BTU Calculator
Calculator
Estimate the BTU (British Thermal Unit) capacity required to effectively cool or heat a room. Based on industry-standard guidelines used by HVAC professionals. Enter your room dimensions and conditions to get a recommended BTU rating for air conditioners and heaters.
Formula
Formula
BTU = Area × 20 × ceiling factor × sun factor + (occupants × 600)
The base rule of thumb is 20 BTU per square foot for cooling. This is adjusted for: ceiling height above 8 ft (proportional volume increase), sun exposure (−10% shaded / +10% sunny), and occupancy (each extra person beyond 1 adds ~600 BTU/hr of heat load). Heating typically requires 30–35 BTU/ft² depending on climate. These are estimates — a professional Manual J calculation is required for HVAC system design.
Worked example
A 15 × 12 ft bedroom with 8 ft ceilings, medium sun, 2 occupants.
- 1.Area: 15 × 12 = 180 ft²
- 2.Base BTU: 180 × 20 = 3,600 BTU
- 3.Ceiling factor (8 ft standard): × 1.0
- 4.Sun exposure (medium): × 1.0
- 5.Occupancy: + (2 − 1) × 600 = + 600 BTU
- 6.Total cooling: 3,600 + 600 = 4,200 BTU/hr
- 7.Tons: 4,200 ÷ 12,000 = 0.35 tons
A 5,000 BTU/hr window unit is a good minimum for this room.
Assumptions and limitations
- Based on ASHRAE/Energy Star rule-of-thumb guidelines, not a full Manual J calculation.
- Standard ceiling height is 8 ft; taller rooms increase BTU proportionally.
- Does not account for insulation quality, climate zone, window size, or humidity.
- For rooms over 400 ft² or open-plan spaces, consult an HVAC professional.
- 1 ton of cooling = 12,000 BTU/hr.