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Find the right AC size in BTU for any room — no more guessing. Free, no signup.

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AC BTU Calculator — What Size Air Conditioner Do I Need?

An undersized AC never cools, and an oversized one wastes money and leaves the room clammy. Enter your room details and instantly get the recommended BTU size, built on the standard 20 BTU per square foot rule with real-world adjustments.

Recommended AC size
3,500 BTU
look for a unit rated at or near this capacity
Room area
180
square feet
Calculated need
3,600
BTU before rounding
Suggested range
3,000 – 4,000
BTU · any unit in this band works
Room areaTypical AC sizeBest for
100 – 150 sq ft5,000 BTUSmall bedroom or office
150 – 250 sq ft6,000 BTUStandard bedroom
250 – 300 sq ft7,000 BTULarge bedroom
300 – 350 sq ft8,000 BTUSmall living room
350 – 400 sq ft9,000 BTULiving room or studio
400 – 450 sq ft10,000 BTULarge living room
450 – 550 sq ft12,000 BTUOpen-plan space
550 – 700 sq ft14,000 – 15,000 BTUGreat room or small apartment
700 – 1,000 sq ft18,000 – 21,000 BTULarge apartment or small house
Buying tip: when your calculated need falls between two standard sizes, choose the larger one if the room is sunny, has high ceilings, or regularly holds more than two people — otherwise the smaller size dehumidifies better and costs less to run.
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All math happens in your browser. Your room measurements are never uploaded or stored anywhere.

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Unlimited AC sizing calculations — free, no limits.

How to use the AC BTU calculator

  1. 1Measure your room

    Enter the length and width in feet, plus the ceiling height. The standard 8-foot ceiling needs no adjustment.

  2. 2Describe the room

    Pick sunlight exposure, room type, and insulation quality — a sunny kitchen needs far more cooling than a shaded bedroom.

  3. 3Count the occupants

    Enter how many people regularly use the room. Every person beyond the first two adds 600 BTU of body heat.

  4. 4Shop to the number

    Use the recommended size and the suggested range to pick a unit — the breakdown below the result shows exactly how each adjustment changed the number.

Why homeowners use this calculator

Standard 20 BTU rule, done right

Starts from the industry-standard 20 BTU per square foot, then adjusts for the factors that actually change cooling load — not a raw square-foot guess.

Transparent breakdown

Every adjustment is shown line by line, so you can see exactly how sunlight, ceilings, kitchens, people, and insulation moved your number.

Kitchen heat accounted for

Kitchens get a 4,000 BTU cooking-heat allowance — the most common reason a bedroom-sized unit fails in a kitchen.

A range, not just one number

ACs come in standard sizes, so you get a recommended size plus a workable band, making it easy to match what stores actually sell.

Full size reference table

The room-area table covers 100 to 1,000 sq ft, so you can sanity-check the result or size a second room without re-running the math.

Works on your phone

Measure the room, run the numbers on the spot, and screenshot the result before heading to the store.

AC BTU calculator FAQs

What size air conditioner do I need for a 12x15 room?
A 12x15 ft room is 180 sq ft. At the standard 20 BTU per sq ft, the base need is 3,600 BTU; a sunny bedroom with average insulation comes out to about 3,960 BTU, so a 4,000 BTU unit (or anything in the 3,500–4,500 BTU range) is the right pick. Type your exact details into the calculator above for your room.
How many BTU per square foot do I need?
The industry rule of thumb is 20 BTU per square foot for a standard 8-foot ceiling with average sunlight and insulation. Sunny rooms need about 10% more, shaded rooms about 10% less, and high ceilings add roughly 10% per extra foot.
Is a bigger BTU air conditioner better?
Not always. An oversized AC cools the room too fast and shuts off before removing humidity, leaving the air cold but clammy, and it cycles on and off more, which wastes energy. Stay within the suggested range above for the best comfort.
How much BTU does a kitchen need?
Kitchens need an extra 4,000 BTU on top of the room-size calculation, because ovens, stoves, and other appliances add significant heat. A 200 sq ft kitchen, for example, needs about 8,000 BTU instead of the 4,000 BTU a same-size bedroom would need.
What happens if my AC is too small?
An undersized unit runs nonstop on hot days, never reaches your set temperature, wears out faster, and still leaves the room uncomfortable. If your calculated need is close to the top of a size band, go up to the next standard size.
Does ceiling height change the BTU I need?
Yes. The 20 BTU per sq ft rule assumes an 8-foot ceiling. This calculator adds 10% for each foot above 8 feet, because taller rooms hold more warm air to cool. A 10-foot ceiling therefore adds about 20% to the base need.
How many BTU for 500 square feet?
A 500 sq ft room needs about 10,000 BTU at the base rule (500 × 20), moving to 12,000 BTU for a sunny room or one with poor insulation. Check the size table above or run your exact room through the calculator.
Do portable and window ACs use the same BTU ratings?
Yes, BTU ratings are comparable across window, portable, and mini-split units — but portable units lose some real-world efficiency through their exhaust hose, so when choosing a portable unit, pick the top of your suggested range.

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