BTU Calculator
This btu calculator estimates cooling or heating BTUs from room square footage, then adjusts for climate, sun, insulation, and ceiling height.
How to use
- Choose cooling or heating, then enter length and width or switch to square feet.
- Set climate, sun, and insulation. Average options keep the plain 20 BTU per sq ft cooling baseline.
- Enter ceiling height in feet. Eight feet is the baseline, so a taller ceiling raises the estimate.
- Read estimated BTUs and tons. Clear empties the room size and ceiling height.
Formula
Cooling starts at 20 BTU per sq ft. Heating starts at 30 BTU per sq ft. BTU = area × baseline × climate × sun × insulation × (ceiling height ÷ 8). One ton is 12,000 BTU per hour. Cooling climate factors are 0.85 cool, 1 moderate, 1.2 hot, and 1.4 very hot. Heating climate factors are 0.8 mild, 1 moderate, 1.5 cold, and 2 very cold. Sun factors for cooling are 0.9 shaded, 1 average, 1.1 sunny, and 1.25 very sunny. Heating sun factors are 1.1 shaded, 1 average, 0.95 sunny, and 0.9 very sunny, because winter sun lowers the heating load. Insulation factors are 1.3 poor, 1 average, 0.9 good, and 0.8 excellent.
Example. A 12 ft by 20 ft room is 240 sq ft. Cooling with a moderate climate, average sun, average insulation, and an 8 ft ceiling is 240 × 20 = 4,800 BTU, or 0.4 tons.
Example. A 15 ft by 20 ft room is 300 sq ft. Cooling, hot (1.2), sunny (1.1), poor insulation (1.3), and an 8 ft ceiling is 300 × 20 × 1.2 × 1.1 × 1.3 = 10,296 BTU, or 0.86 tons.
Example. The 12 ft by 20 ft room in heating mode with a cold climate (1.5), average sun, average insulation, and an 8 ft ceiling is 240 × 30 × 1.5 = 10,800 BTU.
Not a Manual J calculation
Manual J uses room-by-room building loads: windows, orientation, infiltration, and the rest of the house. This page is a quick estimate from square footage and a few multipliers. Hire an HVAC pro before you buy a whole-home system. A slightly undersized air conditioner that runs longer often dehumidifies better than a large unit that short-cycles.
Frequently asked questions
How many BTUs per square foot?
A common rule of thumb is roughly 20 BTU per sq ft for cooling, then adjust for climate and sun.
What is a BTU?
British Thermal Unit — a measure of heat energy. Higher BTU ratings move more heat.
Cooling vs heating BTUs?
Use the cooling mode for AC sizing and heating mode for space heaters or furnace ballparks.
Should I size up?
Slightly undersized with good runtime often dehumidifies better than a large short-cycling unit.
Is this a Manual J calculation?
No. Manual J uses detailed building loads. This is a quick estimate only.
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