.0F | 20.3 |
| 20,000 gallons | 200,000 BTU | 1.0F | 20.3 |
| 25,000 gallons | 266,000 BTU | 1.0F | 19.1 |
| 30,000 gallons | 333,000 BTU | 1.1F | 18.3 |
| 40,000 gallons | 399,000 BTU | 1.0F | 20.4 |
The hours are step 7 at 82 percent, with the heater firing the whole time and nothing lost on the way. Treat the table as the quick answer and the calculator as the careful one.
What the rule leaves out is the heat the surface loses to wind and cold air. Raypak prints a similar check, output divided by gallons x 8.33, and warns beside it that weather is not counted. In Raypak's heat loss table, water at 80F under 60F air gives up 210 BTU an hour per square foot at 3.4 mph and 510 at 13.63 mph. Across 512 square feet that is 107,520 and 261,120 BTU an hour, against the 164,000 a 200,000 BTU heater delivers. Gallons cannot see wind, which is why the makers' charts start from surface area.
Area x rise x 12 is not the makers' rule
You will read that this formula is how the manufacturers size a heater. It is not. Only the Department of Energy prints it, and it calls the answer approximate and says a trained pool professional should do the sizing. We found the formula in no Raypak, Pentair, Hayward or Jandy manual, brochure or catalog.
Pentair and Raypak work from the pool's volume and the hours you allow. Jandy and Hayward chart by surface area. Put one size of heater through them at a 30F rise and the answers spread across 470 square feet.
| Whose figure | Pool a 400,000 BTU heater covers at 30F | What it assumes |
|---|---|---|
| Hayward H400 | 1,200 sq ft | 3 1/2 mph wind, no time given |
| Jandy JXi 400, still air | 990 sq ft | 1F an hour, 4.5 ft deep |
| Department of Energy formula at 82 percent | About 910 sq ft | 1 to 1 1/4F an hour, 3 1/2 mph wind |
| Pentair model 400 | 788 sq ft | 30F gained in 24 hours, 5.5 ft deep, losses left out |
| Jandy JXi 400, 3.5 mph wind | 730 sq ft | 1F an hour, 4.5 ft deep |
Divide each heater's rating by its chart's area and rise, and the number standing where 12 stands is 11.1 for Hayward, 13.4 for Jandy in still air and 18.2 for Jandy in wind. Raypak's heat loss table works out between 8.7 and 13.8. That arithmetic is ours, not the makers'. A chart that differs from the calculator by one size assumes something else.
Input and output are the other trap. The department rates gas heaters by output, Pentair's manual by input, and one Pentair brochure calls the same sizes output. Treat a model number as input.
Pool heat pump sizing
A heat pump is sized by the day, not the hour. The calculator does it in three steps. If you have not chosen between the two yet, read pool heat pump vs gas heater first.
- Daily heat. Gallons x 8.3 x rise x a cover factor x a wind factor. The cover factor is 0.5 with no cover, 0.3 with a liquid cover and 0.2 under a blanket. The wind factor is 1 up to 3.5 mph and 1.25 from 3.6 to 5 mph. The sheet goes no higher and gives the factor for a pool without a blanket, so the calculator holds a stronger wind at 1.25 and adds nothing under a blanket. Those constants are from AquaCal's operating cost worksheet, which gives the run time of a heat pump already chosen and does not size one. Our pool with no cover: 19,200 x 8.3 x 20 x 0.5 = 1,593,600 BTU a day.
- Run hours. You choose 8, 12, 16 or 24 hours a day. That choice and the division are ours. The one figure from a maker is Raypak's: a heat pump may run up to 24 hours a day. 1,593,600 divided by 24 is 66,400 BTU an hour.
- Next size up, from 50,000, 65,000, 85,000, 102,000, 118,000, 138,000 and 143,000 BTU. 66,400 lands on 85,000, which runs 18.7 hours a day and needs 37.7 hours to warm the pool from cold. Hold it to 12 hours and the answer is 138,000. Add a blanket and 12 hours needs 637,440 BTU a day, 53,120 an hour, which a 65,000 BTU heat pump covers.
The calculator also prints the Department of Energy figure, 122,880 BTU an hour here, the output that gains 1 to 1 1/4F an hour. It is the only sizing formula anyone publishes for heat pumps. Pentair says a properly sized heat pump lifts the pool about 1F an hour on average, and is for holding a temperature, not fast heating.
No maker's document we found gives a pool size for heating with a model above 65,000 BTU. Moov rates its 50,000 BTU MI500 for pools up to 14,000 gallons and its 65,000 BTU model for 18,000. At a 20F rise with no cover, over 24 hours, those pools need 48,417 and 62,250 BTU an hour. That arithmetic is ours. It says an "up to" figure is a heat pump running around the clock on a bare pool. Treat the printed pool size as a ceiling, not a recommendation.
The BTU on the label is a best case. Makers rate at 80F air, 80 percent humidity and 80F water. Raypak's 118,000 BTU Crosswind V delivers 110,000 at 63 percent humidity and 75,000 in 50F air.
Electric resistance heaters
These heat spas and small pools. Raypak aims its E3T at compact jobs and at indoor rooms where a gas heater cannot be vented. It comes in 5.5, 11, 18 and 27 kW, which is 18,767 to 92,128 BTU an hour, on single phase 240V.
Raypak's formula is written for spas: degrees an hour = kW x 410, divided by gallons, weather left out. On a 500 gallon spa the 11 kW E3T gives 9F an hour. For pools the only maker's method we found is from Coates, which works from surface area: its printed table gives a 30 kW heater 512 square feet outdoors at a 20F rise. That is our example pool, and more than any single phase E3T.
Above ground pools
Raypak's 105,000 BTU 106A delivers 86,100 BTU an hour. Its 150,000 BTU 156A, which Raypak also offers for inground pools, delivers 123,000. Raypak sets no gallon or square foot limit on either, but its chart gives the pace: on 300 square feet the 106A manages 0.9F an hour and the 156A 1.3F.
The one gallon figure any maker gives is Pentair's: a limit of 15,000 gallons on its 125,000 BTU MasterTemp 125, which is 8.3 BTU a gallon. Hayward's 135,000 BTU above ground heaters carry no pool size guidance.
Altitude, maker by maker
Thin air cuts a gas heater's output, and each maker draws the line at a different height. Find your model, not an average.
| Heater | Threshold | What the maker says |
|---|---|---|
| Raypak Digital 206A to 406A | 2,000 ft | Above it, input drops 4 percent per 1,000 ft above sea level |
| Raypak Digital Low NOx | 5,000 ft | Above it, ask the factory |
| Raypak AVIA | 4,500 ft | Certified to here. Natural gas runs to 10,000 ft. Propane with a kit: 7,800 ft in the manual, 10,000 ft in the catalog |
| Raypak 106A, 156A | 2,000 ft | Brochure: 4 percent per 1,000 ft. Manual: ask the factory. Catalog: natural gas part numbers to 4,999 ft |
| Jandy JXi | 4,500 ft | No derate below it, then 4 percent less output per 1,000 ft over 4,500 ft |
| Hayward Universal H-Series | 2,000 to 10,100 ft | The limit unmodified. Natural gas H200, H300, H400: 10,100 ft. H150: 7,700 ft. Natural gas H250, propane H200: 5,400 ft. The rest: 2,000 ft |
| Hayward H135ID, ABG135 | 2,000 ft | Must be installed below it |
| Pentair MasterTemp | None stated | No limit or derate in the manual |
Raypak gives each elevation band its own part number, and the two Digital heaters linked here are the 0 to 2,000 ft versions. At 5,000 ft Raypak's rule takes 20 percent off, so our 149,854 BTU answer needs 187,317 BTU of rating: a 200,000 BTU heater, not a 150,000.
One heater for a pool and a spa
Size for the pool, then time the spa. Hayward's chart has the H400 lifting 1,000 gallons by 30F in 45 minutes, while the H150 needs 120. Jandy's JXi 400 takes 44 minutes, and Jandy halves its times for a 15F rise. Both assume an insulated, covered spa.
If the pool calls for 150,000 BTU and you want 1,000 gallons hot inside the hour, the spa sets the size.