Heating BTU calculator for a room, garage or shop
The short answer
A heater has to put out as much heat as the space loses on a cold night. This calculator adds up the heat lost through the walls, ceiling, windows and air leaks in BTU per hour. Divide by 12,000 for a mini split's size in tons.
Heating BTU calculator
This BTU calculator works out how much heat a room, garage or shop loses on a cold night, and the mini split size that matches it.
Your numbers
Your answer
About 2,800 BTU per hour
This room loses about 2,800 BTU per hour when it is 0°F outside and 68°F inside.
To keep up, a heater needs to put out at least that much heat. For a mini split or heat pump, that is about 0.2 tons of capacity at 0°F.
- Heat loss2,800 BTU/hr
- Mini split or heat pump0.2 tons
Where it goes: walls 900, ceiling 400, windows and doors 600, air leaks 800 BTU per hour.
At a glance
- January average low
- 23.7°FBonners Ferry, NOAA
- Nights below 0°F
- About 3 a yearBonners Ferry, NOAA
- One ton of heat pump
- 12,000 BTU per hour
- Sizing standard
- Manual J load calculationACCA
How the heat loss calculator works
Heat leaks out of a building four ways: through the walls, the ceiling, the windows and doors, and with air that leaks in and out. The calculator adds them up.
- Temperature difference: the temperature you want inside minus the outside temperature you plan for.
- Walls: outside wall length x ceiling height, minus the windows and doors. Divide by the wall R-value and multiply by the temperature difference.
- Ceiling, if it sits under a cold attic or roof: length x width, divided by the ceiling R-value, times the temperature difference.
- Windows and doors: their area x the U-factor x the temperature difference.
- Air leaks: length x width x height x air changes per hour, divided by 60, gives cubic feet per minute. Multiply that by 1.1 and by the temperature difference.
- Add the four. That is the heat loss in BTU per hour. Divide by 12,000 to get tons.
What it assumes
- The starting numbers are our guesses, not facts about your building. They are R-13 walls, R-30 ceiling, a window U-factor of 0.30, and 0.5 air changes an hour for a room or 1 for a garage or shop. Put in your own.
- Heat lost through the floor is left out. So is the heat it takes to warm a cold shop back up after the big door has been open.
- Heat from people, lights, the sun and appliances is not counted.
- It is a quick estimate, not a Manual J load calculation.
How cold to plan for in Bonners Ferry
A heater is sized for cold nights, not average ones. The calculator starts at 0°F. You can plan for colder if you want room to spare in a cold snap.
BTU calculator for a garage or shop: a Bonners Ferry example
Say you have a 24 by 32 foot shop near Bonners Ferry with 10-foot walls, R-13 in the walls and R-30 in the ceiling. No windows, about 1 air change an hour, and you want it at 55°F on a 0°F night.
- Temperature difference: 55 - 0 = 55.
- Walls: 112 feet of wall x 10 feet = 1,120 square feet. 1,120 / 13 x 55 = 4,738 BTU per hour.
- Ceiling: 24 x 32 = 768 square feet. 768 / 30 x 55 = 1,408 BTU per hour.
- Air leaks: 24 x 32 x 10 = 7,680 cubic feet x 1 / 60 = 128 cubic feet per minute. 1.1 x 128 x 55 = 7,744 BTU per hour.
- Total: 4,738 + 1,408 + 7,744 = about 13,900 BTU per hour, or about 1.2 tons.
For a room, the calculator starts with a 12 by 14 foot corner bedroom under the attic. It has 26 feet of outside wall and 30 square feet of window. At 68°F inside and 0°F outside, that comes to about 2,800 BTU per hour.
Mini split size calculator: BTU to tons
Mini splits and heat pumps are sold by BTU per hour or by tons. One ton is 12,000 BTU per hour, so divide your heat loss by 12,000.
| BTU per hour | Tons |
|---|---|
| 9,000 | 0.75 |
| 12,000 | 1 |
| 18,000 | 1.5 |
| 24,000 | 2 |
| 36,000 | 3 |
| 48,000 | 4 |
Check that the unit you pick is rated to put out that much heat at the outside temperature you plan for, not just at its size on the box.
Facts this tool uses
| Fact | Figure | Source |
|---|---|---|
| Heat through walls, ceiling and windows | U x area x temperature difference, with U = 1 / R | University of Oregon, building science equations (Infiltration: Blower Door), citing ASHRAE FundamentalsConduction and U = 1/R lines checked Oct 28, 2026 |
| Heat lost to air leaks | 1.1 x cubic feet per minute x temperature difference | University of Oregon, building science equations (Infiltration: Blower Door), citing ASHRAE FundamentalsInfiltration heat loss equation checked Oct 28, 2026 |
| One ton of capacity | 12,000 BTU per hour | ACCA, A Little History Behind "Tons" of Air Conditioner CapacityTon definition checked Oct 28, 2026 |
| Bonners Ferry, January average low | 23.7°F | NOAA NCEI, U.S. Climate Normals 1991-2020, Bonners Ferry station (monthly)January minimum temperature normal checked Oct 28, 2026 |
| Bonners Ferry, cold nights a year | Low below 10°F on about 10.4 days; below 0°F on about 3.0 days | NOAA NCEI, U.S. Climate Normals 1991-2020, Bonners Ferry station (annual)Annual days below 10°F and 0°F checked Oct 28, 2026 |
| Bonners Ferry, heating degree days | 6,674 a year (base 65°F) | NOAA NCEI, U.S. Climate Normals 1991-2020, Bonners Ferry station (annual)Annual heating degree days checked Oct 28, 2026 |
| Manual J | The national ANSI-recognized standard for HVAC equipment sizing loads | ACCA, Manual JManual J description checked Oct 28, 2026 |
Next step
Ready to put in a heater or heat pump? Furnace repair and heating businesses in Bonners Ferry