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How the calculation works
Pool equipment mixes unit systems more than almost anything else in a household. A pump is rated in GPM, a heater in BTU, a test kit in ppm, and the pool itself might be measured in metres while the chemicals are sold by the pound.
The conversions used here: 1 US gallon = 3.785 litres, 1 foot = 0.3048 metres, 1 pound = 0.4536 kilograms, °C = (°F − 32) × 5/9, and 1 GPM = 3.785 litres per minute.
Worked example
A 20,000 US gallon pool is 75,708 litres, or 16,653 imperial gallons. A pump rated 42 GPM moves 159 litres a minute. Water at 82°F is 27.8°C.
Assumptions
- US gallons throughout unless imperial is chosen explicitly — a US gallon is about 83% of an imperial one.
Limitations
- ppm is not converted, because it is already a ratio — 3 ppm is 3 ppm in any unit system.
Questions people ask
Are US and imperial gallons the same?
No, and confusing them is a common source of dosing errors. A US gallon is 3.785 litres; an imperial gallon is 4.546. A pool measured in imperial gallons holds about 20% more than the same number of US gallons.
Do I need to convert ppm?
No. Parts per million is a ratio — one milligram per litre — so it reads the same whatever units the pool is measured in.
What is a normal pool temperature?
78–82°F, or 26–28°C, suits most swimmers. Competitive swimming is usually cooler; therapy pools are warmer.