AC BTU Calculator

An AC BTU calculator estimates the cooling capacity, measured in British thermal units (BTU) per hour, that a room air conditioner needs for a room of a given size. A BTU is a unit of heat energy. For air conditioners, it describes how much heat a unit can remove from a room in one hour.

Choosing the right BTU rating matters. An undersized air conditioner runs constantly and may never reach a comfortable temperature. An oversized one cools the room quickly and shuts off before it removes enough humidity from the air, leaving the room cold and damp. ENERGY STAR also notes that an oversized unit costs more to buy and wastes energy.

How to calculate BTU for an air conditioner

This calculator follows the ENERGY STAR room air conditioner sizing chart. ENERGY STAR is a U.S. government program that rates the energy use of appliances. Its chart matches a room's floor area to a cooling capacity.

The calculation has three steps:

Multiply the room's length by its width to get the floor area in square feet. Metric dimensions are converted to square feet first (1 square foot = 0.09290304 square meters).
Find the floor area in the chart and read the capacity.
Adjust the capacity for sun, people and kitchen use.

ENERGY STAR calculates its chart for an 8-foot ceiling. The chart does not use ceiling height, so the calculator does not ask for it. ENERGY STAR suggests a higher-capacity unit for rooms with taller ceilings.

ENERGY STAR sizing chart

Floor area (sq ft)	Capacity (BTU per hour)
100 up to 150	5,000
150 up to 250	6,000
250 up to 300	7,000
300 up to 350	8,000
350 up to 400	9,000
400 up to 450	10,000
450 up to 550	12,000
550 up to 700	14,000
700 up to 1,000	18,000
1,000 up to 1,200	21,000
1,200 up to 1,400	23,000
1,400 up to 1,500	24,000
1,500 up to 2,000	30,000
2,000 up to 2,500	34,000

Each row includes its lower bound and stops just below its upper bound. A room of exactly 150 square feet uses the 6,000 BTU row. A room of exactly 2,500 square feet uses the last row.

Rooms outside the chart

The chart starts at 100 square feet and ends at 2,500 square feet. The calculator handles other sizes as follows:

Under 100 square feet: the calculator shows 5,000 BTU, the smallest size on the chart, with a note.
Over 2,500 square feet: the calculator gives no size and shows a note. A single room unit is not meant for that much space. Large spaces are usually cooled by a central system sized by an HVAC professional.

Adjustments

ENERGY STAR lists four adjustments to the chart capacity. The calculator applies all of them:

Heavily shaded room: subtract 10 percent.
Very sunny room: add 10 percent.
More than two people: add 600 BTU for each person beyond two who regularly uses the room.
Kitchen: add 4,000 BTU, because ovens and stovetops add heat.

ENERGY STAR does not say in which order to combine them. The calculator applies the sun percentage to the chart capacity first. It then adds the fixed amounts for people and the kitchen. The result is rounded to the nearest whole BTU.

Formula

$\text{BTU} = \text{Chart capacity} \times \text{Sun factor} + 600 \times \max(0,\ \text{People} - 2) + \text{Kitchen}$

Variables

Chart capacity: the value from the sizing chart for the room's floor area.
Sun factor: 0.9 for a heavily shaded room, 1.0 for average sun, 1.1 for a very sunny room.
People: the number of people who regularly use the room. Fractions are rounded down.
Kitchen: 4,000 if the unit cools a kitchen, otherwise 0.

Examples

Bedroom. A bedroom is 12 feet by 10 feet, with average sun and two people.

Floor area = 12 × 10 = 120 square feet.
The chart row for 100 up to 150 square feet gives 5,000 BTU.
No adjustments apply. The result is 5,000 BTU.

Sunny living room. A living room is 15 feet by 20 feet. It is very sunny, and four people use it.

Floor area = 15 × 20 = 300 square feet, which falls in the 300 up to 350 row: 8,000 BTU.
Sun: 8,000 × 1.1 = 8,800 BTU.
People: 2 extra people × 600 = 1,200 BTU.
Result: 8,800 + 1,200 = 10,000 BTU.

Shaded kitchen. A kitchen is 12 feet by 14 feet and heavily shaded, with two people.

Floor area = 12 × 14 = 168 square feet, in the 150 up to 250 row: 6,000 BTU.
Shade: 6,000 × 0.9 = 5,400 BTU.
Kitchen: 5,400 + 4,000 = 9,400 BTU.

Metric room. A room is 4 meters by 5 meters, with no adjustments.

Floor area = 20 square meters, or about 215.3 square feet.
The 150 up to 250 row gives 6,000 BTU.

AC size categories

After the BTU figure, the calculator names a size category:

Small (5,000-8,000 BTU): bedrooms, small home offices, dorm rooms.
Medium (8,000-12,000 BTU): master bedrooms, studio apartments.
Large (12,000-18,000 BTU): living rooms, larger bedrooms.
Extra large (18,000-24,000 BTU): big living rooms, finished basements.
Very large (24,000+ BTU): open floor plans and other big spaces.

A result exactly on a boundary goes in the higher category. For example, 8,000 BTU is Medium.

Where the BTU rule comes from

The BTU is the heat needed to raise the temperature of one pound of water by one degree Fahrenheit. It began as a unit for heating and carried over into air conditioning.

Modern air conditioning traces back to Willis Carrier. In 1902 he designed a system to control humidity at a Brooklyn printing company, not for comfort. Window units went on sale in 1932 but were costly. Sales grew after World War II, and by the late 1960s most new US homes had central air conditioning.

A chart like ENERGY STAR's is a shortcut for one room. For an entire house, HVAC contractors use a more detailed method called Manual J, published by the Air Conditioning Contractors of America. It accounts for wall and window insulation, window direction, local climate and duct losses.

Frequently asked questions

How many BTU does a 10-by-10-foot room need?

The floor area is 100 square feet. That is the first row of the ENERGY STAR chart, so the room needs 5,000 BTU. A sunny room would need 5,500 BTU.

What size room can a 12,000 BTU air conditioner cool?

The ENERGY STAR chart pairs 12,000 BTU with 450 up to 550 square feet. Sun, extra people or a kitchen lower the area that a 12,000 BTU unit can cover.

Does ceiling height change the result?

The calculator does not use ceiling height. ENERGY STAR bases its chart on an 8-foot ceiling. For a taller ceiling, it suggests choosing a larger unit.

What happens if an air conditioner has too few BTU?

It runs continuously and may never reach the target temperature on hot days. Constant running also puts extra strain on the compressor.

Is it bad to buy a unit with too many BTU?

Yes. An oversized unit cools the room quickly and shuts off before it removes enough moisture from the air. The room then feels cold and damp.

What is a "ton" of air conditioning?

One ton of cooling equals 12,000 BTU per hour. The term dates to before mechanical refrigeration, when buildings were cooled with blocks of ice. Melting one ton of ice over 24 hours absorbs heat at a rate of about 12,000 BTU per hour.

Formula in code

1const CHART = [
2  [100, 5000], [150, 6000], [250, 7000], [300, 8000], [350, 9000],
3  [400, 10000], [450, 12000], [550, 14000], [700, 18000], [1000, 21000],
4  [1200, 23000], [1400, 24000], [1500, 30000], [2000, 34000],
5];
6
7function sizeRoomAC(areaSqFt, sun = 'average', people = 2, kitchen = false) {
8  if (areaSqFt > 2500) return null; // beyond the chart
9  let btu = 5000; // also used below 100 sq ft
10  for (const [minArea, capacity] of CHART) {
11    if (areaSqFt >= minArea) btu = capacity;
12  }
13  if (sun === 'shaded') btu *= 0.9;
14  if (sun === 'sunny') btu *= 1.1;
15  btu += Math.max(0, Math.floor(people) - 2) * 600;
16  if (kitchen) btu += 4000;
17  return Math.round(btu);
18}
19

References

ENERGY STAR (U.S. EPA). Room Air Conditioners. ENERGY STAR room air conditioners — sizing chart from 100 to 2,500 square feet, based on an 8-foot ceiling; adjustments for shade, sun, extra occupants and kitchens; oversized units leave rooms damp and waste energy.
U.S. Energy Information Administration. British thermal units (Btu). EIA: British thermal units — definition of the Btu.
U.S. Department of Energy. History of Air Conditioning. DOE: History of Air Conditioning — Willis Carrier's 1902 humidity-control system in Brooklyn; spread of home air conditioning.
Air Conditioning Contractors of America. Technical Manuals. ACCA technical manuals — Manual J residential load calculation.
Air Conditioning Contractors of America. A Little History Behind "Tons" of Air Conditioner Capacity. ACCA HVAC blog — one ton of cooling equals 12,000 BTU per hour, from melting a ton of ice over 24 hours.

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