BTU Calculator
Calculate required BTU for heating and cooling a room based on dimensions, insulation, climate zone, windows, sun exposure, and occupants.
Room profile
Updates as you typeRecommended AC units
Closest standard sizes for your room| Unit size | Fit | Headroom vs. required | Match |
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Oversized AC units short-cycle — cooling the air without removing humidity, which leaves the room cold and clammy. Aim for a unit within ±10% of the calculated cooling BTU for best comfort and efficiency.
Monthly running cost
Based on your electricity rate, SEER & runtimePower draw = cooling BTU ÷ SEER. Actual bills depend on weather, thermostat setpoint, and utility demand charges — treat this as a ballpark figure.
Formula
- A
- Room area in square feet (length × width)
- h
- Ceiling height factor (actual height ÷ 8 ft standard)
- i
- Insulation multiplier (0.70 excellent → 1.30 poor)
- c
- Climate multiplier (varies for heating vs. cooling)
- s
- Sun exposure multiplier (0.90 → 1.10 depending on orientation)
- W
- Number of windows; each adds ~1,000 BTU of thermal leak
- P
- Extra occupants beyond the base 2; each adds ~600 BTU
- Area A = —
- Height factor h = —
- Base × h × i × c × s = —
- Windows contribution = —
- Occupants contribution = —
- Cooling BTU/h = —
The 20 BTU / sq ft baseline is the rule-of-thumb used by Energy Star for cooling. Real-world HVAC sizing (Manual J) accounts for window orientation, air infiltration, duct losses, and internal gains from appliances — so treat this as a first-pass estimate, not a replacement for a contractor's load calculation.