Enter the room, its sun and who uses it. You get the BTUs ENERGY STAR's sizing chart asks for, the size of window unit sold that covers it, how wide those units are, and whether they plug into a normal outlet.
Your air conditioner
An 8,000 BTU air conditioner for your 300 sq ft roomENERGY STAR: 8,000 BTU for 300–350 sq ft
Quick answer: ENERGY STAR sizes a room air conditioner by the floor: 5,000 BTU for 100–150 sq ft, 8,000 BTU for 300–350 sq ft, 12,000 BTU for 450–550 sq ft, up to 34,000 for 2,500. Then take off 10% for a heavily shaded room or add 10% for a very sunny one, add 600 BTU for each person past two, and add 4,000 BTU in a kitchen. Buy the next size sold: no ENERGY STAR certified unit comes in 7,000 or 9,000 BTU.
How to use the AC BTU calculator
- Measure the room: its length and width. For an odd shape, split it into rectangles and add them up, as ENERGY STAR says.
- Tell it about the room: how sunny it is, how many people are usually in it, and whether it is a kitchen. Add the ceiling if it is over 8′, and the window’s opening width to see which units fit.
- You get the BTUs ENERGY STAR’s chart asks for, the size sold that covers them, how many certified units come in that size, how wide they are and what outlet they need, with the room drawn from above to scale.
Moving air as well as cooling it? The ceiling fan direction tool says which way to turn the fan in summer, and the ceiling fan size calculator sizes it.
The rules, and where they come from
The chart. ENERGY STAR’s room air conditioner page: find the room’s square footage in the “Area To Be Cooled” column, “then use the “Capacity Needed (BTUs per hour)” column”. “The capacities are calculated based on an 8-foot ceiling.”
The adjustments. ENERGY STAR: “If the room is heavily shaded, reduce capacity by 10 percent. If the room is very sunny, increase capacity by 10 percent. If more than two people regularly occupy the room, add 600 BTUs for each additional person. If the unit is used in a kitchen, increase capacity by 4,000 BTUs.”
Not too big. ENERGY STAR: “If the unit is too large, it will cool the room before it has a chance to remove the humidity. This can lead to discomfort because cool, moist air will make the room feel damp and clammy.”
The sizes sold. ENERGY STAR’s list of certified room air conditioners (538 models on 4 October 2026) gives every model’s capacity, size and voltage. None is rated 7,000 or 9,000 BTU, so a room the chart puts at 7,000 takes an 8,000 BTU unit. Every certified unit of 18,000 BTU or more runs on 230 V.
Mini-splits. ENERGY STAR’s chart is for room units. For a ductless mini-split it says: “Your contractor should verify the proper size system for your home using a tool called ‘Manual J’”.
ENERGY STAR’s sizing chart
| Room | BTU per hour | BTU a sq ft |
|---|---|---|
| 100–150 sq ft | 5,000 | 33.3–50 |
| 150–250 sq ft | 6,000 | 24–40 |
| 250–300 sq ft | 7,000 | 23.3–28 |
| 300–350 sq ft | 8,000 | 22.9–26.7 |
| 350–400 sq ft | 9,000 | 22.5–25.7 |
| 400–450 sq ft | 10,000 | 22.2–25 |
| 450–550 sq ft | 12,000 | 21.8–26.7 |
| 550–700 sq ft | 14,000 | 20–25.5 |
| 700–1,000 sq ft | 18,000 | 18–25.7 |
| 1,000–1,200 sq ft | 21,000 | 17.5–21 |
| 1,200–1,400 sq ft | 23,000 | 16.4–19.2 |
| 1,400–1,500 sq ft | 24,000 | 16–17.1 |
| 1,500–2,000 sq ft | 30,000 | 15–20 |
| 2,000–2,500 sq ft | 34,000 | 13.6–17 |
BTU a square foot falls as rooms grow: there is no single “BTU per square foot” in ENERGY STAR’s chart.
The sizes you can buy
| BTU | Certified models | Plug | Window units, wide |
|---|---|---|---|
| 5,000 | 19 | all 115 V | 16″ |
| 6,000 | 27 | all 115 V | 15.9–20.7″ |
| 8,000 | 94 | all 115 V | 18.5–24.2″ |
| 10,000 | 102 | 94 of 102 on 115 V | 18.5–26.5″ |
| 12,000 | 122 | 114 of 122 on 115 V | 18.9–30″ |
| 14,000 | 40 | 32 of 40 on 115 V | 18.9–23.6″ |
| 18,000 | 24 | all 230 V | 23.6–30″ |
| 24,000 | 26 | all 230 V | 23.6–30″ |
From data.energystar.gov, 4 October 2026: window and through-the-wall units; the width is the unit’s own, so check each maker’s minimum window opening.
Worked examples
An 8,000 BTU air conditioner for your 300 sq ft room: a 20′ × 15′ living room with some sun and two people, straight off the chart.
A 6,000 BTU air conditioner for your 120 sq ft room: a very sunny 12′ × 10′ bedroom needs 5,500 BTU (5,000 + 10%), and 6,000 is the next size sold.
An 8,000 BTU air conditioner for your 180 sq ft room: a 15′ × 12′ room the chart puts at 6,000, plus 1,200 for two more people, needs 7,200; there is no certified 7,000 BTU unit.
A 10,000 BTU air conditioner for your 144 sq ft room: a 12′ × 12′ kitchen for three: 5,000 + 600 + 4,000 = 9,600 BTU.
Common mistakes
- Buying bigger to be safe. ENERGY STAR: a unit that is too large cools the room before it removes the humidity, and the room feels “damp and clammy”.
- Forgetting the kitchen. A kitchen adds 4,000 BTU, almost a whole extra 5,000 BTU unit.
- Not checking the plug. Every certified unit of 18,000 BTU or more needs a 230 V outlet.
- Sizing a mini-split from this chart. A mini-split is sized by the installer’s Manual J.
More on keeping rooms cool at Dreamy Home Style
Planning the room itself? Draw it to scale in the free 2D floor planner.
AC size questions
How many BTU do I need?
Find your room’s square footage on ENERGY STAR’s chart: 5,000 BTU for 100–150 sq ft, 8,000 for 300–350, 12,000 for 450–550. Then adjust for sun, people and a kitchen.
What size air conditioner do I need?
The smallest size sold that covers the chart’s number after the adjustments: certified window units come in 5,000, 6,000, 8,000, 10,000, 12,000, 14,000, 18,000 and 24,000 BTU, with a few in between.
How many BTU per square foot?
It falls as the room grows: 33–50 BTU a square foot at 100–150 sq ft, about 23–27 at 300–350, and 14–17 at 2,000–2,500 on ENERGY STAR’s chart.
What size AC for a 12 × 12 room?
144 sq ft is on the chart’s first row: 5,000 BTU, 5,500 if it is very sunny (so a 6,000 BTU unit), 9,000 if it is a kitchen (so a 10,000 BTU unit).
Is a bigger air conditioner better?
No. ENERGY STAR: “A unit that is too big will be less effective” and leaves the room cool but damp.
How do I size a mini-split?
ENERGY STAR says your contractor should size it with a Manual J calculation of your home’s heating and cooling needs, not from the room unit chart.