BTU Calculator
Size heating and cooling BTUs for a room's volume, climate and insulation quality.
Inputs
Each additional person adds ~600 BTU/h cooling load.
Cooling Load
5,250BTU/h
Heating Load
5,063BTU/h
Suggested AC Tonnage
0.5ton
Next standard unit size at or above the calculated requirement.
Exact Tonnage Required
0.44ton
Room Area
180sq ft
Room Volume
1,620cu ft
Step by step
Room area
15 ft × 12 ft
= 180 sq ft
Ceiling height factor
9 ft ÷ 8 ft
= 1.125
Cooling base load
180 sq ft × 20 BTU/sq ft
= 3600 BTU
Apply climate × insulation × sun × height factors
× 1 × 1 × 1 × 1.125
= 4050 BTU
Occupant load (cooling)
2 × 600 BTU
= 1200 BTU
Total cooling BTU
= 5,250 BTU/h
AC tonnage required
5,250 ÷ 12,000
= 0.44 tons → suggest 0.5 ton unit
How it works
Heating and cooling loads are calculated from the room's floor area using the industry rule-of-thumb of 20 BTU/sq ft for cooling and 25 BTU/sq ft for heating. Three multipliers adjust for climate severity, insulation quality, and sun exposure. Rooms taller than 8 feet have a proportionally higher load. Each occupant adds approximately 600 BTU/h to the cooling load. The result is converted to AC tonnage (12,000 BTU/h = 1 ton) and the next available standard unit size is recommended.
Formulas
Cooling BTU
Cooling BTU = area × 20 × climate × insulation × sun × (height/8) + occupants × 600
Tonnage
Tonnage = Cooling BTU ÷ 12,000
Frequently Asked Questions
What is a BTU?
BTU stands for British Thermal Unit — the amount of heat needed to raise one pound of water by 1°F. HVAC systems are rated in BTU/h (how many BTUs they move per hour).
What is an AC ton?
1 ton of air conditioning = 12,000 BTU/h. The term originates from the cooling power of melting one ton of ice in 24 hours.
Is this calculation accurate enough to buy a unit?
The rule-of-thumb method is sufficient for single-room sizing. For a whole house, a licensed HVAC contractor should perform a Manual J calculation that accounts for local design temperatures, window areas, duct losses and other factors.
Should I size up or down when the result falls between standard sizes?
Always round up for cooling — an undersized unit runs constantly and never achieves setpoint. For heating, rounding up is also safer. The calculator suggests the next standard size above the calculated load.