Air Conditioner BTU Calculator
This calculator sizes a room air conditioner from the well known 20 BTU per hour per square foot rule, then refines it. Enter the room length and width, and adjust for ceiling height, sun exposure, the number of occupants, and whether the space is a kitchen. It reports the base load, a recommended capacity rounded to a tidy 100 BTU, and the equivalent tonnage.
How to size a room air conditioner
- Enter the room length and width in feet.
- Set the ceiling height in feet (the rule assumes 8; taller rooms scale up).
- Pick the sun exposure from the menu: Normal, Sunny, or Shaded.
- Enter how many people regularly use the room (the baseline is 2).
- Tick the Kitchen box if the space has cooking appliances.
- Read the recommended BTU per hour and the tons, then Copy result.
Examples
15 by 20 ft living room, standard conditions
300 sq ft, 8 ft ceiling, normal sun, 2 occupants, not a kitchen
Base 6,000 BTU, recommended 6,000 BTU per hour, about 0.5 tons
12 by 12 ft sunny bedroom with an extra person
144 sq ft, 8 ft ceiling, sunny, 3 occupants, not a kitchen
Base 2,900 BTU, recommended 3,800 BTU per hour, about 0.32 tons
14 by 16 ft kitchen with 9 ft ceilings
224 sq ft, 9 ft ceiling, normal sun, 2 occupants, kitchen
Base 5,000 BTU, recommended 9,000 BTU per hour, about 0.75 tons
Frequently asked questions
How many BTU do I need per square foot?
The common rule is 20 BTU per hour per square foot of floor area. A 300 square foot room starts at about 6,000 BTU, and the tool then adjusts for ceiling height, sun, people, and heat sources.
How does the calculator combine all the adjustments?
It starts from area times 20 BTU, scales that by ceiling height over 8 feet, multiplies by a sun factor, then adds a flat amount for extra people and a kitchen. A sunny 12 by 12 room with a third occupant goes 2880, times 1.1 for sun is 3168, plus 600 for the extra person is 3768, rounded to 3,800 BTU.
Why does sun exposure change the size?
A room with heavy sun gains extra heat, so the tool multiplies the load by 1.1, roughly 10 percent more. A heavily shaded room multiplies by 0.9, about 10 percent less. A normal room is left unchanged.
How much does each extra person or a kitchen add?
Occupancy above the baseline of two people adds 600 BTU per hour each, and ticking Kitchen adds a flat 4,000 BTU for appliance heat. These are added after the sun adjustment, not multiplied by it.
Do high ceilings need more cooling?
Yes. The rule assumes 8 foot ceilings. The tool multiplies the base load by your ceiling height divided by 8, so a 9 foot ceiling raises the load by 12.5 percent because it holds more air to cool.
How many BTU are in a ton of cooling?
One ton equals 12,000 BTU per hour, so a 24,000 BTU unit is a 2 ton air conditioner. The tool divides the recommended BTU by 12,000 to report the tonnage.
Is a bigger air conditioner always better?
No. An oversized unit cools the air fast then shuts off before it pulls out much humidity, leaving the room cold and clammy and cycling on and off. Right sizing keeps the air dry and the run times steady.
How accurate is this compared to a Manual J load calculation?
This is a rule of thumb for quick planning, not a substitute for an ACCA Manual J calculation. Manual J accounts for insulation, window type, air leakage, climate zone, and orientation, which this tool does not, so use it to get in the right ballpark before a contractor sizes the job.
Is anything I enter sent to a server?
No. The sizing runs in your browser, so the room details you type stay on your device and are never uploaded.
Learn more
- Why an oversized air conditioner leaves a room clammy
How short cycling ruins comfort, why bigger is not better for AC sizing, and how to read the BTU rule this tool applies.
Related tools
Concrete Calculator
Calculate concrete for a slab or footing. Enter length, width and thickness to get cubic yards plus 60 and 80 lb bag counts, with a waste allowance.
Paint Calculator
Estimate how many gallons of paint you need for a room or wall. Enter dimensions, coats, doors and windows to get paintable area and gallons.
Flooring Calculator
Find how many boxes of flooring you need for a room. Enter length, width, box coverage and waste to get square footage, boxes and total cost.
Appliance Energy Calculator
Find what an appliance costs to run from its wattage. Enter watts, hours per day and your rate to see kWh and cost per day, month and year.
Aquarium Volume Calculator
Calculate your fish tank volume in gallons and liters from inside dimensions in inches. Set a fill level to estimate the actual water it holds.
Baseboard Calculator
Estimate linear feet and pieces of baseboard or trim for a room. Enter length and width, subtract doorways, set piece length and add waste.