Heater Size Calculator
Calculate the recommended heater BTU capacity for a room from its size, ceiling height, insulation quality, and climate severity.
Recommended Capacity
7,500 BTU
300 sq ft roomHow to Size a Heater in BTUs
This calculator is the heating companion to our Air Conditioner BTU Calculator. It starts from a standard heating rule of thumb of about 25 BTU per square foot of room area (assuming an 8 ft ceiling), then adjusts for ceiling height, how well-insulated the space is, and how cold your climate typically gets — the two factors that matter most for heating but don't apply the same way to cooling.
Poor insulation increases the estimate by 20% and good insulation reduces it by 15%, relative to an average, reasonably insulated room. A mild climate reduces the estimate by 20% and a cold climate increases it by 30%. Ceilings taller than 8 ft add 5% capacity per extra foot, just as with the AC calculator.
Example
A 300 sq ft room (15 ft × 20 ft) with a standard 8 ft ceiling, average insulation, and a moderate climate needs about 300 × 25 = 7,500 BTU of heating capacity — a useful starting point for sizing a space heater or a zone of a larger heating system.
Common Use Cases
- Choosing the right BTU rating for a portable or wall-mounted space heater.
- Sizing a single room or zone of a ductless heat pump system.
- Comparing how much insulation upgrades or a colder climate change your heating needs.
FAQs
How is this different from the AC BTU Calculator?
The AC BTU Calculator sizes cooling capacity using sun exposure and occupant count, since heat gain from sunlight and body heat matters most for cooling. This calculator instead sizes heating capacity using insulation quality and climate severity, since heat loss through walls, windows, and outside temperature matters most for heating. They use different inputs because heating and cooling loads are driven by different factors.
Why does insulation matter so much for heating?
Poorly insulated rooms lose heat quickly through walls, windows, and gaps, so a heater has to work harder and longer to maintain temperature. Well-insulated rooms retain heat better, letting a smaller heater keep up.
Should I round the result up or down?
Round up to the nearest common heater size (1,500, 5,000, 10,000, 15,000 BTU, and so on) rather than down — an undersized heater will run constantly without keeping the room comfortable, especially during the coldest stretches of your climate.