How air conditioner sizing works
Cooling capacity is measured in BTUs per hour — the more BTUs, the more heat a unit can remove. The starting point is roughly 20 BTU per square foot of floor area, but several factors push that number up or down. Sun-baked rooms gain heat through windows; tall ceilings mean more air volume to cool; extra people and heat-producing kitchens add load. This calculator applies the standard adjustments to give a practical estimate.
BTUs to tons
Air conditioners are also rated in "tons," where one ton equals 12,000 BTU per hour. So a room needing 18,000 BTU calls for roughly a 1.5-ton unit. Window and portable units are usually labeled in BTUs; central and mini-split systems are often described in tons.
| Room area | Rough BTU |
|---|---|
| 150–250 sq ft | ~6,000 |
| 250–350 sq ft | ~8,000 |
| 350–550 sq ft | ~12,000 (1 ton) |
| 550–1,000 sq ft | ~18,000–24,000 |
Why bigger isn't better
It's tempting to oversize "just to be safe," but an AC that's too large cools the air quickly and shuts off before it can pull humidity out, leaving the room cold and clammy and the unit short-cycling. Right-sizing keeps the space both cool and dry, and it runs more efficiently.
Frequently asked questions
Does this size a whole-house system?
No — it's meant for a single room or a window, portable, or mini-split unit. Central air for a whole home depends on ductwork, insulation, and air sealing, which a contractor calculates with a Manual J load analysis.
What about heating?
This estimates cooling load only. Heating is sized differently and depends heavily on your climate and insulation.
Should I round up to the next unit?
Pick the closest common unit size to the BTU estimate. A small amount over is fine; significantly oversizing causes the humidity and short-cycling problems described above.
BTU calculation example
A 15-by-20-foot room has 300 square feet. A typical starting estimate is about 20 BTU per square foot, or 6,000 BTU. Strong afternoon sun, high ceilings, extra occupants, or heat-producing appliances can raise the requirement, which is why the calculator adjusts more than floor area alone.
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