Air conditioners are rated in BTUs (or tons — one ton equals 12,000 BTU/hr). Sizing matters more than people expect: too small and the unit runs constantly without ever cooling the room; too big and it short-cycles, blasting cold air and shutting off before it has run long enough to pull humidity out of the air, leaving the room cold but clammy. The goal is a unit matched to the room’s actual heat load.
The rule of thumb
A common starting point is about 20 BTU per square foot of living space. So a 500 sq ft room needs roughly 10,000 BTU, and 1,000 sq ft needs about 20,000 BTU (just under 2 tons). Use the chart below as a baseline, then adjust for the room’s specifics.
| Room size | Rough BTU |
|---|---|
| 150–250 sq ft | 6,000 |
| 300–450 sq ft | 9,000–10,000 |
| 550–700 sq ft | 14,000 |
| 1,000 sq ft | ~20,000 |
Adjust for your room
Add roughly 10% capacity for rooms that get a lot of direct sun, and reduce about 10% for heavily shaded rooms. Add for high or vaulted ceilings (you’re cooling more air volume), for kitchens (appliances throw off heat), and for rooms that regularly hold several people. For a single room, a window or portable unit sized this way is fine; for cooling a whole house, a proper Manual J load calculation — which accounts for insulation, windows, climate, and orientation — is what HVAC pros use and beats any rule of thumb.
Frequently asked questions
How many BTU per square foot? Roughly 20 as a starting estimate; adjust for sun, ceilings, and occupancy.
What’s a ton of cooling? 12,000 BTU/hr.
Is bigger better? No — an oversized unit short-cycles and leaves rooms humid and uncomfortable.