Air Conditioner Size Calculator

Air conditioner sizing uses BTU (British Thermal Units) per hour as the measure of cooling capacity. An undersized AC runs constantly and can’t reach temperature; an oversized AC cools too fast without dehumidifying properly, leaving the room feeling clammy.

The Formula:

Required BTU = Room area (sq ft) × 20

Twenty BTU per square foot is the figure the US Department of Energy publishes, and it assumes a standard 8-foot ceiling. Every adjustment below is a correction to that assumption.

BTU to Tonnage Conversion:

1 ton of cooling = 12,000 BTU/hour

Worked Example:

Living room: 20 ft × 15 ft = 300 sq ft, 8 ft ceiling. South-facing windows. 3 regular occupants.

Base BTU: 300 × 20 = 6,000 BTU Sun adjustment: ×1.1 = 6,600 BTU Extra person: +600 BTU Total: 7,200 BTU → the next unit up the shelf is 8,000 BTU

That last step matters. Window units come in fixed steps, so you almost never buy the exact number the arithmetic gives you. Round up, never down: an undersized unit runs flat out all afternoon and still loses ground on the hottest day of the year, which is the only day you actually care about.

Common AC Unit Sizes:

This is the same 20 BTU per square foot rule the calculator uses, rounded up to the sizes window units are actually sold in. No sun, kitchen, ceiling or occupancy adjustment is applied here.

One caveat worth knowing. The Energy Star chart you will see in stores runs about one size step higher than this at the small end, recommending 6,000 BTU for a 200 sq ft bedroom where the flat rule says 5,000. Their chart assumes older, leakier construction. In a well-insulated modern room the flat rule holds up; in a 1960s house with single-pane windows, go up a step.

Practical Tips:

• SEER (Seasonal Energy Efficiency Ratio) matters: SEER 16–20 costs significantly less to run than SEER 10
• Mini-split systems offer zoned control with no ductwork, which makes them the usual answer for a room addition
• Insulate and air-seal before upsizing. Sealing leaks can cut the required capacity by 20–30%, and a smaller unit that runs longer dehumidifies better than a big one that does not