BTU Calculator
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Rough BTU/h and tons from floor area — planning estimate only.
Rates last reviewed: July 2026
Room area
Cooling load
10,000 BTU/h
0.83 tons · 400 sq ft
- Tons of cooling
- 0.83
- Area (m²)
- 37.2
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Cooling load rule of thumb
HVAC contractors use detailed Manual J calculations for proper sizing. A common rule of thumb for quick estimates is BTU/h ≈ floor area × factor — often 20–25 BTU per square foot for moderate climates, higher for sunny rooms or poor insulation.
One ton of cooling = 12,000 BTU/h. Oversized units short-cycle and dehumidify poorly; undersized units run constantly. This tool is for ballpark planning — not code compliance or equipment selection.
Example: 400 sq ft × 25 BTU/sq ft = 10,000 BTU/h (~0.83 tons). A sunny, vaulted room may need a higher factor; a shaded basement may need less.
Official sources
Rates and formulas in this calculator reference the documentation below. Confirm current numbers on the source site before relying on them. Links do not imply endorsement by those organizations of Calcometry or this tool.
Common questions
Is this a Manual J calculation?
No. Manual J accounts for insulation, windows, orientation, and infiltration. Use this only as a rough starting point.
What factor should I use?
Try 20 for shaded or mild climates, 25 for average rooms, 30+ for hot climates or high solar gain. Adjust for ceiling height separately if well above 8 ft.