About the Heater Size Calculator

Heaters are rated in BTU/hr (British thermal units per hour) or watts. The sizing rule of thumb runs from about 20 BTU per square foot in mild coastal climates to 55 in frigid northern regions, with ceiling height and insulation quality as multipliers. A drafty old house with 10-foot ceilings needs roughly 40% more heat than the same area in a modern well-sealed home.

Right-sizing matters

An undersized heater runs continuously and never reaches setpoint; an oversized one short-cycles, wasting energy and causing temperature swings. The common pitfalls — ignoring ceiling height, using the wrong climate factor, or forgetting insulation — all push you off by 20–30%. The calculator's climate comparison chart shows exactly how the same room's demand changes from San Diego to Minnesota, so you can see which factor dominates your number.