How to estimate BTUs
The simplest sizing rule is based on floor area:
BTU per hour = room area (sq ft) × BTU per sq ft
About 20 BTU per square foot is a common baseline for cooling. Divide the result by 12,000 to express it in tons of air conditioning (1 ton = 12,000 BTU/hr).
BTU by room size (cooling)
The U.S. ENERGY STAR window-AC chart maps room size straight to capacity — a handy cross-check for the area rule:
| Room size | Cooling capacity |
|---|---|
| 100–150 sq ft | 5,000 BTU |
| 150–250 sq ft | 6,000 BTU |
| 250–350 sq ft | 7,000–8,000 BTU |
| 350–450 sq ft | 9,000–10,000 BTU |
| 450–550 sq ft | 12,000 BTU (1 ton) |
| 550–700 sq ft | 14,000 BTU |
| 700–1,000 sq ft | 18,000 BTU (1.5 tons) |
Adjust for your room
| Condition | Adjustment |
|---|---|
| Very sunny room | add ~10% |
| Heavily shaded room | subtract ~10% |
| Kitchen | add ~4,000 BTU |
| Each person over two | add ~600 BTU |
Window unit, mini-split, or central?
- Window or portable unit. The cheapest way to cool a single room — match the BTU to the room from the chart above.
- Ductless mini-split. Efficient and quiet for a room or addition with no ductwork; sized in the same BTU/tons but far more efficient at part load.
- Central air. Cools the whole house through ducts. Size it with a professional Manual J load calculation, not a rule of thumb — whole-home sizing depends on insulation, windows, and air sealing, not just floor area.
What the square-foot rule misses
The 20-BTU rule is a starting point, not a final answer. It assumes ~8 ft ceilings and average insulation. Real load also depends on ceiling height (tall rooms hold more air to cool), climate, window area and direction, insulation and air sealing, and how many people and heat-producing appliances are in the room. Heating needs more capacity in cold climates — roughly 30–35 BTU/sq ft in mild zones up to 50–60 BTU/sq ft in cold northern ones, versus the ~20 used for cooling.
Turning a BTU number into a unit
Once you have a BTU target, here's how it maps to what's on the shelf:
- Window and portable units are labeled directly in BTU — match your number to the nameplate (e.g. an 8,000 BTU unit).
- Mini-splits and central AC are sold in tons — divide your BTU by 12,000 (so 24,000 BTU = 2 tons). They come in half-ton steps, so round to the nearest size.
- Check the efficiency. The SEER2 rating (cooling) and HSPF2 or AFUE (heating) set the running cost — a higher number costs more up front but less every month, which pays off in a hot or cold climate.
Between two sizes, lean to the smaller one for cooling: a right-sized unit runs longer and pulls more humidity than an oversized one that short-cycles.