How it works
Heating equipment is rated by how much heat it delivers per hour. In the US and Canada that is BTU per hour (BTU/hr); in Australia and most other countries it is kilowatts. This calculator uses a widely used rule of thumb: a number of BTU per square foot that rises as winters get colder, adjusted for ceiling height and insulation.
BTU/hr = floor area (sq ft) × BTU per sq ft for your climate × (ceiling height ÷ 8 ft) × insulation factor
kW = BTU/hr ÷ 3,412
The ceiling adjustment matters because you heat air, not floor. A room with 10 ft ceilings holds 25 percent more air than the same room with 8 ft ceilings.
Worked example
A 300 sq ft family room with 8 ft ceilings, average insulation, in a cold climate: 300 × 50 × (8 ÷ 8) × 1.0 = 15,000 BTU/hr, or 4.4 kW. In metric terms that is about 158 watts per square meter of floor.
Give the same room 10 ft ceilings and poor insulation and the estimate climbs to 300 × 50 × 1.25 × 1.3 = 24,375, shown as 24,400 BTU/hr (7.14 kW). Insulation and ceiling height can change the answer by more than half.
BTU per square foot by climate
| Climate | BTU/hr per sq ft | W per m² |
|---|---|---|
| Hot (mild winters) | 30 | 95 |
| Warm | 35 | 110 |
| Moderate | 40 | 126 |
| Cold | 50 | 158 |
| Very cold | 60 | 189 |
These figures assume 8 ft (about 2.4 m) ceilings and average insulation. They are starting points used across the trade for quick estimates, not engineering values, and they tend to run high: a proper heat-loss calculation for a newer or upgraded home often comes in well below them.
Quick reference: moderate climate
| Floor area | BTU/hr | kW |
|---|---|---|
| 150 sq ft (14 m²) | 6,000 | 1.76 |
| 300 sq ft (28 m²) | 12,000 | 3.52 |
| 500 sq ft (46 m²) | 20,000 | 5.86 |
| 1,000 sq ft (93 m²) | 40,000 | 11.72 |
Turning BTU into an electric heater size
Electric resistance heat turns every watt into heat: 1 watt equals 3.412 BTU/hr. A common 1,500 W portable heater delivers about 5,118 BTU/hr. The 15,000 BTU/hr room in the example would need about 2.9 of those heaters running flat out on the coldest night, which is why a single plug-in heater rarely heats a large room in a cold climate. To see what that heat costs, use the space heater cost calculator.
When the rule of thumb is not enough
- Whole-house equipment. Furnaces, boilers and heat pumps should be sized with a room-by-room heat-loss calculation. In the US this is ACCA Manual J. Oversized equipment short-cycles, wears faster and heats unevenly.
- Lots of glass. Large windows, skylights and glass doors lose heat much faster than insulated walls. Choose the poor insulation setting or add a margin.
- Rooms over unheated space. A room above a garage or a crawlspace loses more heat through the floor.
- Basements. Below-grade walls lose less heat than exposed walls, so a finished basement often needs less than the estimate.
Practical tips
- Fix air leaks and add insulation before buying a bigger heater. Lowering the heat loss is permanent; extra capacity is not. The insulation calculator shows how deep to go.
- Heaters are sold in steps. Choose the next size up from the estimate, but avoid going far beyond it.
- Gas and propane heaters are rated by input and output. Compare the output figure with this estimate.
Safety and code
Fuel-burning heaters, furnaces and boilers need correct venting and combustion air. Install carbon monoxide alarms on every level of the home and near sleeping areas. Unvented heaters are restricted or banned in some places, so check local code. Hard-wired electric heaters need a circuit sized for their load; have a licensed electrician install them.
Frequently asked questions
How many BTU do I need to heat 1,000 square feet?
With the rule of thumb used here and 8 ft ceilings with average insulation, about 40,000 BTU/hr in a moderate climate and 50,000 BTU/hr in a cold climate. Insulation and windows can move that up or down considerably.
How do I convert BTU to kW?
Divide BTU/hr by 3,412. For example, 15,000 BTU/hr is about 4.4 kW. To go the other way, multiply kW by 3,412.
How many watts of electric heat per square foot?
A common rule of thumb is 10 watts per square foot, which equals about 34 BTU/hr per square foot. Colder climates and poorly insulated rooms need more.
Can I use this to size a furnace?
Use it for a ballpark only. Whole-house heating equipment should be sized from a room-by-room heat-loss calculation such as Manual J.