Pool heater size calculator

How pool heaters get sized

The pool industry uses one formula for sizing gas and heat-pump pool heaters:

BTU per hour = pool surface area (sq ft) × temperature rise (°F) × 12 × wind factor

The 12 in that formula is a baked-in industry constant from the Department of Energy. It assumes a 3.5 mph baseline breeze and represents 1 degree of water temperature rise per hour for every 12 square feet of pool surface per 1,000 BTU of heater output. The wind factor adjusts up for windier conditions: 1.0 for a sheltered backyard, 1.25 for a typical suburban yard with light breezes, 1.50 for exposed lots or hilltops.

Worked example: a 15 by 30 foot pool (450 sq ft) needing to raise from a 60 degree morning to an 82 degree target (22 degree rise) in a light breeze needs 450 × 22 × 12 × 1.25 = 148,500 BTU per hour. Round up to the next standard heater size, which is 150,000 BTU on the gas side or 90,000 BTU on the heat pump side (heat pumps are rated differently because they pump heat rather than burn fuel).

A pool cover changes the answer dramatically. Per Department of Energy data, a solar cover cuts pool heat loss by 50 to 70 percent because most of a pool's heat loss is evaporation off the surface. Using a cover overnight means you can run a smaller (cheaper) heater and your fuel bill drops by half. The calculator above applies a 60 percent reduction when the cover box is checked.

Gas vs heat pump vs solar: which one fits

The three pool heater types optimize for different things. Pick by how you actually use the pool, not by which is "best."

• Gas (natural or propane): Fastest heat-up by a wide margin. A 400,000 BTU gas heater warms a typical 15 by 30 pool from 60 to 82 degrees in 8 to 12 hours. Best for sporadic weekend use where you want the pool ready Friday night. Install cost $1,500 to $4,500 (plus $1,000 to $2,500 for a new gas line if you do not have one). Monthly fuel cost $200 to $500 for natural gas, $300 to $600 for propane.
• Electric heat pump: Slowest heat-up but lowest run cost. The same 15 by 30 pool takes 36 to 60 hours from cold with a typical heat pump. Best for regular daily use in mild climates where the heat pump runs constantly to maintain temperature rather than recovering from cold. Loses effectiveness below 50 degrees outdoor and stops working below about 45 degrees. Install $2,500 to $7,500. Monthly cost $50 to $200.
• Solar: Zero fuel cost but completely sun-dependent. Adds heat only during sunny hours, no help at night or on cloudy days. Best as a season extender on top of another heater, or as the sole heater in hot climates with reliable summer sun. Install $2,500 to $9,500. Monthly cost $10 to $25 for the pump electricity.

The combination most pool owners in moderate climates land on is heat pump for daily-use maintenance plus a small gas heater for fast weekend recovery. Pure gas costs the most to run; pure heat pump cannot recover from a cold pool fast enough to be the only option in cooler climates.

Standard pool heater sizes by pool size

Most residential pool heaters ship in 5 or 6 common sizes. Match your calculated BTU to the nearest standard size up:

• Small pool, 200-400 sq ft: 150,000-200,000 BTU gas (Hayward H150FDN, Raypak 156A, Pentair MasterTemp 175); 70,000-90,000 BTU heat pump (Pentair UltraTemp 70, AquaCal SQ80).
• Medium pool, 400-600 sq ft: 250,000-300,000 BTU gas (Hayward H250FDN, Raypak 266A, Pentair MasterTemp 250); 110,000-125,000 BTU heat pump (Hayward HP21104T, AquaCal SQ120).
• Medium-large pool, 600-800 sq ft: 300,000-400,000 BTU gas (Hayward H300FDN, Raypak 336A, Pentair MasterTemp 300); 125,000-140,000 BTU heat pump (Hayward HP21124T, Pentair UltraTemp 140).
• Large pool, 800+ sq ft: 400,000+ BTU gas (Hayward H400FDN, Raypak 406A, Pentair MasterTemp 400); 140,000+ BTU heat pump (Hayward HP21404T) or paired units.

All three major brands (Hayward, Pentair, Raypak) make reliable equipment. Pentair's MasterTemp series is generally quietest, Raypak's Digital series has the most readable controls, Hayward has the widest dealer network for parts and warranty service.

What a pool heater costs to run each month

Operating cost depends on heater type, pool size, climate, and whether you use a pool cover. For a typical 15 by 30 pool (450 sq ft) held at 82 degrees, running about 8 hours of heater time per day during the use season:

• Natural gas: $200-$400 per month without a cover, $90-$180 with a cover.
• Propane: $300-$600 per month without a cover, $135-$270 with a cover.
• Electric heat pump: $80-$160 per month without a cover, $36-$72 with a cover.
• Solar (after install): $10-$25 per month for pump electricity, regardless of cover.

A pool cover is the single highest-ROI pool accessory you can buy. A $200 solar cover saves $100-$300 per month in heating costs, paying back in about a month of regular use. The Department of Energy estimates 50 to 70 percent of pool heat loss is evaporation, which a cover stops almost entirely.

Heat-up time: how long until you can swim

The difference between gas and heat pump becomes obvious when you want a cold pool ready for an unplanned weekend. Approximate heat-up times to take a 15 by 30 pool from 60 to 82 degrees:

• 400,000 BTU gas heater: 8 to 12 hours.
• 250,000 BTU gas heater: 14 to 20 hours.
• 140,000 BTU heat pump: 48 to 72 hours.
• 110,000 BTU heat pump: 60 to 90 hours.
• Solar (4 to 6 sun hours per day at 75% pool coverage): 4 to 7 days.

The math: a 450 sq ft pool with 5-foot average depth holds about 16,800 gallons. Raising 16,800 gallons by 22 degrees needs about 3.1 million BTU of net heat input. A 400,000 BTU gas heater at 83 percent efficiency delivers about 332,000 useful BTU per hour. Total time: about 9 hours, matching the field experience above. Heat pumps deliver more useful BTU per dollar (COP of 4 to 5) but have lower absolute output, so they take much longer to recover from cold.

Common questions about sizing a pool heater

What size pool heater do I need for a 15 by 30 pool?

For a 15 by 30 pool (450 sq ft) raising from 60 to 82 degrees in a typical suburban yard with light breeze, the calculator returns about 148,500 BTU per hour, which rounds up to a 150,000 BTU gas heater or a 110,000 BTU heat pump. If you use a pool cover overnight, you can drop a size to 100,000 BTU gas or 70,000 BTU heat pump and still maintain temperature.

How long does a pool heater take to heat a pool?

For a typical 15 by 30 pool from a 60-degree cold start to 82 degrees: 8 to 12 hours with a 400,000 BTU gas heater, 14 to 20 hours with a 250,000 BTU gas heater, 48 to 72 hours with a 140,000 BTU heat pump. Solar alone takes 4 to 7 days. Heat-up time scales directly with heater output, not pool size, so a bigger heater always warms faster.

Is a heat pump pool heater cheaper than gas?

Cheaper to run, yes. Much cheaper. A heat pump uses about a quarter of the fuel cost of natural gas and a sixth of the cost of propane to deliver the same heat. The catch is heat-up time. If you swim daily and just need to maintain temperature, the heat pump wins on lifetime cost. If you use the pool weekends only and need to bring a cold pool up to temperature on Friday afternoon, gas wins because the heat pump cannot heat fast enough.

How much does it cost to run a pool heater per month?

Roughly $200-$500 for natural gas, $300-$600 for propane, $50-$200 for an electric heat pump, and $10-$25 for solar (just the pump). All ranges assume a typical 15 by 30 pool at 82 degrees with 8 hours of heater run time per day. Adding a pool cover overnight cuts all of these numbers by about 60 percent.

What temperature should I set my pool heater to?

78 to 82 degrees is the standard range for recreational use. Lap swimming feels cool at 78, family use is comfortable at 82, therapy or older swimmers prefer 84 to 88. Every degree above 78 costs about 10 to 15 percent more in monthly fuel. Setting the heater 4 degrees lower (84 to 80) saves about 40 percent on fuel.

Do pool covers really make a difference?

Yes, dramatically. A $200 solar cover saves $100-$300 per month in heating costs for a typical pool. The Department of Energy cites 50-70 percent reduction in heat loss and 90-95 percent reduction in evaporation (which is where most of the heat goes). It is the single best investment you can make if you are paying to heat a pool.

Will a pool heat pump work in cold weather?

Not really. Pool heat pumps lose effectiveness below 50 degrees outdoor and basically stop working below 45 degrees. They are designed for the same temperature range a pool is comfortable in, which means they work great during the swimming season but cannot extend the season into cold weather. For cold-shoulder season pool use, gas is the answer.

Natural gas vs propane for a pool heater?

Natural gas if you have a line available, propane if you do not. Performance is identical, but natural gas costs about half as much per BTU delivered, so monthly fuel bills are roughly half on natural gas. Running a new gas line to a pool heater costs $1,000 to $2,500 depending on how far the line runs; for any heater you plan to use more than 50 days per year, the line pays back in 1 to 2 seasons.

Is a solar pool heater worth it?

In hot, sunny climates: yes, easily. Payback runs 1 to 5 years for the install, then operating cost is essentially zero forever. In moderate climates: payback stretches to 5 to 10 years, still profitable but slow. In cool climates: solar only works as a season extender alongside another heater, not as the primary unit.

For matching a heat pump to home heating rather than pool heating, the heat pump sizing calculator handles whole-house heating loads. For estimating broader HVAC operating costs across the home, the operating cost calculator converts BTU consumption to monthly dollars at your local fuel rates.