How it works
| Area (sq ft) | BTU/h | Area (sq ft) | BTU/h |
|---|---|---|---|
| 100–150 | 5,000 | 450–550 | 12,000 |
| 150–250 | 6,000 | 550–700 | 14,000 |
| 250–300 | 7,000 | 700–1,000 | 18,000 |
| 300–350 | 8,000 | 1,000–1,200 | 21,000 |
| 350–400 | 9,000 | 1,200–1,400 | 23,000 |
| 400–450 | 10,000 | 1,400–1,500 | 24,000 |
| 1,500–2,000 | 30,000 | ||
| 2,000–2,500 | 34,000 |
ENERGY STAR room air conditioner sizing chart. On a boundary (for example, exactly 400 sq ft) this calculator uses the lower row.
Then ENERGY STAR's adjustments are applied: −10% if heavily shaded, +10% if very sunny, +600 BTU for each person beyond two, and +4,000 BTU in a kitchen. Finally the total is rounded down to the nearest thousand.
Tons = BTU/h ÷ 12,000
One ton of refrigeration is 12,000 BTU/h, which is the heat needed to melt one ton of ice over 24 hours (ASHRAE).
Worked example
A 400 sq ft very sunny room used by 3 people: the chart gives 9,000 BTU/h. Sun adds 10% (9,900), the third person adds 600 (10,500), and rounding down gives 10,000 BTU/h ≈ 0.83 ton.
Assumptions and limits
- This is a rule of thumb for a single room or open area, the way ENERGY STAR intends it for room and window units.
- For central air or heat pumps, contractors should run an ACCA Manual J load calculation, which accounts for insulation, windows, air leakage, orientation and climate. Square-foot rules often oversize whole-house systems.
- Canada's ENERGY STAR (NRCan) chart differs slightly in the 450–700 sq ft rows; this page uses the U.S. EPA version.
- An oversized AC cools the air before it removes humidity, leaving the room clammy. Bigger is not better.