How to size a room air conditioner without guessing

The wrong air conditioner can be uncomfortable in two directions. An undersized unit may run continuously without catching up. A substantially oversized unit can satisfy the thermostat quickly but remove less moisture, leaving the room cool and clammy.

For conventional room air conditioners, ENERGY STAR publishes a useful starting table based on floor area and an 8-foot ceiling.

Start with floor area

Measure the conditioned space, not the entire home. For a rectangle, multiply length by width. Divide irregular rooms into smaller rectangles or triangles and add their areas.

Room area	Starting capacity
100–150 sq. ft.	5,000 Btu/h
150–250 sq. ft.	6,000 Btu/h
250–300 sq. ft.	7,000 Btu/h
300–350 sq. ft.	8,000 Btu/h
350–400 sq. ft.	9,000 Btu/h
400–450 sq. ft.	10,000 Btu/h
450–550 sq. ft.	12,000 Btu/h
550–700 sq. ft.	14,000 Btu/h
700–1,000 sq. ft.	18,000 Btu/h

These are baseline capacities. A 375-square-foot bedroom starts near 9,000 Btu/h, but the room itself determines whether that baseline is sensible.

Make the documented adjustments

ENERGY STAR recommends several simple changes:

• Reduce capacity by 10% when the room is heavily shaded.
• Increase capacity by 10% when the room is very sunny.
• When more than two people regularly occupy the room, add 600 Btu/h for each additional person.
• For a kitchen, add 4,000 Btu/h.

Do not mechanically apply opposing adjustments and assume the answer is precise. A sunny kitchen with poor insulation and a high ceiling has more variables than this table captures.

Know when square feet are not enough

Floor area cannot describe ceiling height, window area, air leakage, insulation, local design temperature, connected spaces, or heat from electronics. An attic bedroom with a roof directly above it can have a very different load from a first-floor bedroom with the same dimensions.

For a window unit, the table is a practical screening tool. For a permanent mini-split or whole-home system, ask for a recognized load calculation such as Manual J. The cost of correcting permanently oversized equipment is much higher than exchanging a room unit.

Portable AC requires another number

Portable units may display both an older ASHRAE capacity and the lower federal DOE SACC rating. Use DOE SACC when comparing portable models. Do not compare a portable unit’s larger ASHRAE number directly with the rated capacity of a window unit.

Our room AC sizing calculator follows the ENERGY STAR table and adjustments. Its result is a starting range, not a guarantee that a particular unit will hold a particular temperature.

A useful purchasing checklist

Before choosing a capacity, record:

1. Conditioned floor area and ceiling height.
2. Window direction and afternoon sun exposure.
3. Typical number of occupants.
4. Whether the space is a kitchen or contains substantial heat-producing equipment.
5. Whether doors to adjoining spaces will remain open.
6. For portable units, the DOE SACC value rather than only the largest number on the box.

The goal is not to buy the largest machine that fits the budget. It is to choose enough net cooling capacity for the actual room while preserving useful moisture removal and reasonable cycling.