What size air conditioner do I need? Plan on roughly 20 BTU per square foot. ENERGY STAR’s chart puts a 150 to 250 sq ft bedroom at 6,000 BTU, a 350 to 400 sq ft room at 9,000 BTU and 450 to 550 sq ft at 12,000. Add 10% for a sunny room, 600 BTU per extra person and 4,000 BTU for a kitchen.
It’s 92 degrees, the bedroom feels like a sauna, and the store has a wall of window units ranging from 5,000 to 25,000 BTU. Bigger must be better, right? Nope. A unit that’s too big leaves the room cold and damp. Too small and it hums all day without catching up. Getting what size air conditioner you need right is mostly about one measurement and a few honest adjustments.
Start by finding the room’s square footage. Length times width, in feet. If you need help, our square footage calculator handles odd shapes, or read how to calculate square footage.
What size air conditioner do I need? The room chart
This is the sizing chart ENERGY STAR publishes for room air conditioners (window, through-the-wall and similar units). It assumes a normal room with 8-ft ceilings, about two people and average sun.
| Room area | Cooling capacity (BTU/hr) | BTU per sq ft (mid-range) |
|---|---|---|
| 100 to 150 sq ft | 5,000 | 40 |
| 150 to 250 sq ft | 6,000 | 30 |
| 250 to 300 sq ft | 7,000 | 25 |
| 300 to 350 sq ft | 8,000 | 25 |
| 350 to 400 sq ft | 9,000 | 24 |
| 400 to 450 sq ft | 10,000 | 24 |
| 450 to 550 sq ft | 12,000 | 24 |
| 550 to 700 sq ft | 14,000 | 22 |
| 700 to 1,000 sq ft | 18,000 | 21 |
| 1,000 to 1,200 sq ft | 21,000 | 19 |
| 1,200 to 1,400 sq ft | 23,000 | 18 |
| 1,400 to 1,500 sq ft | 24,000 | 17 |
| 1,500 to 2,000 sq ft | 30,000 | 17 |
| 2,000 to 2,500 sq ft | 34,000 | 15 |
Capacity bands from ENERGY STAR. The last column is our own math: the BTU figure divided by the middle of each area band, so you can see how the ratio falls as rooms get bigger.
Notice the per-square-foot number. Small rooms need around 30 BTU per square foot, big open spaces under 20. That’s because a small room has more wall and window per square foot of floor, so more heat sneaks in. It’s also why the plain “20 BTU per sq ft” rule undersizes small bedrooms a little.
What size air conditioner do I need for a sunny room or kitchen?
ENERGY STAR gives four adjustments. Apply the percentages first, then add the fixed amounts:
- Heavily shaded room: cut capacity by 10%.
- Very sunny room: add 10%.
- More than two people regularly in the room: add 600 BTU for each extra person.
- Kitchen: add 4,000 BTU. Stoves, ovens and dishwashers pump out a surprising amount of heat.
What counts as “very sunny”? I’d use it for any room with big south- or west-facing windows and no shade trees or awnings, or a top-floor room under a dark roof. A north-facing room under a big maple counts as shaded.
Worked examples
A living room is 16 × 23.5 ft, with big west windows. Three people watch TV there most evenings.
- Area: 16 × 23.5 = 376 sq ft, which falls in the 350 to 400 band: 9,000 BTU
- Very sunny, +10%: 9,000 × 1.10 = 9,900 BTU
- One person over two, +600: 10,500 BTU
Units come in steps like 8,000, 10,000 and 12,000. Here 10,000 BTU is within 5% of the target and the closest fit. Honestly, I’d only jump to 12,000 if the room also opens onto a hallway or kitchen with no door.
A 320 sq ft studio apartment, shaded by the next building, one person, kitchen in the same room.
- 320 sq ft falls in the 300 to 350 band: 8,000 BTU
- Heavily shaded, −10%: 7,200 BTU
- Kitchen, +4,000: 11,200 BTU
A 12,000 BTU unit is the sensible pick. That kitchen adjustment is the one people skip, then wonder why the place hits 85 every time they cook pasta.
High ceilings, open plans and odd rooms
“What size air conditioner do I need” gets trickier once the room isn’t a plain box. Charts are built around 8-ft ceilings. With higher ceilings you’re cooling more air, and warm air pools up there. A simple fix: multiply your result by your ceiling height divided by 8. At 10 ft that’s × 1.25; at 12 ft, × 1.5. It’s a rough adjustment, not a formal method, but it’s a lot closer than ignoring the ceiling.
A few other situations the chart doesn’t cover:
- Open floor plans. If the living room flows into the kitchen and dining area with no doors, size for the whole open area.
- Attic bedrooms and sunrooms. Lots of glass or a hot roof right overhead can push you 20% or more above the chart. Treat these as “very sunny” and then some.
- Basements. Often need less than the chart, but they’re damp, so a smaller unit that runs longer (and pulls out more moisture) is usually better.
- Hot, humid climates. Pick the lower end of what fits rather than the higher, so the unit runs long enough to dry the air.
What size air conditioner do I need for a whole house?
Central systems are sold in tons. One ton equals 12,000 BTU per hour, and residential systems usually run from 1.5 to 5 tons. The quick rule of thumb many contractors start from is about 20 BTU per square foot of living space:
| Home size | BTU at 20/sq ft | Tons | Rule-of-thumb size |
|---|---|---|---|
| 1,000 sq ft | 20,000 | 1.7 | 2.0 ton or less |
| 1,250 sq ft | 25,000 | 2.1 | 2.5 ton or less |
| 1,500 sq ft | 30,000 | 2.5 | 2.5 ton or less |
| 1,750 sq ft | 35,000 | 2.9 | 3.0 ton or less |
| 2,000 sq ft | 40,000 | 3.3 | 3.5 ton or less |
| 2,500 sq ft | 50,000 | 4.2 | 4.5 ton or less |
| 3,000 sq ft | 60,000 | 5.0 | 5.0 ton or less |
Our math: square feet × 20 ÷ 12,000, rounded up to the nearest half ton. Use it to sanity-check a quote, not to buy equipment.
Why “or less”? Because the rule of thumb ignores everything that matters: insulation, windows, air leaks, climate and which way the house faces. A tight, newer home in a mild climate can need far less than this table. An older, leaky house in Phoenix might need more. Contractors’ rules vary from about 400 to 600 square feet per ton for exactly that reason.
Why a Manual J beats any chart
For a central system or a ductless mini-split for the whole house, ask for a Manual J load calculation. It’s the residential load method from the Air Conditioning Contractors of America (ACCA), and most U.S. building codes require it for new equipment. It looks at your actual walls, attic insulation, window sizes and types, orientation, air leakage and local design temperatures, room by room.
If a contractor sizes your system by looking at the old unit’s label, or just multiplies your square footage by a number, get another quote. The old unit might have been oversized for 20 years.
And here’s a sequencing tip almost nobody mentions. If you plan to add insulation, seal air leaks or replace windows, do those first. A 2014 National Renewable Energy Laboratory study looked at exactly this: replacing the AC before efficiency upgrades leaves you with a system sized for a leakier house, and in their simulations oversizing could raise cooling energy use by 10% to 21% when the unit draws standby power while off.
Is an Air Conditioner That Is Too Big or Too Small Worse?
| Too big | Too small | |
|---|---|---|
| Temperature | Hits the setting fast, then shuts off | Runs constantly, may never get there |
| Humidity | Short runs leave air damp and clammy | Usually removes moisture well |
| Energy | Can waste energy, especially with standby losses | Long run times on hot days |
| Wear | Frequent on/off cycles | Mostly fine, just working hard |
| Comfort | Cold spots, uneven rooms | Warm afternoons |
A unit that is too big hits the setting fast, shuts off and leaves air damp and clammy, while one that is too small runs constantly and may never reach the setting. So when you ask what size air conditioner do I need and land between two sizes, here’s the tiebreaker. If you’re stuck between two sizes, go smaller in humid places and larger only if the room is very sunny or packed with people. A unit that runs steady for 15 to 20 minutes at a time is doing its job.
Before you buy: four things to check
- Your outlet. Small window units run on a standard 115-volt outlet. Many larger units (often around 15,000 BTU and up) need a 230-volt circuit. Check the spec sheet before you buy.
- Portable unit ratings. Portable ACs now show a lower SACC rating beside the bigger ASHRAE number. Use the SACC number with the chart above.
- Window size. Measure the opening. Big units won’t fit narrow windows, and casement windows need a different design.
- Efficiency. An ENERGY STAR certified room unit uses less power for the same cooling. Look at the yellow EnergyGuide label for the yearly cost estimate.
Running a unit all summer adds up. If you’re comparing models, our percentage calculator shows how much cheaper one is to run, and the Celsius to Fahrenheit converter helps when a spec sheet lists temperatures in °C.
What size air conditioner do i need: quick recap
- What size air conditioner do I need for a 300 sq ft bedroom? About 7,000 BTU.
- What size air conditioner do I need for a 500 sq ft living room? About 10,000 BTU, more if it’s sunny or has a kitchen.
Frequently asked questions
What size air conditioner do I need for a 12×12 room?
A 12 × 12 room is 144 sq ft, which falls in ENERGY STAR’s 100 to 150 sq ft band, so a 5,000 BTU unit. If it gets afternoon sun, 6,000 BTU is reasonable.
What size air conditioner do I need for 1,500 square feet?
For a room or open space of 1,400 to 1,500 sq ft, ENERGY STAR’s chart says about 24,000 BTU. For central air in a 1,500 sq ft house, the 20-BTU-per-square-foot rule of thumb gives 30,000 BTU, or 2.5 tons, but a Manual J calculation often comes in lower in a well-insulated home.
How many square feet does a 1 ton air conditioner cool?
One ton is 12,000 BTU per hour. By ENERGY STAR’s room chart that covers about 450 to 550 sq ft. For whole houses, rough rules land between about 400 and 600 sq ft per ton depending on climate and insulation.
Is it bad to buy an air conditioner that's too big?
Yes, usually. An oversized unit cools the air fast and shuts off before it has pulled much moisture out, so the room feels cold and clammy. It also cycles on and off more, which can waste energy and wear parts.
What happens if my AC is too small?
It runs nearly nonstop on hot afternoons and may never reach your setting. It won’t hurt the unit much, but you’ll be warm and pay for long run times.
How many BTU do I need per square foot?
Roughly 20 BTU per square foot is the usual starting point. ENERGY STAR’s room chart works out to about 20 to 40 BTU per sq ft for small rooms and about 14 to 17 for very large spaces.
Does ceiling height change what size AC I need?
Yes. Sizing charts assume 8-ft ceilings. With higher ceilings there’s more air to cool, so a simple adjustment is to scale up in proportion: a 10-ft ceiling needs roughly 25% more capacity than the chart shows.
Do portable air conditioners use the same sizing chart?
Use the chart, but check the label carefully. Newer portable units list a lower SACC rating next to the bigger ASHRAE number. Size by the SACC number, since that’s closer to what the unit actually delivers.
Sources
- Room Air Conditioners: sizing chart, ENERGY STAR, U.S. EPA (BTU by area and adjustments)
- Manual J Residential Load Calculation, Air Conditioning Contractors of America
- Energy Impacts of Oversized Residential Air Conditioners, Booten, Christensen and Winkler, National Renewable Energy Laboratory, 2014