Pool & spa
Pool Heater Size Calculator
Sizes a gas heater from surface area and temperature rise, where the surface is where the heat goes.
Result
Most heat leaves a pool through its surface, which is why surface area matters more than depth and why a cover is the single most effective thing you can add. Wind, humidity and night air temperature all change the loss substantially, so treat this as a size band rather than a precise figure.
Get a Free Business AuditHow the calculation works
Heater output in BTU per hour is driven by the pool's surface area and the temperature rise wanted, because heat is lost from the surface rather than through the walls. The calculation accounts for the volume to be raised and the rate at which the surface loses heat, with evaporation being the dominant loss. A larger surface loses more heat for the same volume, which is why a wide shallow pool is harder to heat than a deep narrow one of the same capacity.
What moves the number
- Surface area, which governs heat loss more than volume does.
- Temperature rise wanted above ambient.
- Whether a cover is used, which transforms the heat loss.
- Wind exposure, which accelerates evaporative loss substantially.
Where this one goes wrong
A cover is worth more than extra heater capacity. Evaporation is the largest single heat loss from a pool, and a cover largely stops it, so an owner choosing between a bigger heater and a cover is usually better served by the cover and often needs less heater as a result. Sizing up to compensate for an uncovered, wind-exposed pool means buying and running capacity to replace heat that need not be lost. On gas sizing, the heater's input determines the gas line and meter capacity required, and an existing supply may not support a large unit, which is worth establishing before specifying.
Before you quote
- Size from surface area and temperature rise, not volume alone.
- Recommend a cover before recommending more heater capacity.
- Confirm the gas supply and meter can carry the heater's input.
Where the rules come from
Questions
About this calculator
Why does surface area matter more than volume for heating?
Because heat leaves a pool from its surface, mainly through evaporation, rather than through the walls. A wide shallow pool loses more heat than a deep narrow one of the same capacity, which is why sizing from surface area and temperature rise is the correct approach.
Should I buy a bigger heater or a cover?
The cover, in most cases. Evaporation is the largest heat loss from a pool and a cover largely stops it, which means less heat to replace and often a smaller heater needed. Buying capacity to replace heat that a cover would have retained is the more expensive path.
Can my gas supply run a pool heater?
Not always. A pool heater has a substantial input and the existing gas line and meter may not have capacity alongside the house's other appliances. That is worth establishing before choosing a unit, because upgrading the supply can cost as much as the heater.
The formula
energy = gallons x 8.34 x temperature rise; heater = energy / hours + surface loss
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How to use this calculator
- Pool length (ft) (ft). Enter your own figure; the page starts at 32 ft.
- Pool width (ft) (ft). Enter your own figure; the page starts at 16 ft.
- Pool volume (gallons). Enter your own figure; the page starts at 20,000.
- Temperature rise wanted (F) (F). Enter your own figure; the page starts at 12 F.
- Cover. Enter your own figure; the page starts at Covered when not in use.
- Hours to reach temperature. Enter your own figure; the page starts at 24.
The result updates as you type, so you can try a figure, see what it does to heater size, and change it back. Nothing you enter is sent anywhere or saved.
A worked example
These are the figures the calculator opens with. Every number in the results column below is produced by the same calculation the page runs, so you can follow it through before putting your own figures in.
What goes in
| Input | Value |
|---|---|
| Pool length (ft) | 32 ft |
| Pool width (ft) | 16 ft |
| Pool volume (gallons) | 20,000 |
| Temperature rise wanted (F) | 12 F |
| Cover | Covered when not in use |
| Hours to reach temperature | 24 |
What comes out
| Result | Value |
|---|---|
| Heater size main result | 85,858 |
| Surface area | 512.0 |
| Energy to heat the water | 2,001,600 |
| Heat lost per hour | 2,458 |
Raising 20,000 gallons by 12 degrees in 24 hours needs about 85,858 BTU/h, allowing 2,458 BTU/h for surface losses.
How sensitive the answer is
Holding everything else steady and changing only pool length (ft), this is what happens to heater size. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Pool length (ft) (ft) | Heater size |
|---|---|
| 16 ft | 84,629 |
| 24 ft | 85,243 |
| 32 ft example above | 85,858 |
| 48 ft | 87,086 |
| 64 ft | 88,315 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Pool length (ft) (ft)
- Measured in ft, starting at 32 ft.
- Pool width (ft) (ft)
- Measured in ft, starting at 16 ft.
- Pool volume (gallons)
- A number, between 0 and 2000000, starting at 20,000.
- Temperature rise wanted (F) (F)
- Measured in F, between 0 and 50, starting at 12 F.
- Cover
- Choose from Covered when not in use, No cover.
- Hours to reach temperature
- A number, between 0 and 240, starting at 24.
What the results tell you
| Result | What it is |
|---|---|
| Heater size | The headline result. A number. |
| Surface area | A decimal figure. |
| Energy to heat the water | A number. |
| Heat lost per hour | A number. |
Keep going
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