How it works
A walkthrough, end to end.
- 1
Enter the room's square footage and ceiling height.
- 2
Pick the climate zone (mild, moderate, hot/cold) and load type (cooling or heating).
- 3
See the recommended BTU/h capacity, with sun and occupancy adjustments.
Reference
BTU sizing
Base = sq ft × climate factor × (ceiling/8). +600 BTU per extra person, +10% for very sunny rooms.
Use cases
What you can do with this.
Window AC
Pick the right window unit.
Mini-split
Size each indoor head correctly.
Space heater
Match heater output to room size.
Whole-home HVAC
Sum room BTUs for total system size.
Garage / shop heat
Estimate uninsulated zone BTU needs.
FAQ
Frequently asked.
Both use similar base sizing; the calculator adjusts factor by load type and climate.
Older / poorly insulated rooms need 20–30% more capacity.
No. All math runs in your browser.
1 ton AC = 12,000 BTU/h. Output also shown in tons.