What size air conditioner do I need?

By Nazrul Islam. Updated October 11, 2026.

Short answer: BTU/h = the ENERGY STAR chart value for your square footage, ±10% for shade or strong sun, plus 600 per person beyond two, plus 4,000 for a kitchen. A very sunny 12 × 20 ft bedroom (240 sq ft) used by three people needs 6,000 + 600 (sun) + 600 (third person) = 7,200 BTU/h.

1. Your room

Measure the floor of the room the unit will cool. The chart assumes an 8-foot ceiling.

Measure byLength (ft)Width (ft)People regularly in the roomThe unit will cool a kitchen

That is 180 sq ft.

How much sun does the room get?NormalNo change to the chart numberHeavily shadedENERGY STAR: reduce capacity by 10%Very sunnyENERGY STAR: increase capacity by 10%

2. The size you need

Recommended capacity: 6,000 BTU/h
Chart value: 6,000 BTU/h

Step	BTU/h
Chart value for 180 sq ft	6,000
Sun exposure (normal)	0
People beyond two (0 × 600)	0
Kitchen	0
Total	6,000

Bigger is not better. ENERGY STAR says an oversized unit cools the room before it removes the humidity, so the room feels damp and clammy. It also costs more and wastes energy.

How this is calculated

Area = length × width. The chart value is the BTU/h listed for that area range, assuming an 8-foot ceiling.
Heavily shaded: chart value × 0.9. Very sunny: chart value × 1.1.
Add 600 BTU/h for each person beyond two who regularly uses the room.
Add 4,000 BTU/h if the unit cools a kitchen.
ENERGY STAR does not say which order to apply these in. We apply the sun percentage to the chart value only, then add the fixed amounts.
An area that falls exactly on a range boundary, such as 150 sq ft, uses the higher range, since each row runs "up to" the next.

The ENERGY STAR room air conditioner sizing chart

ENERGY STAR bases these capacities on an 8-foot ceiling. Each row runs from the first number up to the second.

Area to be cooled (sq ft)	Capacity needed (BTU/h)
100 up to 150	5,000
150 up to 250	6,000
250 up to 300	7,000
300 up to 350	8,000
350 up to 400	9,000
400 up to 450	10,000
450 up to 550	12,000
550 up to 700	14,000
700 up to 1,000	18,000
1,000 up to 1,200	21,000
1,200 up to 1,400	23,000
1,400 up to 1,500	24,000
1,500 up to 2,000	30,000
2,000 up to 2,500	34,000

Adjust for your room

ENERGY STAR lists four adjustments to the chart value.

Condition	Adjustment
Room is heavily shaded	Reduce capacity by 10%
Room is very sunny	Increase capacity by 10%
More than two people regularly use the room	Add 600 BTU/h per extra person
Unit cools a kitchen	Add 4,000 BTU/h

ENERGY STAR does not say what order to apply these in. Our calculator applies the 10% to the chart value, then adds the fixed amounts.

Worked examples

Room	Chart value	Adjustments	Total (BTU/h)
12 × 15 ft bedroom (180 sq ft), two people	6,000	None	6,000
20 × 20 ft shaded living room (400 sq ft)	10,000	−1,000 shade	9,000
10 × 20 ft kitchen (200 sq ft)	6,000	+4,000 kitchen	10,000
20 × 25 ft sunny family room (500 sq ft), four people	12,000	+1,200 sun, +1,200 people	14,400

Why a bigger unit is not better

An oversized unit cools the room before it has a chance to remove humidity, per ENERGY STAR. Cool, moist air feels damp and clammy. ENERGY STAR adds that it costs more, wastes energy and does not cool better.

A U.S. Department of Energy fact sheet makes the same point for central systems. It says short cycling can lead to poor humidity control, and excessive cycling lowers overall efficiency.

Common questions

How many BTUs do I need for a 12 × 12 room?

That is 144 sq ft, which falls in the 100 up to 150 row. ENERGY STAR lists 5,000 BTU/h before adjustments.

What if my ceilings are higher than 8 feet?

The chart assumes an 8-foot ceiling. ENERGY STAR only says you may want a unit with a higher CADR for taller ceilings. It gives no formula, so treat the chart number as a floor.

What size do I need for more than 2,500 sq ft?

The ENERGY STAR chart stops at 2,500 sq ft. For larger spaces or a whole house, ask an HVAC contractor for a Manual J load calculation. The DOE fact sheet explains that Manual J sets loads from the building's envelope, insulation and air leakage.

My room is an odd shape. How do I measure it?

Split it into rectangles, multiply length by width for each, and add them. ENERGY STAR also suggests asking the store where you buy the unit for help.