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Pool Heater Sizing Calculator

Estimate the heater output needed to raise a pool to temperature in a target time, and what a cover saves.

Computed in your browser — nothing you enter is uploaded. Figures are presented for United States against IRC 2024, and every formula is cited under regulatory standards below.

Last verified 2026-08-26 · v1.0.0

Market
Imperial · sales tax

Heater output required

22.71 kW

Low confidence

A planning figure. Standing loss depends on wind, humidity, air temperature and shelter, none of which are inputs here — it varies by a factor of two or more between a sheltered and an exposed site.

Energy to raise the water
465.11 kWh
Heat-up power
19.38 kW
Standing loss allowance
3.33 kW
BTU/h
77499.85 BTU/h
Approximate surface area used
358.8 ft²

Running these inputs gives 22.7 kW as the heater output required. Heat-up power carries the most weight in this calculation, at 19.4 kW. Note that pool volume sits at the low end of the range this calculator was checked against, so treat the output as indicative rather than settled. Low confidence on these inputs, so use the number to plan rather than to order. Currently reading for United States under IRC 2024 — pick a different market above and the figures re-cast accordingly.

Add the equipment this sizes

This result is a specification — 22.713 kW — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — confirmed fixes become pinned regression tests.

[Schema Verified] Computed in alignment with American Concrete Institute (ACI 318-19) formulas and International Residential Code (IRC 2024) spatial boundaries.

Regulatory standards & verification citations

  • Specific heat capacity of water 4.186 kJ/kg·K; 1 m³ of water = 1000 kg
  • Evaporation is the dominant heat loss from an uncovered pool, typically 50-70% of the total — ASHRAE Handbook, Applications: Natatoriums

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Frequently asked questions

Why does a cover make such a difference?
Because evaporation carries most of the heat out of an outdoor pool — commonly 50-70% of the total loss. Every kilogram of water that evaporates takes about 2.26 MJ with it. A cover suppresses that almost entirely, which is why it is the cheapest and most effective thing you can add to a heated pool.
Should I size for fast heat-up or for holding temperature?
Mostly for holding, with an acceptable heat-up time. A heater sized to raise the pool eight degrees overnight is enormously oversized for maintaining it, and will cycle. Longer initial heat-up and a heater matched to the standing loss is both cheaper to buy and better to run.
Why is the confidence low?
Because standing loss depends on wind speed, humidity, air temperature and how sheltered the pool is, and none of those are inputs here. The energy to raise the water is exact physics; the allowance to hold it is a planning figure that can be out by a factor of two between a sheltered courtyard and an exposed hillside.