Miscellaneous
BTU Calculator - Room Air Conditioning & Heating Needs
Calculate BTUs needed to heat or cool a room based on dimensions, sun exposure, and occupancy.
What is a BTU?
A British Thermal Unit (BTU) is the amount of energy needed to raise 1 pound of water by 1°F. In HVAC, BTUs measure the cooling or heating capacity of air conditioners and furnaces.
Room sizing guide
| Room size | BTU needed (cooling) |
|---|---|
| 150–250 sq ft | 6,000 BTU |
| 250–350 sq ft | 8,000 BTU |
| 350–450 sq ft | 10,000 BTU |
| 450–550 sq ft | 12,000 BTU |
| 700–1,000 sq ft | 18,000 BTU |
| 1,000–1,200 sq ft | 21,000 BTU |
Adjustments
- Add 10% for sunny rooms; reduce 10% for heavily shaded rooms
- Add 600 BTU per person beyond two regular occupants
- Kitchen: add 4,000 BTU for cooking heat
Heating BTU guide
The table above covers cooling; heating requirements are different. As a general starting point, heating typically requires 10–20 BTU per square foot depending on climate and insulation level, compared to 20–30 BTU/sq ft for cooling.
| Climate zone | BTU per sq ft (heating) |
|---|---|
| Mild (Southern US, Zone 1–2) | 10–15 BTU/sq ft |
| Moderate (Mid-Atlantic, Zone 3–4) | 15–20 BTU/sq ft |
| Cold (Midwest, Northeast, Zone 5–6) | 20–30 BTU/sq ft |
| Very cold (Upper Midwest, Zone 7+) | 30–40 BTU/sq ft |
EER and SEER
BTU capacity tells you how powerful a unit is; efficiency ratings tell you how much electricity it uses to produce that output.
- EER (Energy Efficiency Ratio): BTU output divided by watts consumed at a single test condition. Higher is more efficient. A window unit with EER 12 uses less power than one with EER 8 to cool the same space.
- SEER (Seasonal Energy Efficiency Ratio): similar to EER but measured over a range of temperatures representing a full cooling season. Central air conditioners are rated by SEER. The US federal minimum is SEER 14 (2023+). High-efficiency units reach SEER 20–25.
HVAC sizing warning
The BTU tables here are rough estimates for single-room window or portable units. For a whole-home central HVAC system, a proper Manual J calculation performed by a licensed HVAC professional is the correct sizing method. Manual J accounts for local climate data, insulation values, window area and orientation, ceiling height, infiltration rates, and internal heat gains. Oversized systems short-cycle (turn off before fully dehumidifying), while undersized systems run continuously and cannot maintain setpoint on peak days.