BTU Calculator
Calculate the BTU needed to cool or heat a room based on square footage, ceiling height, and insulation quality. Get AC unit size in tons.
sq ft
ft
Cooling BTU/hr
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Heating BTU/hr —
AC Unit Size —
Extended More scenarios, charts & detailed breakdown ▾
sq ft
ft
Cooling BTU/hr
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AC Unit Size —
Est. kWh/yr (SEER 16) —
Professional Full parameters & maximum detail ▾
sq ft
ft
BTU
%
ft
Load Breakdown
Base Load (BTU/hr) —
Occupant Load (BTU/hr) —
Window Load (BTU/hr) —
Equipment Load (BTU/hr) —
Subtotal (BTU/hr) —
Final Sizing
Total w/ Duct Loss & Altitude —
Recommended Unit Size —
Unit Size w/ 15% Buffer —
How to Use This Calculator
- Enter the room square footage (length × width).
- Enter the ceiling height in feet.
- Select insulation quality to refine the estimate.
- Results show cooling BTU, heating BTU, and recommended AC unit size in tons.
Formula
Base BTU = Area (sq ft) × BTU factor
BTU factor by insulation: Poor = 25, Average = 20, Good = 17, Excellent = 14 BTU/sq ft (cooling). Ceiling adjustment: multiply by (ceilingHeight / 8). Heating BTU ≈ cooling BTU × 1.25.
Example
Example: 300 sq ft room, 9 ft ceilings, average insulation → Cooling: 6,750 BTU (0.56 tons) → Heating: 8,438 BTU.
Frequently Asked Questions
- A common rule of thumb is 20 BTU per square foot for cooling under average conditions (8-foot ceilings, moderate insulation, moderate climate). Poor insulation, high ceilings, many windows, or hot climates require more — up to 25–30 BTU per sq ft. Well-insulated modern homes may need as little as 15 BTU per sq ft. For heating with a furnace or heat pump, typical sizing is 30–60 BTU per sq ft depending on climate zone (use 30 in mild climates, 60 in very cold climates). This calculator lets you adjust for ceiling height, insulation quality, and climate to refine the estimate.
- One ton of cooling capacity equals 12,000 BTU per hour (BTU/hr). The term originates from the amount of heat needed to melt one ton of ice in 24 hours. A 2-ton unit provides 24,000 BTU/hr, a 3-ton unit provides 36,000 BTU/hr, and so on. Residential central AC systems typically range from 1.5 to 5 tons. Mini-split units are commonly 9,000 to 24,000 BTU (0.75–2 tons). Window units range from 5,000 to 25,000 BTU (0.4–2 tons). To convert BTU to tons, divide by 12,000.
- Divide the BTU/hr rating by 12,000 to get tons. For example, 24,000 BTU/hr ÷ 12,000 = 2 tons. To go from tons to BTU/hr, multiply by 12,000. An 18,000 BTU mini-split = 18,000 ÷ 12,000 = 1.5 tons. For heating with a heat pump or furnace, the output is typically rated in BTU/hr directly (e.g., a 60,000 BTU furnace). For cooling equipment, the ton rating is more common in the HVAC industry. This calculator shows results in both BTU/hr and tons for convenience.
- Yes — ceiling height directly affects the air volume that must be cooled or heated. A room with 10-foot ceilings has 25% more volume than the same floor area with 8-foot ceilings, requiring proportionally more BTU. The standard rule of thumb (BTU per sq ft) assumes 8-foot ceilings. To correct for higher ceilings, multiply the basic BTU estimate by (actual ceiling height ÷ 8). For example, a 500 sq ft room at 8 ft = 10,000 BTU estimate; at 10 ft = 10,000 × (10÷8) = 12,500 BTU. Rooms with cathedral ceilings or open loft areas should be calculated by total cubic volume.
- No — avoid oversizing your AC unit. An oversized unit cools the air too quickly and shuts off before completing a full humidity-removal cycle (short-cycling). This leaves the air cool but damp, creating uncomfortable conditions and potential mold issues. Short-cycling also stresses the compressor, reducing equipment lifespan. A properly sized unit runs longer cycles, removes more moisture, and maintains more even temperatures. The target is for the AC to run about 15–20 minutes per cycle. Adding a 10–15% safety buffer above your calculated BTU is fine; going 25–50% over is not recommended.