Pool heater size calculator

How the pool heater input and heat-up time are calculated

This page uses two published methods. One sizes the heater input from the surface area of the pool and the temperature rise, which is Raypak's rule and rests on a 3.5 MPH average wind and a heat-up of 1 to 1.25°F per hour. The other counts the heat that the water itself needs from the volume, which is Pentair's time formula, and gives the hours a given heater takes and the input that would do it in a target time. Losses to evaporation, wind and the night sit inside the first method as an allowance and are left out of the second, and the page does not calculate them. Gas heaters need the maker's rated input and its derates, and heat pumps are not sized here.

Temperature rise

rise = desired pool temperature - mean air temperature of the coldest month usedRaypak sizes on the rise from the coldest month's mean to the pool temperature you want. 82°F against 57°F is a 25°F rise. The maker's table has columns for 20, 25, 30 and 35°F.

Input by pool area

BTU/h input = pool area x rise x 15The factor is 15 BTU/h of input per square foot per °F of rise. 800 sq ft at a 25°F rise is 300,000 BTU/h. The same rule reproduces the maker's own table, since each printed row works out to within 1 percent of its model's input.

Faster heating and commercial pools

input x 1.5 for 1-1/2°F per hour, x 2 for 2°F per hour; input / 1.25 for a commercial pool on a 30-hour recoveryFor 1-1/2°F per hour Raypak adds 50 percent to the input, which makes 450,000 BTU/h here, and for 2°F per hour it adds 100 percent, which makes 600,000 BTU/h. Raypak's table rests on a 24-hour recovery, and for a 30-hour recovery it multiplies the listed pool size by 1.25. A heater therefore serves a larger pool, and the input for this pool is the area rule divided by 1.25, which gives 240,000 BTU/h.

Volume

gallons = area x average depth x 7.5Pentair estimates gallons as length times width times average depth times 7.5 for a rectangle, which is the area times the depth times 7.5. 800 sq ft at 5.5 ft is 33,000 gal, the 40 by 20 ft pool in Pentair's own example.

Heat for the water

BTU = gallons x 8.34 x riseA gallon of water weighs 8.34 lb in Pentair's formula and takes 1 BTU per pound for each °F, so 33,000 gal raised 20°F needs 5,504,400 BTU before any loss.

Heat-up time

hours = gallons x 8.34 x rise / (heater BTU/h input x efficiency)Dividing the heat by what the heater puts into the water gives the hours. A heater of 300,000 BTU/h at 84 percent puts 252,000 BTU/h into the water, so 5,504,400 BTU takes 21.8 hours, a rise of 0.92°F per hour before losses.

Input for a target time

BTU/h input = gallons x 8.34 x rise / (hours x efficiency)Solving the same formula for the input gives 273,036 BTU/h to heat 20°F in 24 hours at 84 percent. The two methods need not agree, because the first is a rule for the whole pool and the second counts only the water.

Run time to hold the temperature

run hours a day = gallons x 8.34 x daily loss / (input x efficiency)Pentair says a pool will usually lose 2 to 4°F per day in cooler periods. Winning back 2°F is 550,440 BTU and takes 2.2 hours of run time at 300,000 BTU/h, and 4°F takes 4.4 hours, against the 4 to 6 hour daily filter cycle that Pentair names.

Worked examples from published pool heater data

The default case is the 40 by 20 ft pool of Pentair's gallons example, 800 sq ft at 5.5 ft average depth, which holds 33,000 gal. At 82°F against a 57°F coldest-month mean, Raypak's rule gives 300,000 BTU/h. Heating the water from 62°F to 82°F needs 5,504,400 BTU by Pentair's formula. At 84 percent, 300,000 BTU/h takes 21.8 hours and 273,036 BTU/h would take 24 hours. A 400,000 BTU/h heater takes 16.4 hours.

The table below runs every printed row of Raypak's P-514, P-624 and P-824 sizing table through the area rule. Each model number is read as its input in thousands of BTU/h. The pool area times the rise times 15 lands within 1 percent of that input in every row, for example 1,360 sq ft at 25°F for the P-514, which is 510,000 BTU/h against 514,000.

Eight cases below run through the same functions the form uses. Five of them use the Pentair pool and change one input at a time. A 15°F rise from a 67°F month cuts the area rule to 180,000 BTU/h and stretches that heater to 36.4 hours. The faster rates raise the input to 450,000 and 600,000 BTU/h, and a commercial pool on a 30-hour recovery divides the input by 1.25, which makes 240,000 BTU/h for the Pentair pool.

Standards and data sources

• Raypak, P-514 Manual, pool heater sizing page - The rule BTUH input required = pool area x desired temperature rise x 15, based on 1 to 1.25°F per hour and a 3.5 MPH average wind. It adds 50 percent to the input for 1-1/2°F per hour and 100 percent for 2°F per hour, says to select a heater one size larger where high wind exists, and, for a commercial pool on a 30-hour recovery against the 24 hours of its table, multiplies the listed pool size by 1.25. Its table of pool area by rise at 20, 25, 30 and 35°F is reproduced in the worked examples.
• Pentair, Product Catalog 2025-2026, Engineering Data, Heater Sizing Information - The time formula in hours with the 8.34 lb per gallon, the estimate of gallons in a pool with 7.5 gallons per cubic foot for a rectangle and its 40 by 20 ft example, the 2 to 4°F daily loss in cooler periods, the 4 to 6 hour filter cycle for maintenance heating, and a time formula that counts only the heat that raises the water. It sends commercial heaters of 500,000 BTU/h and over to the factory.
• Fluidra, Jandy Gas Heaters (JXiQ and JXi), 2024 Product Catalog Addendum - A pool sizing chart that assumes 4.5 ft of average depth and 1°F per hour, and Energy Guide labels with a thermal efficiency of 83 and 84 percent for its gas models. The 84 percent default comes from these labels.
• U.S. Department of Energy, Energy Savers, Heat Pump Swimming Pool Heaters (archived 2004 fact sheet) - The statements that evaporation turns out to be 70 percent of the heat loss from a pool, that a cover can reduce a pool's energy consumption by 50 to 75 percent, and that the typical range for pools is 78 to 82°F. These are used as text only, and no number from the sheet enters a formula.

Limitations of this calculator

• Evaporation, wind and night losses are not calculated. The Department of Energy's archived fact sheet puts evaporation at 70 percent of a pool's heat loss, but the sources quoted here give no coefficients for it, so Raypak's area rule stands in for all of it and the time formula leaves it out.
• Raypak's rule rests on a 3.5 MPH average wind and a heat-up of 1 to 1.25°F per hour, and it has no depth term. A windier site, a deeper pool or an uncovered pool in a cold month can need more than it gives, which is why Raypak says to select one size larger where high wind exists.
• The heat-up time counts only the heat that raises the water. Pentair's time formula has no loss term, so a real heat-up is longer, and a pool that loses heat as fast as the heater adds it never reaches temperature. Results beyond 168 hours are refused.
• Heater models come in fixed sizes, and a maker's model number is not the input this page prints. Take the figure to the manufacturer's data for the model, its rated input, its efficiency and any derate for elevation, venting or gas type. This page applies none of them.
• Gas, propane and electric resistance heaters only. A heat pump is rated by a COP above 1, not by an efficiency under 100 percent, and solar collectors depend on sun and roof area, so neither is sized here. The efficiency field takes 50 to 100 percent.
• An indoor pool is outside Raypak's outdoor basis. Its evaporation goes into the room air and has to be removed by the building's dehumidification and ventilation, which is a separate design with its own load. Enter the room air temperature for an indoor pool, since the outdoor mean would give a rise the room never has. A rise above 10°F is refused.
• A cover is not credited. The Department of Energy's fact sheet says a cover can reduce a pool's energy consumption by 50 to 75 percent, which would shorten real heat-ups and lower the heater needed, and the page gives no figure for it.
• Volume is the area times the average depth times 7.5, so an irregular pool needs a fair average depth. Spas and hot tubs heat on other tables and are not sized here, and a pool above 500,000 BTU/h of input goes to the manufacturer.
• This is a planning estimate, not a design drawing. A licensed contractor, the manufacturer's data and the authority having jurisdiction decide the heater, the gas or electric supply and the installation.

Common pool heater sizing mistakes

1. Using the pool temperature as the rise. Raypak's rule takes the rise from the coldest month's mean air temperature to the pool temperature, and not the pool temperature alone. 82°F against 57°F is 25°F and gives 300,000 BTU/h, while a milder 67°F month gives a 15°F rise and 180,000 BTU/h.
2. Leaving the efficiency out of the heat-up time. Pentair divides by the input times the efficiency of the heater. At 84 percent a 300,000 BTU/h heater puts 252,000 BTU/h into the water, and the heat-up takes 21.8 hours. A heater taken as 100 percent efficient would show 18.3 hours.
3. Treating the heat-up time as a promise. The formula gives 21.8 hours for 20°F with no loss at all. Pentair's formula has no loss term, so a cool night, wind or an uncovered pool lengthens it. Plan on more than the figure, and keep the pool covered while it heats to hold what the heater adds.
4. Ignoring wind. Raypak's formula assumes a 3.5 MPH average wind and says to go one size larger where high wind conditions exist. A pool in an exposed yard sized at 300,000 BTU/h should be checked against the next model up.
5. Sizing a deep pool on its area alone. The area rule has no depth in it. Jandy's chart assumes 4.5 ft of average depth. The default pool is 5.5 ft deep, holds 33,000 gal, and at 300,000 BTU/h heats about 0.92°F per hour before losses, which is under the 1°F per hour that the rule is based on. A deeper pool should be checked with the time formula.
6. Forgetting the gas line, the air and the vent. A heater of 300,000 BTU/h burns gas at a rate set by its input, so the gas line, the combustion air and the venting are sized from the maker's input for the model chosen. The gas pipe sizing calculator and the combustion air calculator take that input.

Common questions about pool heater size

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Used on the same job

• Motor circuit size calculator The branch circuit for the pool pump motor.
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