Sizing a pool heat pump correctly is what determines whether your pool warms up when you want it to, and stays warm without your heater running nonstop.
If a heat pump is too small for the pool and your climate, a few predictable things happen:
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It takes a long time to heat up, especially after cool nights or when you first start the season.
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It runs for very long hours, because it is working near its limit just to keep up with heat loss.
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It may never reach your target temperature in normal weather, so the pool stays a few degrees cooler than you want even though the unit is running.
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It struggles more in cooler or windy conditions, making performance feel inconsistent day to day.
A properly sized unit (or slightly oversized one) avoids those problems. It heats faster, maintains temperature with fewer runtime hours, and keeps performance steady across normal weather changes.
The goal is not just to heat the pool once. The goal is to hold your swimming temperature reliably, day after day, with reasonable runtime.
Step One: Calculate Your Pool Volume
Heat pump capacity is tied directly to how much water you are heating, so gallons come first. If you already know your pool volume from builder paperwork or a pool app, use that number. If not, you can calculate it.
Rectangular or square pools
Length (ft) × Width (ft) × Average Depth (ft) × 7.5 = gallons
Round pools
Diameter (ft) × Diameter (ft) × Average Depth (ft) × 5.9 = gallons
Oval pools (common above ground shape)
Length (ft) × Width (ft) × Average Depth (ft) × 5.9 = gallons
Average depth is the shallow end depth plus deep end depth divided by two. Once you have gallons, you have the foundation for sizing.
Step Two: Pick a BTU Range Using a Reliable Rule of Thumb
Once you know gallons and climate expectations, you can get a solid BTU range quickly.
A widely used sizing guideline is:
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About 4 BTU per gallon for typical summer heating conditions
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About 5 BTU per gallon if you want reliable heating in cooler parts of the season
So:
Pool Gallons × 4 = summer sized BTU range
Pool Gallons × 5 = extended season BTU range
Example for a 20,000 gallon pool
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Summer sizing: 20,000 × 4 = 80,000 BTU
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Cooler month sizing: 20,000 × 5 = 100,000 BTU
You would choose the closest model at or above that number.
Factors That Push Towards a Larger Unit
Rules of thumb assume average conditions. Real pools sometimes lose heat faster than average. If any of the following apply, stepping up a size is usually the right move:
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Windy or very open backyard
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Limited cover use, or no cover at all
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Heavy shade or low sun exposure
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Higher target temperatures
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You want faster initial warm up
These conditions increase heat loss, which increases the amount of heating your unit must replace each day.
Why Sizing Up is Usually the Safe Choice
If you are between two models, the larger unit is almost always the better long term experience.
A slightly oversized heat pump:
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heats up faster at the start of the season
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spends fewer hours per day running to hold temperature
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handles cooler or drier weather more comfortably
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operates under less strain over time
Even though the unit is larger, it does not automatically mean higher running cost. Because it reaches and maintains temperature more easily, it often runs less total time.
Quick Note on Ratings and Real Output
Heat pump BTU ratings are measured under standardized test conditions. AHRI Standard 1160 is the main U.S. performance rating standard for pool heat pumps.
That rating is useful for comparing models, but real outdoor conditions vary. Cooler air and lower humidity reduce both heating output and efficiency. This is normal behavior for all heat pumps, and it is another reason why sizing slightly above the minimum calculation improves real world comfort.
Simple Sizing Examples
Example A: Summer only use
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Pool volume: 15,000 gallons
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Target temperature: 82°F
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You mainly swim in warm summer weather
Sizing: 15,000 × 4 = 60,000 BTU
Recommended unit range: roughly 60,000 to 80,000 BTU.
Example B: Heating in cooler months too
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Pool volume: 25,000 gallons
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Target temperature: 84°F
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You want dependable heating beyond peak summer
Sizing: 25,000 × 5 = 125,000 BTU
Recommended unit range: roughly 125,000 to 140,000 BTU.
These numbers are not meant to be to-the-BTU precise. They are meant to get you into the correct model class quickly.
Bottom Line
Sizing a pool heat pump comes down to three inputs: pool gallons, your target temperature, and the coolest air conditions you plan to swim in. Once you know those, the 4 to 5 BTU per gallon guideline gets you into the right capacity range fast.
If you are unsure or between sizes, size up. The difference shows up as faster warm up, easier temperature holding, and a better experience throughout the season.
