Pool Heater Size Calculator (BTU)

How pool heater BTU sizing is calculated

The energy needed to heat water follows a simple physical relationship: it takes 8.34 BTU to raise one gallon of water by 1°F (since a gallon of water weighs 8.34 lbs and water's specific heat is 1 BTU per pound per degree). The formula here is BTU/hr = gallons × 8.34 × desired temperature rise ÷ hours to heat.

A 20,000 gallon pool being heated 20°F over 24 hours needs about 139,000 BTU/hr -- close to a common 150,000-400,000 BTU residential gas or heat-pump pool heater rating, which illustrates why real heaters are usually sized above the bare theoretical minimum.

This formula only accounts for heating the water mass itself -- it doesn't include ongoing heat loss to evaporation, wind, and cooler night air, which a pool loses continuously while heating (and while just sitting at temperature). That's why manufacturer sizing charts, which factor in regional climate, pool surface area and whether you use a pool cover, typically recommend a heater somewhat larger than this raw calculation.

Using a pool cover dramatically reduces both the time and the ongoing energy needed to reach and hold a target temperature, since evaporation is the single largest source of heat loss from an outdoor pool.

Where these numbers come from

The formulas and default values in this calculator are based on:

• standard water-heating physics (1 gallon of water = 8.34 lbs
• 1 BTU raises 1 lb of water by 1°F), the basis for BTU sizing formulas published by pool heater manufacturers
• manufacturer sizing charts additionally account for surface heat loss, climate and pool covers

These are planning estimates. Test your actual water, follow product labels, and check your equipment manufacturer's guidance before you buy or dose chemicals.