AC BTU Calculator
What size air conditioner do you need? This free BTU calculator estimates the cooling capacity your room needs, based on square footage, ceiling height, sun exposure, occupants, kitchens, and insulation. It starts from the standard 20 BTU per square foot rule of thumb, applies each adjustment, and recommends the nearest standard AC size — in BTU and tons — so you don't buy a unit that's too weak or wastefully oversized.
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Enter your room details to see your AC size.
How to use this calculator
Measure your room's length × width to get the area in square feet, and enter your ceiling height (most rooms are 8 feet). Then pick your sun exposure — choose Sunny if the room gets several hours of direct sunlight through the windows, Shady if it's mostly shaded — and your insulation quality (Good for a well-insulated modern home, Poor for an older home with drafty windows).
Set the number of occupants who regularly use the room, and mark Yes for kitchen if you're sizing an AC for a kitchen or an open-plan room that includes one. The results update instantly as you change anything.
The calculator shows the BTU capacity your room actually needs, the nearest standard AC size to shop for, and the equivalent size in tons. If your needs exceed 24,000 BTU, consider two smaller units or a central air system instead of one giant unit.
How it works
The industry rule of thumb is 20 BTU per square foot for a room with a standard 8-foot ceiling and average sun. The calculator starts with area × 20, then scales it by your ceiling height (× height ÷ 8) and your sun exposure (× 0.9 shady, × 1.1 sunny). Each extra person beyond two adds 600 BTU of body heat, and a kitchen adds 4,000 BTU for appliances. Finally, insulation quality multiplies the total (× 1.15 poor, × 0.9 good), and the required capacity is rounded up to the nearest 100 BTU.
ACs are only sold in standard sizes — 5,000, 6,000, 8,000, 10,000, 12,000, 14,000, 18,000, and 24,000 BTU — so the calculator recommends the smallest standard size that covers your required BTU, rounding up because a unit can't do work it isn't rated for.
Why not just buy the biggest unit? An oversized air conditioner cools the room so fast that it shuts off before running long enough to remove humidity — a problem called short-cycling. The result is a cold but clammy room, uneven temperatures, more wear on the compressor, and higher energy bills. Aim for the recommended size, not the maximum.
Estimates: this is a sizing estimate, not an engineering calculation. Vaulted ceilings, large glass doors, poor ductwork, and extreme climates all shift real-world needs — for whole-home systems, get a professional Manual J load calculation.
Frequently asked questions
What size AC do I need for 500 sq ft?
A 500 sq ft room needs roughly 10,000 BTU using the common 20 BTU per square foot rule of thumb (500 × 20 = 10,000). Adjust upward for sunny rooms, high ceilings, kitchens, or extra occupants, and downward for shade or good insulation. The calculator above applies all of these adjustments automatically and rounds up to the nearest standard unit size.
Is a bigger BTU air conditioner always better?
No. An oversized air conditioner cools the room so quickly that it shuts off before running long enough to remove humidity — a problem called short-cycling. The result is a cold but clammy room, uneven temperatures, and higher energy bills. It is better to size the unit as close to your calculated BTU needs as possible.
How many BTUs per square foot do I need?
About 20 BTUs per square foot is the standard baseline for a room with an 8-foot ceiling and average sun exposure. Taller ceilings, sunny windows, kitchens, extra people, and poor insulation can push that number higher, while shade and good insulation push it lower.
What does BTU mean?
BTU stands for British Thermal Unit. For air conditioners, a unit's BTU rating measures how much heat it can remove from a room in one hour — so a higher BTU rating means more powerful cooling capacity.
How many BTUs are in a ton of cooling?
One ton of cooling equals 12,000 BTU per hour. The term comes from the amount of heat needed to melt one ton of ice in 24 hours. Central air systems are usually described in tons, while window and portable units use BTU directly.