BTU Calculator — HVAC Sizing
Determine the right BTU capacity for heating or cooling any room. This calculator factors in room size, ceiling height, insulation quality, climate zone, sun exposure, and occupancy to give you an accurate HVAC sizing estimate.
Room Details
Full Breakdown
Room Area
Room Volume
Base BTU
Ceiling Adjustment
Insulation Factor
Climate Factor
Sun Adjustment
Occupant Load
Recommended BTU
Tonnage (AC)
How to Use This Calculator
- Enter the room length and width in feet.
- Set the ceiling height (8 ft is standard).
- Choose your insulation quality, climate zone, and sun exposure.
- Enter the typical number of occupants.
- Select heating or cooling mode.
- Click "Calculate BTU" for your recommendation.
The Formula
Base BTU = Room Sq Ft × 20 (cooling) or × 30 (heating)
Ceiling Adj = +12.5% per foot above 8 ft
Insulation Factor: Poor ×1.3, Average ×1.0, Good ×0.8
Climate Factor: Hot ×1.2, Warm ×1.1, Moderate ×1.0, Cold ×1.1, Very Cold ×1.3
Sun: Shaded −10%, Average ×1.0, Sunny +10%
Occupants: +600 BTU per person beyond 2
1 Ton AC = 12,000 BTU
Ceiling Adj = +12.5% per foot above 8 ft
Insulation Factor: Poor ×1.3, Average ×1.0, Good ×0.8
Climate Factor: Hot ×1.2, Warm ×1.1, Moderate ×1.0, Cold ×1.1, Very Cold ×1.3
Sun: Shaded −10%, Average ×1.0, Sunny +10%
Occupants: +600 BTU per person beyond 2
1 Ton AC = 12,000 BTU
This uses the ACCA Manual J simplified method. The 20 BTU/sq ft cooling baseline is industry-standard for moderate climates. For a precise load calculation, consult an HVAC professional.