BTU Calculator
Size heating and cooling in BTUs for a room's dimensions, climate and insulation.
About BTU Calculator
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
Worked example
With the default values below, BTU Calculator returns Cooling Load of 5,250. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Room Lengthminimum 1 ft
- 15 ft
- Room Widthminimum 1 ft
- 12 ft
- Ceiling Heightaccepts 6–20 ft
- 9 ft
- Climate Zoneone of Hot & Humid (e.g. Gulf Coast, Florida), Hot & Dry (e.g. Arizona, Nevada), Moderate (e.g. mid-Atlantic, Pacific NW), Cold (e.g. Minnesota, Upper Midwest), Very Cold (e.g. Alaska, Canada border)
- Moderate (e.g. mid-Atlantic, Pacific NW)
- Insulation Qualityone of Excellent (new construction, R-20+ walls), Average (typical residential), Poor (older home, minimal insulation)
- Average (typical residential)
- Sun Exposureone of Shaded (north-facing or heavily shaded), Average, Very sunny (south/west-facing, skylights)
- Average
- Number of Regular Occupantsaccepts 0–20
- 2
Results
- Cooling Load
- 5,250 BTU/h
- Heating Load
- 5,063 BTU/h
- Suggested AC TonnageNext standard unit size at or above the calculated requirement.
- 0.5 ton
- Exact Tonnage Required
- 0.44 ton
- Room Area
- 180 sq ft
- Room Volume
- 1,620 cu ft
Step by step
- 1Room area= 180 sq ft
- 2Ceiling height factor= 1.125
- 3Cooling base load= 3600 BTU
- 4Apply climate × insulation × sun × height factors= 4050 BTU
- 5Occupant load (cooling)= 1200 BTU
- 6Total cooling BTU= 5,250 BTU/h
- 7AC tonnage required= 0.44 tons → suggest 0.5 ton unit
Assumptions and limits
- The result is derived only from the 7 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax or compounding frequency into account.
- Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
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.