Common garage sizes
What Size Heater for a 2-Car Garage?
A two-car garage needs roughly 17,000 to 24,500 BTU/hr with average insulation and a 40 F temperature rise, depending on whether it is a compact 20 x 20 or a full 24 x 24. "Two-car" is the least precise label in garage sizing, so start from your real dimensions.
Size your two-car garage| Scenario | Dimensions | Insulation | Temp rise | Required output | Standard unit size |
|---|---|---|---|---|---|
| 20 x 20, mild winter | 20 x 20 x 8 ft | Average | 30 F | 13,000 BTU/hr | 15,000 BTU/hr about 3,800 W electric |
| 20 x 20, cold winter | 20 x 20 x 8 ft | Average | 40 F | 17,000 BTU/hr | 20,000 BTU/hr about 5,000 W electric |
| 20 x 20, very cold winter | 20 x 20 x 8 ft | Average | 50 F | 21,500 BTU/hr | 25,000 BTU/hr about 6,300 W electric |
| 24 x 24, cold winter | 24 x 24 x 8 ft | Average | 40 F | 24,500 BTU/hr | 25,000 BTU/hr about 7,200 W electric |
| 24 x 24 well insulated, cold winter | 24 x 24 x 8 ft | Good | 40 F | 18,500 BTU/hr | 20,000 BTU/hr about 5,400 W electric |
All rows use the same formula as the calculator. "Cold winter" means a 40 F rise, for example 20 F outside warmed to 60 F inside.
Why 20 x 20 and 24 x 24 need different heaters
Adding four feet to both sides sounds minor, but 24 x 24 x 8 holds 4,608 cubic feet against 3,200 for a 20 x 20: 44% more air to heat. That is the difference between a 20,000 and a 25,000 BTU/hr unit. Tandem and storage-heavy layouts stretch further still, which is why a chart alone can put you a full size off.
The electric ceiling sits in two-car territory
A 24 x 24 garage at a 40 F rise needs about 7,200 watts, which still fits the largest common residential 240V heaters (7,500 to 10,000 W on a dedicated 40 to 50 A circuit). Push the conditions harder, poor insulation, a 50 F rise, or a bigger footprint, and the requirement passes 10,000 W. At that point a natural gas or propane unit heater usually wins on running cost and wiring, or you can insulate first and shrink the load back into electric range.
Garage doors are the weak point
Two overhead doors can be a quarter of the wall area. Uninsulated steel doors with tired seals leak heat faster than any wall, so judge the insulation setting by the whole envelope, doors included. Upgrading door insulation and bottom seals is often cheaper than the next heater size up, and it improves every winter after.
Size for how you actually use the space
A daily workshop held at 60 F all winter and a garage warmed to 50 F for weekend projects are different jobs. Enter the indoor temperature you truly want: every 10 F of extra rise adds roughly a quarter to the load at typical winter temperatures. If you only need short sessions, a slightly larger unit also shortens warm-up time from cold.