Pool Heater Size Calculator

Size a pool heater in BTU from the water surface area, not the gallons. This pool heater size calculator compares gas, heat pump and solar for hours to heat and cost.

By CalcBuilt Editorial TeamUpdated September 9, 2026Formulas checked against manufacturer specs, see how we calculate.

Quick answer: Size a pool heater from surface area, not gallons: 5 BTU per hour per sq ft for each degree the water sits above the air, plus the heat-up. A 16 by 32 ft pool held at 84 F in 65 F air, warmed from 70 F in a day, needs 142,000 BTU per hour of output.

Your measurements

Plan view from above: area is length times width.

Pool shapeOnly the water surface matters here, because that is where the heat leaves. A round or oval pool is taken as 78.5 percent of its bounding rectangle, a kidney or freeform pool as 85 percent.Pool lengthftWater surface at the coping line, not the deck. Ignored for a round pool, which uses the diameter below.Pool width or diameterftMeasure inside the wall at the water line. A nominal 24 ft above-ground pool measures a few inches less inside.Average water depthftShallow end plus deep end divided by two is close enough. Depth does not change the holding load at all, only how long the first heat-up takes.Water temperature you want (F)Most families settle between 80 and 86 F. The Florida Solar Energy Center calls 78 to 80 F comfortable for spring and fall. Every degree above the air costs money every hour.Water temperature now (F)Read the pool thermometer, not the weather app. This drives the first heat-up only, never the running cost.Average air temperature (F)Use the average for the coldest month you actually want to swim, day and night together, not the afternoon high. Roughly 72 F for a south Florida winter, 65 F for a Carolina October, 55 F for a Pacific Northwest June.Wind exposureWind at the water surface, not on the roof. A solid fence or a hedge on the windward side is worth more than a bigger heater, because evaporation climbs with air speed over the water.Pool coverA solid, foam or bubble cover cut measured evaporation by 94 to 96 percent in the Cal Poly study, and solar disks by 50 percent. The cover only counts for the hours it is actually on the water.Time allowed for the first heat-upA shorter heat-up buys a bigger heater. Once the pool is at temperature the heater only replaces surface loss, so this choice does not change the running cost.Natural gas price per therm ($)A therm is 100,000 BTU. Residential gas typically runs $1.10 to $2.20 a therm in 2026 US retail once the fixed monthly charge is spread over a heating bill. For propane, divide the price per gallon by 0.915.Electricity price per kWh ($)The EIA put the US residential average at 18.34 cents per kWh in June 2026. Check your own bill, since a pool heat pump can push a house into a higher rate tier.

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Heater output needed142,000 BTU per hourAll results

Results

Heater output needed: 142,000 BTU per hour; 48,640 BTU per hour holds 512 sq ft of surface at 84 F in 65 F air. Another 93,165 lifts 19,150 gal by 14 F in 24 hours
Gas heater to buy: 200,000 BTU per hour input; Gas heaters are sold by input. At the 82 percent thermal efficiency floor in 10 CFR 430.32(k) this one puts 164,000 BTU per hour into the water and reaches 84 F in 16.8 hours
Heat pump to buy: 140,000 BTU nameplate; Larger than any single residential unit, so this is the biggest one made. At 65 F air it delivers about 98,700 BTU per hour, 71 percent of the label, and reaches 84 F in 32.9 hours
Solar collector area: 1,075 sq ft; 210 percent of the 512 sq ft water surface. The Florida Solar Energy Center puts the collector ratio at 0.83 to 1.25 for a covered pool swum year round, and 2.1 times that with no cover
Cost to hold the temperature: $649 a month on gas; $398 a month on a heat pump at COP 4.7, and $22 on solar, which is only the extra filter pump power. That is 433 therms or 2,213 kWh on the heat pump, held around the clock with no credit for daytime sun
What a cover would save: $379 a month; 58 percent off the gas bill if it went on whenever nobody is swimming. EPA WaterSense credits a pool cover with stopping up to 95 percent of evaporation. IECC 2021 R403.10.3 asks for a vapor-retardant cover on an outdoor heated pool unless a heat pump or solar carries more than 75 percent of the heat
Water lost to evaporation: 2,842 gal a month; About 0.29 in off the surface a day at these conditions, carrying 34,048 BTU per hour with it. Evaporation is taken as 70 percent of the surface loss
Planning estimate: Verify on site; Sizing is one line of the job. A gas heater needs the line resized for its input, a heat pump needs a dedicated 240 V circuit and bonding, and both need a permit and an inspection

Pool Heater Size formula

Surface area (sq ft) = Length x Width   (x 0.785 oval, x 0.85 freeform; round = Diameter x Diameter x 0.785)
Holding load (BTU/h) = Area x 5 x Wind factor x Cover factor x (Target F - Air F)
Heat-up load (BTU/h) = Gallons x 8.34 x (Target F - Current F) / Hours allowed
Output needed = Holding load + Heat-up load
Gas heater input = Output needed / 0.82   (82% is the federal minimum)
Heat pump nameplate = Output needed / (0.57 + 0.009 x (Air F - 50))
Solar collector area = Pool surface area x 1.0 covered, x 2.1 uncovered

Worked example

An 18 by 36 ft pool is 648 sq ft of surface and about 26,660 gallons at a 5.5 ft average depth. The owner wants 82 F water through October, when the average air temperature is 60 F. Uncovered, the holding load is 648 x 5 x 22 = 71,280 BTU per hour. A bubble cover pulled on overnight, 14 hours out of 24, drops it to 42,174. The cover leaves about 30 percent of the loss in place while it sits on the water.

Add the first heat-up from 68 F to 82 F in a day: 222,349 lb of water x 14 degrees / 24 hours = 129,704 BTU per hour. The covered pool then needs 172,000 BTU per hour of output and the uncovered one 201,000. Both land on the same 250,000 BTU input gas heater, which delivers 205,000 BTU per hour into the water. It reaches 82 F in about 17.7 hours with the cover on, 19.9 hours without. The cover does not change the heater, but at $1.50 a therm it cuts the monthly holding cost from about $951 to $563.

Quick reference

Holding load in BTU per hour, uncovered pool in average wind
Pool size	10 F above air	15 F above air	20 F above air	25 F above air
12 x 24 ft, 288 sq ft	14,400	21,600	28,800	36,000
14 x 28 ft, 392 sq ft	19,600	29,400	39,200	49,000
16 x 32 ft, 512 sq ft	25,600	38,400	51,200	64,000
18 x 36 ft, 648 sq ft	32,400	48,600	64,800	81,000
20 x 40 ft, 800 sq ft	40,000	60,000	80,000	100,000

Why surface area sizes the heater and gallons do not

Heat leaves a pool at the water surface. Evaporation carries most of it and convection to the air takes the rest. The walls of a below-grade shell give up very little to soil already near the water temperature. Two pools with the same outline lose heat at the same rate whether they are 3 ft deep or 9 ft. That is why a sizing rule built on gallons gets the running cost wrong, and why this page starts from length times width.

Gallons matter for exactly one number, the first heat-up. Water takes 1 BTU per pound per degree, and a gallon weighs 8.34 lb. A 19,150 gallon pool therefore needs 2.2 million BTU to move 14 degrees, no matter how fast you do it. Spread that over a day and it is 93,000 BTU per hour. Spread it over half a day and it doubles. Add the holding load on top and you have the output the heater has to make.

The old trade rule, surface area times temperature rise times 12, lands in the same neighborhood for a one-day heat-up. It hides which rise it means. Use the gap between the water you want and the average air for the holding load. Use the gap between the water you have and the water you want for the heat-up. They are different numbers and they add.

Gas, heat pump or solar

A gas heater is rated by input and sold by that number. A 400,000 BTU unit is really about 328,000 BTU per hour of heat at the 82 percent federal minimum. It does not care what the air is doing, which is why it is the honest choice for a pool that has to come up 20 degrees by Saturday. It is also the most expensive to run, because every therm is bought outright.

A heat pump is rated by output, and the rating is taken at 80 F air. Pentair publishes 120,000 BTU per hour at 80 F for its UltraTemp 140 and 82,000 at 50 F, with the COP sliding from 5.4 to 4.0. The same box is a different heater in April than in July. At a COP near 5 it moves heat for roughly a third of what gas costs. It still fills a cold pool slowly, and most units shut down near 45 to 50 F air.

Solar is sized by collector area rather than BTU. The Florida Solar Energy Center puts the collector ratio at 0.83 to 1.25 times the pool surface for year-round swimming with a cover. An uncovered pool multiplies that by 2.1, which is more roof than many houses have. Fuel cost is zero and only the extra pump hours show on the bill. Under IECC 2021 R403.10.3 a heat pump or solar system carrying more than 75 percent of the heat also excuses the pool from the cover requirement.

What this calculator leaves out

It gives no credit for daytime solar gain, which on a sunny day offsets a large share of the loss. The monthly running cost here is a no-sun upper bound. It uses one lumped loss coefficient instead of separate evaporation, convection and night sky radiation terms, so it does not respond to humidity, dew point or cloud cover. Wind is three coarse steps rather than a measured speed at the water. Ground and wall conduction, backwash makeup water and an attached spa all sit outside it.

It also stops at the heater. Gas line sizing under IFGC 402.4, combustion air and vent clearances are separate work. So are the dedicated 240 V circuit and equipotential bonding a heat pump needs under NEC 680.26. Roof structure and plumbing for a solar array, the permit and the inspection are also separate line items. Prices are national averages, and a local gas or electric tariff with tiered rates can move the comparison between gas and a heat pump a long way.

Code limits that apply

IECC 2021 R403.10.3checked
Outdoor heated pools need a vapor-retardant cover, unless a heat pump or solar carries 75%
If you miss it: An uncovered heated pool fails the energy inspection, and most of the fuel bought leaves the water as vapor
IECC 2021 R403.10.1
A readily accessible on-off switch on the heater, and no continuously burning gas pilot
If you miss it: A standing pilot burns gas every day of the off season and the heater fails at final inspection
10 CFR 430.32(k)checked
Gas-fired pool heaters made since April 16, 2013 run at 82% thermal efficiency or better
If you miss it: Sizing on the input rating hands the water 18 percent less heat than the label implies
IFGC 402.4
Gas piping sized for the total connected input, with the pool heater added to the house load
If you miss it: An undersized line drops the manifold pressure and the heater trips on flame loss on cold nights
NEC 680.26(B)
Pool heater and pump bonded into the equipotential grid with 8 AWG solid copper
If you miss it: An unbonded heat pump leaves a voltage difference a swimmer can feel at the pool edge

Provision	Limit	If you miss it
IECC 2021 R403.10.3checked	Outdoor heated pools need a vapor-retardant cover, unless a heat pump or solar carries 75%	An uncovered heated pool fails the energy inspection, and most of the fuel bought leaves the water as vapor
IECC 2021 R403.10.1	A readily accessible on-off switch on the heater, and no continuously burning gas pilot	A standing pilot burns gas every day of the off season and the heater fails at final inspection
10 CFR 430.32(k)checked	Gas-fired pool heaters made since April 16, 2013 run at 82% thermal efficiency or better	Sizing on the input rating hands the water 18 percent less heat than the label implies
IFGC 402.4	Gas piping sized for the total connected input, with the pool heater added to the house load	An undersized line drops the manifold pressure and the heater trips on flame loss on cold nights
NEC 680.26(B)	Pool heater and pump bonded into the equipotential grid with 8 AWG solid copper	An unbonded heat pump leaves a voltage difference a swimmer can feel at the pool edge

Rows marked checked produce a warning in the results when an entry crosses the limit. The others are on the plans or on site and this calculator does not test them. Local amendments can be stricter; the adopted edition and the inspector govern.

Mistakes that cost money

Sizing on gallons: a 3 ft wading pool and a 9 ft diving pool of the same outline lose heat at the same rate.
Buying a gas heater on its input rating, when a 400,000 BTU label delivers about 328,000 BTU per hour at the 82 percent federal minimum.
Reading a heat pump nameplate as a year-round number, when the same unit makes about 57 percent of it at 50 F air.
Entering the July average air temperature when the goal is swimming in October, which halves the holding load and undersizes the heater.
Hanging a 400,000 BTU heater on the line that fed a 250,000 BTU one, so manifold pressure sags and the heater trips.
Planning a solar array without measuring the roof, since an uncovered pool swum year round wants about 2.1 times its own surface area in collectors.

Key facts

EPA WaterSense states that using a pool cover can prevent up to 95 percent of pool water evaporation.
In the Cal Poly NPIRC evaporation study a foam cover cut pool evaporation by 95.9 percent and a bubble cover by 94.9 percent. A solid track cover reached 93.9 percent, and solar disks only 50.1 percent.
Federal rule 10 CFR 430.32(k) requires gas-fired pool heaters made on or after April 16, 2013 to reach 82 percent thermal efficiency. A 400,000 BTU input heater therefore puts about 328,000 BTU per hour into the water.
Pentair rates the UltraTemp 140 heat pump at 120,000 BTU per hour with 80 F air and only 82,000 BTU per hour with 50 F air. Its COP falls from 5.4 to 4.0 across that range.
The Florida Solar Energy Center sizes solar pool collectors at 0.83 to 1.25 times the pool surface area for year-round swimming with a cover. An uncovered pool takes 2.1 times that figure.

Frequently asked questions

What size pool heater do I need?

Take the water surface area, multiply by 5 BTU per hour for each degree you want the water above the average air temperature, then add the first heat-up. A 16 by 32 ft pool held at 84 F in 65 F air, warmed from 70 F in a day, needs about 142,000 BTU per hour of output. That is a 200,000 BTU input gas heater.

How long does it take to heat a pool?

A day or two for a typical first heat-up. The same 16 by 32 ft pool reaches 84 F from 70 F in about 17 hours on a 200,000 BTU gas heater. The largest residential heat pump takes about 33 hours, because it only makes 98,700 BTU per hour at 65 F air.

Is a heat pump cheaper than a gas heater for a pool?

To run, yes, usually by half or better. At a COP near 5 a heat pump moves five units of heat for one unit of electricity, while a gas heater burns every BTU it delivers. The catch is speed and cold weather, since a heat pump makes about 57 percent of its nameplate at 50 F air and most units stop below that.

Does a pool cover really cut heating costs?

Yes, and it is the cheapest change available. EPA WaterSense credits a cover with stopping up to 95 percent of evaporation. The Cal Poly NPIRC study measured 93.9 to 95.9 percent for solid, foam and bubble covers. Because evaporation carries most of the heat out of a pool, a cover left on whenever nobody is swimming takes roughly half off the fuel bill.

Sources and references

Pool Covers, WaterSense (U.S. EPA)
Effectiveness of Pool Covers to Reduce Evaporation from Swimming Pools (Cal Poly NPIRC and the National Plasterers Council)
10 CFR 430.32(k), energy conservation standards for pool heaters (U.S. Government Publishing Office)
UltraTemp High Performance Heat Pump, rated BTU and COP at 80/80/80, 80/63/80 and 50/63/80 (Pentair)
Solar Swimming Pool Heating in Florida: Collector Sizing and Economics, FSEC-GP-13-81 (Florida Solar Energy Center)
Chapter 4 [RE] Residential Energy Efficiency, Section R403.10 pools and permanent spas, 2021 IECC (ICC)
Electric Power Monthly, Table 5.6.A average retail price of electricity (U.S. Energy Information Administration)

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