BTU Calculator
Get a quick rule-of-thumb BTU estimate for a room or space by square footage and climate — useful for rough sizing, not a substitute for a Manual J load calculation.
This calculator gives an estimate for planning purposes only. Verify quantities and code requirements with a qualified professional before ordering materials or performing work.
How it's calculated
Base load
BTU = square footage × climate factor
The climate factor (20–35 BTU per sq ft) is a rough multiplier for how hard a space is to condition — mild climates need less, extreme heat/cold needs more.
Adjustments
+400 BTU per occupant over 2, +1,000 per exterior door, +4,000 for a kitchen
People, doors, and cooking appliances all add heat load. Sun exposure adjusts the total up to 10% either direction for a sunny, south-facing space or a shaded one.
BTU by room size (moderate climate, 2 occupants)
| Room size | Estimated BTU/hr |
|---|---|
| 100 sq ft | 2,500 |
| 150 sq ft | 3,750 |
| 250 sq ft | 6,250 |
| 400 sq ft | 10,000 |
| 600 sq ft | 15,000 |
| 1,000 sq ft | 25,000 |
Skip the manual math on your next takeoff
Struvia reads your plans and pulls quantities like this automatically — upload a plan and see it measure a real takeoff.
BTU Calculator FAQ
Is this a Manual J load calculation?
No. This is a quick rule-of-thumb estimate for rough sizing or budgeting conversations. A real HVAC install should use a proper Manual J load calculation, which accounts for insulation R-values, window specs, air infiltration, and local design temperatures in detail.
Why do occupants and doors affect BTU need?
Every person in a space adds body heat, and exterior doors add infiltration and heat transfer every time they open. Both increase the load a system needs to handle.
What climate factor should I use?
Mild (20) suits temperate coastal climates, moderate (25) suits most of the continental US, hot (30) suits the Southwest and Deep South, and extreme (35) suits desert or very high-humidity regions with large seasonal swings.