PoolHeatMath: free poolheatmath planning calculators

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What this site does

PoolHeatMath is a set of free planning calculators and plain-language guides for people who own, or are about to own, a backyard pool or spa and want warmer water without guessing. We focus on one job: turning the physics of heating water into numbers you can use when you talk to a licensed installer, compare quotes, or decide whether a cover is a better first step than new equipment. We do not sell heaters, we do not rank brands, and nothing here is an installation instruction.

The three numbers that matter

Almost every pool heating question comes back to three numbers. The first is volume, the gallons or litres of water you are heating. It decides how much energy the first heat-up takes. The second is surface area, because a warm pool loses most of its heat from the top, not through the walls. It decides how hard the heater has to work to hold temperature. The third is the temperature gap, the difference between the water you want and the air around it. A 10°F gap and a 25°F gap are very different jobs. Our pool volume calculator gives you the first two from a few tape-measure readings, and every other tool builds on them.

How to use the calculators in order

1. Measure length, width and the shallow and deep water depths, then run the pool volume calculator.
2. Choose a target temperature. Our pool temperature guide lists common ranges for lap swimming, family use and therapy.
3. Run the pool heater size calculator for gas or propane, or the pool heat pump size calculator for electric units.
4. Check how long a first heat-up will take with the pool heating time calculator.
5. Estimate a month of running with the pool heater operating cost calculator, using your own utility prices.

Doing them in that order means each answer feeds the next, and you arrive at a quote meeting with a size range, a rough monthly cost and a list of questions.

Choosing a heater type

There are four common ways to warm a residential pool. Natural gas heaters burn fuel in a combustion chamber and heat water quickly regardless of air temperature. Propane heaters work the same way but use a tank, which is useful where there is no gas line, at a higher cost per unit of heat in many regions. Electric heat pumps move heat from the air into the water; they are slower but often far cheaper to run when the air is mild. Solar heaters circulate pool water through collectors, usually on a roof, and have no fuel cost but depend on sun and collector area. Our heat pump vs gas pool heater and solar pool heater guide walk through the trade-offs, and natural gas vs propane covers the fuel question.

Why a cover often comes first

Evaporation is usually the largest single source of heat loss from an outdoor pool. Every pound of water that evaporates carries away roughly a thousand BTU. A cover that sits on the water at night blocks most of that, which can shrink the heater you need and the hours it runs. Before you upsize equipment, read pool cover heat loss and the solar cover guide; they explain why a cover is often the highest-return step for a heated pool.

Worked example: a typical backyard pool

Figures below are illustrative examples (2026-10-29), not quotes or guarantees.

Assumptions: a 16 × 32 ft rectangle, 3.5 ft shallow and 6 ft deep, so 512 sq ft and about 18,200 US gallons. The owner wants 82°F water in a month when nights average 60°F. Warming from 65°F to 82°F needs about 18,200 × 8.34 × 17 ≈ 2.58 million BTU. Holding temperature uncovered, by the rule of about 12 BTU/h per sq ft per °F, needs about 512 × 22 × 12 ≈ 135,000 BTU/h of output during cool nights; with a nightly cover, perhaps half that. A gas heater around 150,000 to 200,000 BTU/h input or a heat pump with enough output at 60°F air would be the range to discuss with a professional.

Safety and who does the work

Pool heaters combine water, electricity, and in many cases gas and combustion exhaust. Gas and propane heaters must be installed and connected by a licensed HVAC or pool professional to local code; gas piping, venting and clearances are not DIY work. Electric heat pumps usually need a dedicated 240 V circuit, which a licensed electrician should size and install with the correct bonding and disconnect. Pool chemicals interact with heater internals, so keep water balanced and follow every chemical product label exactly. If you smell gas, leave the area and call your gas utility from a safe place.

What our numbers are and are not

Our calculators use published physics and long-standing rules of thumb, and they are only as good as your inputs. They are not a site survey, a load calculation, a gas line sizing, or a quote. Local climate, wind, shade, plumbing, pump schedules and how often you use a cover all move the real result. Use the outputs as a starting range and confirm with a pro who can see your yard. The installation cost guide explains what a quote usually includes.

Where to go next

New to pool heating? Start with the pool heater FAQ. Comparing options? Try best pool heaters, which is a how-to-choose guide rather than a brand list. Having trouble with an existing unit? See pool heater not heating and pool heater lifespan. For hot tubs and spas, read hot tub vs pool heater.

FAQ

Is PoolHeatMath free?

Yes. The calculators and guides are free and need no account.

Do you recommend specific brands?

No. We explain how to compare models using spec sheets and quotes, and leave the final choice to you and your installer.

Can I install a pool heater myself?

Not the gas, venting or 240 V electrical parts. Those need licensed professionals working to local code.

Do the calculators work in metric?

Inputs are in feet, gallons and °F; volume results also show litres.