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Water volume.
The same water as the turnover calculation, for a completely different reason — heating is about mass and about losses, not circulation. Volume decides how long the pool takes to warm up from cold. What it costs to KEEP warm is decided mostly by the surface, which is why a large shallow pool is harder to heat than a small deep one holding the same water, and why a cover changes the running cost more than the heater does.
How far the water must be raised.
From current water temperature to target. Eight degrees is a typical early-season lift from 20 °C to 28 °C.
How quickly you want to reach temperature.
A shorter heat-up needs a larger heater that is then oversized for holding temperature. Twenty-four to forty-eight hours is a reasonable target for a domestic pool at the start of the season.
A cover suppresses evaporation, which is most of the loss.
Evaporation carries 50-70% of an outdoor pool's heat loss. A cover is the single most effective intervention available and costs a fraction of the heater.
Heater output required
23 kW
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
- 465.16 kWh
- Heat-up power
- 19.38 kW
- Standing loss allowance
- 3.33 kW
- BTU/h
- 77,508.05 BTU/h
- Approximate surface area used
- 358.83 sq ft
They open the calculator with your figures already in it
Pool Heater Sizing Calculator: 22.72 kW — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- 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
Inputs used
- Pool volume
- 13210 gal
- Temperature rise
- 14.4 °F
- Heat-up time (hours)
- 24
- Cover used overnight
- Yes — cover on when not in use
Intermediate steps
- Energy to raise the water
- 465.16 kWh
- Heat-up power
- 19.38 kW
- Standing loss allowance
- 3.33 kW
- BTU/h
- 77,508.05 BTU/h
- Approximate surface area used
- 358.83 sq ft
Confidence note: 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.
What this calculation does not cover
- Surface area is inferred from volume at an assumed 1.5 m (5 ft) average depth. A wide shallow pool loses far more heat than this suggests, and a deep narrow one less.
- Wind is the largest uncontrolled variable in evaporation and is not modelled. An exposed pool can lose twice what a sheltered one does.
- Solar gain is ignored, and on a sunny day it can exceed the heater's output entirely.
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This result is a specification — 23 kW — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-08-26 · in the site-wide review of 2026-09-06 · v1.0.0
Regulatory standards & verification citations2
- 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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