Work out how many BTUs a room needs to stay comfortable in summer and winter. Enter the room size and ceiling height, pick your climate, insulation, and sun exposure, and the calculator returns the cooling load, the heating load range, the window air conditioner size that fits, and the wattage of electric heat you would need — all from published rules of thumb, including the ENERGY STAR room AC sizing chart.
Sizing a whole house instead of one room? The AC tonnage calculator converts the same method into tons of central air. Upgrading the attic first will shrink both numbers — the insulation calculator shows the R-value your zone calls for, and the propane usage calculator turns a furnace's BTU input into gallons per hour. Our guide to how long a propane tank lasts covers how BTU ratings turn into runtime, and why a furnace's input rating isn't its output.
Frequently asked questions
How many BTUs do I need per square foot?
For cooling, the common rule of thumb is about 20 BTU per square foot in a moderate climate, rising to roughly 25 in warm regions and 30 in hot ones, and dropping to about 17 in cold ones. Heating needs more: roughly 30–35 BTU per square foot in hot climates, 40–50 in moderate ones, and 50–60 in cold ones for a home with average insulation. Those figures assume 8-foot ceilings, so this calculator scales them by your actual ceiling height, then adjusts for insulation, sun, people, and kitchen heat. Treat the result as a starting point for shopping, not an engineering answer.
What size window air conditioner do I need?
Take the cooling BTU from the result and pick the next standard size up — window units commonly come in 5,000, 6,000, 8,000, 10,000, 12,000, 14,000, 15,000, 18,000, 24,000, and 25,000 BTU. The ENERGY STAR chart shown under the calculator is a useful cross-check: it suggests 6,000 BTU for 150–250 sq ft, 12,000 for 450–550 sq ft, and so on, then adds 10% for a very sunny room, 600 BTU for each regular occupant beyond two, and 4,000 BTU for a kitchen. Resist the urge to oversize: a unit that is too big cools the air fast but leaves the room damp.
How do I convert BTU to watts for a space heater?
Divide BTU per hour by 3.412 to get watts. A room that needs 8,000 BTU/h of heat needs about 2,345 W of electric resistance heat. A standard 120-volt plug-in heater is capped at 1,500 W, which is about 5,118 BTU/h, so the calculator also tells you how many 1,500 W heaters it would take. If the answer is more than one, each usually needs its own circuit — or consider a hardwired 240 V heater, a heat pump, or a mini-split instead of stringing heaters together.
Why does the calculator say Manual J is the real method?
ACCA Manual J is the industry-standard load calculation that HVAC designers and many building codes require. It works room by room from the actual wall and ceiling insulation, window size, type and orientation, air leakage, duct location, and local design temperatures. BTU-per-square-foot rules ignore most of that, and studies of real homes show they tend to oversize equipment. This calculator is perfect for picking a window unit or space heater; for a furnace, central AC, or heat pump, ask your contractor for a Manual J report.
Does ceiling height really matter?
Yes. Heating and cooling rules of thumb, including the ENERGY STAR chart, assume standard 8-foot ceilings. A room with 10-foot ceilings has 25% more air and more wall area exposed to the outside, so this calculator multiplies the load by your ceiling height divided by 8. Vaulted and cathedral ceilings also collect hot air up high, so in a room like that, lean toward the upper end of the heating range and consider a ceiling fan to push warm air back down in winter.