SizeDial

AC / BTU Size Calculator

What size air conditioner do I need?

Tell us about the space you want to cool. If you don’t know the square footage, we’ll help you figure it out.

EPA ENERGY STAR basisAbout 30 secondsRoom AC sizing

Let’s size the room ACFour everyday questions. We’ll handle the BTU math.

How big is the space you want to cool?

If you know the square footage, type it below. If not, we can figure it out from room measurements.

Use whichever area you have.

Room size sq ft

How much direct sun does the room get?

Choose the closest match. No need to measure anything.

Mostly shadedLittle direct sun Some sunA normal mix Lots of direct sunSunny for much of the day

How many people are usually in the room?

Count the people who are there regularly.

Usually people

Is this space a kitchen?

Cooking adds extra heat, so kitchens need more cooling power.

No Yes

Anything unusual about the room?

Most rooms can use SizeDial’s standard 8-foot-ceiling assumption. If your ceiling is unusually high, enter it here so we can flag the result appropriately.

Ceiling measurement

Ceiling height ft

The short version

Room AC sizing starts with square footage, then room conditions change the answer.

EPA ENERGY STAR publishes a cooling-capacity chart for room air conditioners based on floor area and an 8-foot ceiling. That is the calculator's base. Sun exposure, regular occupancy, and kitchen use can then move the requirement up or down.

Why an oversized air conditioner is not automatically better

ENERGY STAR warns that a unit that is too large can cool the room before it has enough time to remove humidity, leaving the air cool but damp. Correct sizing is therefore about comfort and moisture removal as well as raw cooling power.

ENERGY STAR room AC capacity chart

The following base capacities are the published ENERGY STAR starting points before room-condition adjustments.

Area (sq ft)Base capacityArea (sq ft)

100–1505,000 BTU700–1,000 → 18,000

150–2506,000 BTU1,000–1,200 → 21,000

250–3007,000 BTU1,200–1,400 → 23,000

300–3508,000 BTU1,400–1,500 → 24,000

350–4009,000 BTU1,500–2,000 → 30,000

400–45010,000 BTU2,000–2,500 → 34,000

450–55012,000 BTU550–700 → 14,000

What changes the BTU recommendation?

For heavily shaded rooms, ENERGY STAR says to reduce capacity by 10%. For very sunny rooms, increase it by 10%. If more than two people regularly occupy the room, add 600 BTU for each additional person. For a kitchen, add 4,000 BTU.

What about high ceilings?

The ENERGY STAR chart is calculated for an 8-foot ceiling and advises considering a higher-capacity room AC when ceilings are higher. Because it does not publish a universal percentage multiplier for ceiling height on this chart, SizeDial flags the condition instead of pretending there is a precise one-size-fits-all formula.

Ready when you are

Ready to shop the size you calculated?

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AC / BTU sizing FAQ

Is this calculator for central air?

No. This launch tool follows EPA ENERGY STAR guidance for room air conditioners. Whole-home central-air sizing requires a proper HVAC load calculation rather than a simple room-size chart.

Should I just buy the next larger AC?

Not automatically. Oversizing can reduce dehumidification. Use the calculated capacity as the starting point and confirm the product is intended for the space and installation type.

Does the room shape matter?

It matters for calculating total floor area. ENERGY STAR advises breaking irregular spaces into rectangles, squares, or triangles and adding their areas together.

Primary sizing source: EPA ENERGY STAR Room Air Conditioners, verified September 8, 2026.