HVAC BTU Calculator
Rough-size heating and cooling load in BTU/hr from room dimensions and adjustment factors. Starting point only — not a Manual J replacement.
Assumptions
- Default 20 BTU/ft² for moderate mixed climates.
- Single zone, standard ceiling height near 8 ft.
- Adjustment factors are user-selected multipliers.
- Does not replace ACCA Manual J or professional load calculations.
- Infiltration, duct losses, and appliance loads are not modeled.
Base BTU = floor area (ft²) × BTU per ft². Adjusted BTU = base × climate × insulation × sun factors. Tonnage = BTU ÷ 12,000.20 × 15 ft room, 20 BTU/ft²
- Floor area = 300 ft². Base = 300 × 20 = 6,000 BTU/hr. At 1.0 factors = 6,000 BTU ≈ 0.5 tons. Large open plans need whole-house analysis.
How to measure
Measure room length and width at the floor. For sizing a whole floor, sum rooms or use building footprint. Note sun exposure and insulation quality honestly in the factors.
Common mistakes
- Sizing only by bedroom count for an entire house.
- Oversizing AC (short cycling) by ignoring insulation factor.
- Using this result as final equipment selection without Manual J.
Frequently asked questions
How many BTU per square foot?
Rules of thumb range 15–25 BTU/ft² depending on climate. Hot humid zones trend higher.
What is a ton of cooling?
One ton = 12,000 BTU/hr of cooling capacity.
Does ceiling height matter?
Yes for volume. This tool uses floor area with factors — high ceilings may need higher BTU/ft².
Heat pump sizing?
Heat pumps need balance of heating and cooling loads. Consult a professional for dual-mode sizing.