AC Tonnage Calculator
Find out what size air conditioner you need. This AC tonnage calculator gives the cooling capacity in BTU and tons for a whole house with central air, or for a single room using the ENERGY STAR sizing chart for window, portable and mini-split units.
Your results will appear here.
How to use the AC tonnage calculator
- Choose whole house or one room. Use Whole house for central air and ducted heat pumps. Use One room for a window unit, portable AC or a single mini-split head.
- Enter the area to cool in square feet. Multiply length by width for each room and add them together.
- Set the ceiling height. Rooms taller than 8 ft have more air to cool, so the calculator scales the result up.
- Describe the space. Pick your climate, sun exposure and insulation for a house, or the number of people and whether the room is a kitchen.
BTU and tons explained
Air conditioner capacity is measured in BTU per hour. One ton of cooling equals 12,000 BTU per hour. Central AC units are sold in half-ton steps, usually from 1.5 to 5 tons; 4.5-ton units are uncommon, so the calculator moves from 4 tons straight to 5.
Tons = BTU per hour ÷ 12,000
Whole-house rule of thumb
For a whole house, the calculator multiplies your area by a cooling load per square foot for your climate, then adjusts for ceiling height, sun and insulation:
| Climate | BTU per sq ft | Square feet per ton |
|---|---|---|
| Cool (northern states) | 18 | about 667 |
| Moderate (Mid-Atlantic, Midwest) | 20 | 600 |
| Warm (Southeast, inland California) | 25 | 480 |
| Hot (Gulf Coast, Southwest desert) | 30 | 400 |
These loads are HomeQuantly rule-of-thumb values, not a substitute for a professional load calculation.
AC size by square footage
Results from this calculator with 8 ft ceilings, average sun and average insulation:
| Home size | Cool | Moderate | Warm | Hot |
|---|---|---|---|---|
| 1,000 sq ft | 1.5 tons | 2 tons | 2.5 tons | 2.5 tons |
| 1,500 sq ft | 2.5 tons | 2.5 tons | 3.5 tons | 4 tons |
| 2,000 sq ft | 3 tons | 3.5 tons | 5 tons | 5 tons |
| 2,500 sq ft | 4 tons | 5 tons | 2 systems | 2 systems |
Room air conditioner sizing chart
For a single room, the calculator uses the ENERGY STAR room air conditioner chart, which assumes 8 ft ceilings:
| Room area | Capacity | Room area | Capacity |
|---|---|---|---|
| 100 – 150 sq ft | 5,000 BTU | 450 – 550 sq ft | 12,000 BTU |
| 150 – 250 sq ft | 6,000 BTU | 550 – 700 sq ft | 14,000 BTU |
| 250 – 300 sq ft | 7,000 BTU | 700 – 1,000 sq ft | 18,000 BTU |
| 300 – 350 sq ft | 8,000 BTU | 1,000 – 1,200 sq ft | 21,000 BTU |
| 350 – 400 sq ft | 9,000 BTU | 1,200 – 1,400 sq ft | 23,000 BTU |
| 400 – 450 sq ft | 10,000 BTU | 1,400 – 1,500 sq ft | 24,000 BTU |
| — | — | 1,500 – 2,000 sq ft | 30,000 BTU |
| — | — | 2,000 – 2,500 sq ft | 34,000 BTU |
ENERGY STAR recommends these adjustments, which the calculator applies:
- Heavily shaded room: reduce capacity by 10%.
- Very sunny room: increase capacity by 10%.
- More than two people regularly use the room: add 600 BTU for each extra person.
- Kitchen: add 4,000 BTU.
Example: a sunny 300 sq ft bedroom used by three people needs 7,000 × 1.10 + 600 = 8,300 BTU. The calculator rounds up to 8,500 BTU, so a 9,000 BTU unit would be a good fit.
Why the right size matters
An undersized air conditioner runs constantly and still cannot keep up on the hottest days. An oversized unit cools the air quickly and shuts off before it removes enough humidity, which leaves rooms cool but clammy and causes more wear from frequent starts.
For central air, ask your HVAC contractor for a Manual J load calculation. It accounts for insulation, windows, orientation, air leaks and local weather, and it is the standard way to size a system.
Frequently asked questions
What size AC do I need for a 1,500 sq ft house?
In a moderate climate with average insulation, this calculator suggests about 2.5 tons (30,000 BTU) for 1,500 sq ft. Hot climates may need 3.5 to 4 tons, and cool climates can often use 2.5 tons or less.
What size AC do I need for a 2,000 sq ft house?
This calculator suggests about 3 tons in a cool climate, 3.5 tons in a moderate climate and 5 tons in warm or hot climates for 2,000 sq ft with average insulation. A Manual J calculation should confirm the size.
How many BTU is a ton of air conditioning?
One ton of cooling equals 12,000 BTU per hour. So a 3-ton unit provides 36,000 BTU per hour.
What size window AC do I need for a 12 x 12 room?
A 12 x 12 ft room is 144 sq ft, which falls in the ENERGY STAR range for a 5,000 BTU unit. Add 10% for a very sunny room and 600 BTU for each person beyond two.
Is it bad to oversize an air conditioner?
Yes. An oversized unit short-cycles, which removes less humidity, wastes energy and can shorten the life of the compressor. Choose the closest size at or slightly above the calculated load.
Sources and methodology
The formulas on this page were written and checked by the HomeQuantly Team against the worked examples shown above. Results are estimates for planning; always confirm measurements on site before you buy materials.