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AC Sizing Calculator

Get a rough idea of what size air conditioner you need, in BTU/h and tons. It uses your floor area, sun, number of people and whether there's a kitchen.

Level: Grades 6–7

Printable worksheet

AC Sizing Calculator

Inputs
Area units

One room, or the total conditioned floor area of a home.

Typical rule-of-thumb value: 20. Some guides use more for hot climates or high ceilings.

Sun exposure

Each person beyond two adds about 600 BTU/h.

Result

Estimated cooling capacity

8,000 BTU/h

A rule-of-thumb estimate is about 8,000 BTU/h (0.67 tons). A Manual J load calculation gives the professional figure.

In tons
0.67 tons
In kilowatts (cooling)
2.34 kW
Base load before adjustments
8,000 BTU/h
Area used
400 ft²

See the step-by-step solution ↓

Step-by-step solution

  1. Start with the base load: the floor area times the cooling needed for each square foot

    400×20=8,000 BTU/h

  2. Add 600 BTU/h of body heat for each person beyond the first two (0 extra)

    0×600=0 BTU/h

  3. Add it all up: the adjusted base plus the people (no kitchen to add)

    8,000+0+0=8,000 BTU/h

  4. Change to tons, the unit central systems are sold in (1 ton = 12,000 BTU/h)

    8,00012,000=0.666667 tons

Test yourself

20 quick questions built from this calculator's formula: work out results, predict what happens when an input changes, and work backward from an answer. Every answer comes with the working.

What this calculator does

Find out roughly how big an air conditioner your room or home needs. Air conditioners are rated by how much heat they can remove each hour. That's measured in BTU per hour (BTU/h); a BTU is a small unit of heat. Central systems are also sized in tons, and one ton is 12,000 BTU/h.

The calculator starts from a widely published rule of thumb: about 20 BTU/h for every square foot of floor. It then adjusts for sunlight, the number of people and whether the space includes a kitchen.

Use the answer as a starting point for shopping, or for a talk with a contractor. It isn't a design figure. In North America, professionals size home cooling with a Manual J load calculation. That checks climate, insulation, windows, air leaks, ceiling height and which way each room faces. A rule of thumb skips most of that, so it can be well off for very old or very efficient buildings.

Where it's used

Why this math matters outside the classroom:

  • Window units. A renter cooling a 400 ft² living room multiplies by 20 BTU/h per ft² to get 8,000 BTU/h before any sun or occupant adjustment.
  • Sunny rooms. A 300 ft² sunroom starts at 6,000 BTU/h, and the 10% sun allowance raises it to 6,600 BTU/h.
  • Tons for central air. Homeowners divide a 30,000 BTU/h load by 12,000 to see that it is 2.5 tons of cooling.
  • Busy rooms. A home office shared by four people adds 600 BTU/h for each of the two people above the first two, or 1,200 BTU/h.
  • Kitchen allowance. Cooling an open kitchen and dining area adds a flat 4,000 BTU/h to cover heat from the stove and oven.

How to use it

  1. Enter the floor area of the room, or of the whole space you want to cool, in square feet or square meters.
  2. Leave the base rate at 20 BTU/h per ft² unless you have a reason to change it, such as a guide for your climate.
  3. Choose how sunny the space is and enter how many people usually use it. Tick the box if it includes a kitchen.
  4. Read the estimate in BTU/h and tons. Then pick the nearest unit size that isn't far below it.

What the inputs mean

Floor area to cool
One room, or the total conditioned floor area of a home.
Base cooling per square foot
20 BTU/h per ft² is a typical rule-of-thumb figure. It fits an average room with standard ceilings in a mild climate. You can change it, because published guides vary, and hot, humid areas often call for more.
Sun exposure
A very sunny room, such as one with large south- or west-facing windows, gets 10% added. A room that's shaded most of the day gets 10% taken off.
People who regularly use the space
The rule already allows for two people. Each extra person who regularly uses the space adds about 600 BTU/h of body heat. Fewer than two people doesn't lower the estimate.
The space includes a kitchen (+4,000 BTU/h)
Stoves and ovens give off a lot of heat. So a space that includes a kitchen gets a flat 4,000 BTU/h added.

Formula and method

base load = floor area (ft²) × 20 BTU/h per ft²
adjusted = base load × 1.10 (very sunny), × 0.90 (shaded) or × 1.00 (average)
people = 600 × max(0, occupants − 2)
total BTU/h = adjusted + people + 4,000 (if there is a kitchen)
tons = total BTU/h ÷ 12,000

The base load (floor area × 20) stands in for the heat that comes in through the walls, roof and windows. The sun setting raises or lowers that base. People and the kitchen are added as fixed amounts, because their heat doesn't depend on the size of the room.

If you enter square meters, the calculator converts them with the exact factor 1 ft² = 0.09290304 m². One ton of cooling is exactly 12,000 BTU/h. The kilowatt figure uses 1 BTU = 1,055.05585262 joules, a joule being a standard unit of energy.

Worked example

A 350 ft² living room gets strong afternoon sun and is usually used by four people. It has no kitchen.

  1. Base load: 350 × 20 = 7,000 BTU/h.
  2. Very sunny: 7,000 × 1.1 = 7,700 BTU/h.
  3. Two people beyond the first two: 2 × 600 = 1,200 BTU/h.
  4. Total: 7,700 + 1,200 + 0 = 8,900 BTU/h.
  5. Tons: 8,900 ÷ 12,000 = 0.74 tons.

Result: The rule of thumb suggests about 8,900 BTU/h. So a room unit in the 9,000–10,000 BTU/h range would be a sensible place to start.

Understanding the result

Window and portable units come in steps such as 6,000, 8,000, 10,000 and 12,000 BTU/h. Central systems come in half-ton steps. Choose the nearest size.

Going much bigger isn't better. An oversized unit cools the air fast, shuts off, and leaves the dampness behind.

For a whole house, treat the result only as a quick check. If a contractor suggests a very different size, ask to see the load calculation behind it.

Assumptions and limitations

  • Ceilings are an ordinary height, about 8 feet (2.4 m).
  • Insulation, windows and sealing against drafts are typical for the area, so 20 BTU/h per ft² is a fair base rate.
  • People and the kitchen are added as fixed rule-of-thumb amounts.
  • This is a rule of thumb, not a Manual J load calculation. It doesn't account for your climate zone, insulation, window size and type, air leaks, or heat lost from the ducts.
  • High ceilings, top-floor rooms under a hot roof, and rooms with lots of electronics or lighting can need noticeably more cooling.
  • Very well insulated or shaded homes may need much less than the estimate.

Frequently asked questions

How many square feet does one ton of air conditioning cool?

At the rule-of-thumb rate of 20 BTU/h per ft², one ton (12,000 BTU/h) covers about 600 square feet of average space. Real homes vary a lot. A tight, modern house might need only one ton per 800 ft² or more. A sunny, poorly insulated one might need a ton for every 400–500 ft².

Is a bigger air conditioner better?

Not usually. An oversized unit reaches the thermostat setting quickly and switches off. It doesn't run long enough to pull moisture out of the air, so the room can feel cool but clammy. It also turns on and off more often, which wastes energy and wears out parts.

What is a Manual J calculation?

It's a detailed, room-by-room count of how much heat a home gains in summer and loses in winter. Heating and cooling (HVAC) professionals in North America use it. It looks at local design temperatures, meaning the hot and cold weather the system is planned around. It also looks at how the walls, roof and windows are built, air leaks and the number of people. It's the accepted way to choose the equipment size for a home.

How do I convert BTU/h to tons or kilowatts?

Divide BTU/h by 12,000 to get tons of cooling. For kilowatts, multiply BTU/h by about 0.000293. So one ton is about 3.52 kW of cooling.

How this calculator is checked

Method: Commonly published residential rule of thumb: 20 BTU/h per ft², ±10% for sun or shade, +600 BTU/h per occupant above two, +4,000 BTU/h for a kitchen; 1 ton = 12,000 BTU/h by definition. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.

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