Updated October 10, 2026
Sizing is the most common mistake in pool heating. An undersized heater runs around the clock, burns money, and dies years early. This chart gives you the right starting point for both heat pumps and gas heaters.
Step 1: Find your pool's gallons
Rectangular pool: length × width × average depth × 7.5 = gallons.
Example: 16 ft × 32 ft × 5 ft average depth × 7.5 = 19,200 gallons.
For freeform pools, break the shape into rectangles, or check your builder's specs — most owners know their gallonage from chemical dosing.
Step 2: Match BTUs to gallons
| Pool volume | Heat pump (BTU) | Gas heater (BTU) | Notes |
|---|---|---|---|
| Up to 10,000 gal | 90K–110K | 150K–250K | Small pools, spas, plunge pools |
| 10,000–15,000 gal | 110K–140K | 250K–400K | Average small residential |
| 15,000–20,000 gal | 140K | 400K | The sweet spot — most pools land here |
| 20,000–30,000 gal | 140K+ (longer cycles) | 400K | Heat pump works but heats slowly |
| 30,000+ gal | Two units or go gas | 400K | Single heat pump will struggle |
Step 3: Adjust for your situation
The chart assumes average conditions. Size up one step if any of these apply:
- Cool climate — heat pumps lose output as air temperature drops
- No pool cover — an uncovered pool loses most of its heat overnight to evaporation
- Wind exposure — wind strips heat off the surface fast
- Shade — shaded pools start colder every morning
- Fast warm-ups wanted — heating for Saturday's party, not maintaining all season
- Attached spa — spas need high BTU output to heat quickly
Golden rule: when in doubt, go bigger. An oversized heater costs a bit more upfront but reaches temperature faster, cycles less, and lasts longer. An undersized heater is the expensive mistake — it never catches up.
Heat pump vs gas sizing — why the numbers differ
You'll notice gas heaters are rated much higher (400K BTU) than heat pumps (140K BTU) for the same pool. That's normal — gas heaters are meant to blast heat quickly, while heat pumps maintain temperature efficiently over long run times. Don't compare the BTU numbers across types; size each according to its own chart.
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Frequently asked questions
What size pool heater do I need?
For heat pumps, a 140K BTU unit suits most 15,000–20,000 gallon pools. For gas heaters, 400K BTU is the standard for full-size residential pools. Smaller pools (under 10,000 gallons) can use 90K–110K BTU heat pumps or 150K–250K BTU gas heaters.
What happens if my pool heater is undersized?
An undersized heater runs constantly to reach temperature, which drives up operating costs and shortens the unit's lifespan. It also can't recover temperature after cool nights or heavy use.
How do I calculate my pool's gallons?
For a rectangular pool: length × width × average depth × 7.5 = gallons. For example, a 16×32 ft pool averaging 5 ft deep holds about 19,200 gallons.
Can a pool heater be too big?
Oversizing is far less harmful than undersizing. A slightly larger heater reaches temperature faster and cycles less. The main downside is higher upfront cost — there's little operating penalty.
Does climate affect heater sizing?
Yes. In cooler climates or for pools that need fast warm-ups, size up one step. Wind exposure, lack of a pool cover, and shaded pools all increase heat loss and call for more BTUs.