AC Size Calculator
Enter the room size in square feet and a few details about it. The calculator gives the cooling load in BTU per hour, the nearest standard unit size and the equivalent in tons and kilowatts.
- BTU and tons
- Sun and kitchen adjustments
- Standard unit sizes
For window, portable and single-room mini split units. Whole-house systems need a Manual J load calculation.
Bigger is not better with air conditioning. An oversized AC cools the air quickly and shuts off before it removes the moisture, so the room feels cold and damp. An undersized one runs all day and never catches up. Matching BTU to the room gives steady, dry, comfortable air.
AC size chart by room area
The ENERGY STAR room sizing guide. Find your room size to see the BTU you need.
| Room area (sq ft) | BTU per hour | Tons | Typical room |
|---|---|---|---|
| 100 to 150 | 5,000 | 0.42 | Small bedroom |
| 150 to 250 | 6,000 | 0.5 | Bedroom |
| 250 to 300 | 7,000 | 0.58 | Large bedroom |
| 300 to 350 | 8,000 | 0.67 | Home office |
| 350 to 400 | 9,000 | 0.75 | Living room |
| 400 to 450 | 10,000 | 0.83 | Living room |
| 450 to 550 | 12,000 | 1 | Open plan |
| 550 to 700 | 14,000 | 1.17 | Large lounge |
| 700 to 1,000 | 18,000 | 1.5 | Studio flat |
| 1,000 to 1,200 | 21,000 | 1.75 | Small apartment |
| 1,200 to 1,400 | 23,000 | 1.92 | Apartment |
| 1,400 to 1,500 | 24,000 | 2 | Small house zone |
These figures assume an 8 ft ceiling, normal sun and two people. Adjust with the calculator for anything different.
Room size and cooling load
Sun and cooking heat shift the answer. Slide the room size and compare four situations.
A west-facing room in afternoon sun can need 10% more cooling than the same room in shade. A kitchen adds about 4,000 BTU because of the heat from cooking.
How to size an air conditioner
Five quick steps. Follow them in order.
Measure the floor
Multiply length by width in feet. 20 × 17.5 ft = 350 sq ft.
If the room has large single-glazed windows or a poorly insulated roof above it, round up to the next unit size.
Window, portable or mini split
Same BTU, different results. Compare the common room units.
Sits in a window and vents directly outside. Good value and efficient for one room.
Rolls between rooms and vents through a hose. Easy to install, least efficient.
Indoor head and outdoor unit linked by pipes. Quiet, efficient and can also heat.
Portable units with one hose pull warm outside air into the room to replace the air they exhaust, so they often deliver 30% to 40% less cooling than their label suggests.
Before you buy an AC
Check the room and the power supply. Tick these off.
Tick each item as you go.
Units over about 12,000 BTU often need a dedicated 20 A circuit or a 240 V outlet. Check the plug type before you buy.
What the calculator tells you
Four ways to express the same cooling. Each tile explains one result.
The cooling load for your room.
The next standard size to buy.
BTU divided by 12,000.
For comparing with metric spec sheets.
Sun, people and cooking all included.
One ton of cooling is 12,000 BTU per hour, the heat needed to melt a ton of ice in a day. It is the unit most HVAC contractors use for central systems.
Frequently Asked Questions
No question matches that search.
How many BTU do I need for a 12 x 12 room?
144 sq ft needs about 5,000 BTU. Round up to 6,000 if the room gets strong afternoon sun.
What size AC for 500 square feet?
About 10,000 to 12,000 BTU, depending on sun and how many people use the room.
How many square feet does a 1 ton AC cool?
Roughly 450 to 600 sq ft for a single room with an 8 ft ceiling.
What happens if my AC is too big?
It cycles on and off too often, leaves the air damp and wears out faster.
Do high ceilings need a bigger AC?
Yes. A 10 ft ceiling has 25% more air than an 8 ft ceiling, so add about 25%.
How many BTU per person should I add?
About 600 BTU for each person above two who regularly uses the room.
Is BTU the same as BTU per hour?
On AC labels, BTU means BTU per hour. It is the rate of heat the unit removes.
Cool the room, not your wallet
Measure the floor, adjust for ceiling, sun, people and cooking, then buy the nearest standard size. A correctly sized unit runs longer cycles, removes more moisture and costs less to run than a bigger one.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.