How much power does an air conditioner need for a room?

The required capacity is calculated by using a guideline of about 20 BTU per hour and square foot. So for a room that measures 300 square feet, you would need approximately 6,000 BTU/hour. This number can be increased depending on the amount of sunlight, people in the room, cooking, ceiling height, and climate. Then choose the next highest standard size.

How many BTUs are needed per square foot?

For typical rooms the rule of thumb is 20 BTUs per hour and square foot assuming an eight-foot ceiling. Rooms with lots of sun or poor insulation may need more, while well-shaded and insulated rooms may require less.

What happens if the air conditioner is too big?

An air conditioner with too much capacity will cool the air quickly and turn off before it has removed moisture. This is called short cycling. The room will end up being cooler and more humid, which can cause the compressor to wear out faster and increase operating costs. It's better to choose an air conditioner with the right capacity for your needs.

How do you convert BTUs to tons?

Divide the number of BTUs per hour by 12,000. One ton of cooling is equal to 12,000 BTUs/hour. A unit with 24,000 BTUs/hour has a capacity of two tons while one with 6,000 BTUs/hour has half the capacity of one ton.

Does the required air conditioning capacity change depending on ceiling height?

Yes. The square footage is based on a standard ceiling height of 8 feet but the higher your ceilings are, the more air you will need to cool. If you select the option to customize your ceiling height, the estimated value will be adjusted accordingly for the increased volume of space.

Air Conditioner Room Size Calculator

Room details

Room length

Room width

Adjust for ceiling height

The base estimate assumes a standard 8 ft ceiling.

Adjust for sun exposure

A sunny room needs more cooling; a shaded room needs less.

Account for the number of people

Each person beyond two adds about 600 BTU per hour.

This room is a kitchen

Cooking appliances add roughly 4,000 BTU per hour of heat.

Fine-tune for climate and insulation

Adjust the estimate for your local climate and how well the room is insulated.

Enter your room's length and width to estimate the air conditioner size you need.

Your AC size

Required cooling capacity

Please input values into the fields

The room size calculator for air conditioners will show you the cooling capacity needed by your room in BTU per hour. Enter the dimensions of your room and answer some questions about sunlight, number of people, and layout to determine the cooling load and appropriate AC size.

It is important to select the right size. If the capacity is too small, even if it runs all the time, the air conditioner cannot cool the room enough. If the capacity is too large, it will blow cold air at first and then stop quickly. Since the running time is not long enough, dehumidification also cannot be done, which may make the room feel damp and cold.

How is air conditioner size determined?

The cooling capacity is expressed in British Thermal Units (BTUs) per hour. A BTU is the amount of energy required to raise one pound of water by one degree Fahrenheit. The higher the BTU rating, the more heat the air conditioner can remove from a space in an hour.

A general rule of thumb is 20 BTUs per hour and square foot. So for a room that's 300 square feet you would need about 6,000 BTUs/hour. This guideline assumes an eight-foot ceiling and normal environmental conditions, so the numbers may vary depending on your actual situation.

Here's how to use this calculator.

First enter the length and width of your room. With these two measurements you will get a clear estimate on how much power is required and what size air conditioner to recommend. The dimensions can be entered in feet or meters and the result units can be switched between BTU/hr, tons, and kilowatts.

Next, activate the appropriate customization options for your room. Take into account ceiling height, direct sunlight or shading, additional people, a kitchen, and local climate and insulation conditions as applicable. Each switch reflects that influence in the estimate so you can see how the numbers change.

Formula for calculating capacity:

This calculator combines a surface area estimate with standard adjustment factors.

BTU = 20 \times A \times \frac{h}{8} \times f_{sun} \times f_{climate} \times f_{ins} + 600 \times \max(0, P - 2) + 4000 \times K

A is the floor area in square feet, h is the ceiling height in feet and f is a factor that takes into account sunshine, climate and insulation. P is the number of people, K is 1 if there is a kitchen and 0 if there isn't one.

For example, a bedroom that is 20 feet long by 15 feet wide with an eight-foot ceiling and normal sunlight exposure would have a surface area of 300 square feet. This means the load would be 6,000 BTU/hr, which is the result of multiplying 20 by 300. Divided by 12,000, this gives an air conditioner size of half ton.

Symbol	Meaning	Example
A	Floor area (length times width)	300 sq ft
h	Ceiling height	8 ft
f (sun)	Sun exposure factor	1.1 for a sunny room
P	Number of people	2
K	Kitchen flag	0 or 1
BTU	Cooling load	6,000 BTU/hr

Recommended capacity according to room size:

The ENERGY STAR guidelines give recommended capacities based on floor area. As the values are rounded up to standard equipment capacities and slightly increased, the results may be somewhat larger than a calculation that is only based on area.

Room area	Suggested capacity
100 to 150 sq ft	5,000 BTU/hr
250 to 300 sq ft	7,000 BTU/hr
350 to 400 sq ft	9,000 BTU/hr
450 to 550 sq ft	12,000 BTU/hr
700 to 1,000 sq ft	18,000 BTU/hr
1,500 to 2,000 sq ft	30,000 BTU/hr

Why more capacity isn't always better.

Buying an air conditioner with a large capacity may seem appealing on hot days, but too much capacity can be counterproductive. When the unit cools down the air quickly and reaches the temperature set by your thermostat, it will turn off before enough time has passed to effectively dehumidify.

This pattern of repeated on-off cycling is known as "short-cycling." It results in a cooler and more humid room, accelerates compressor wear, and consumes more electricity than units with an appropriate capacity and longer, steadier operating cycles.

Factors that affect values:

Sunlight is an important factor. A room with large windows that receive direct sunlight in the afternoon may need about 10 percent more cooling capacity, while a shaded room can reduce it by 10 percent.

The number of people and electrical appliances also increase the heat load. For each person in addition to the first two, about 600 BTU/hr is added. In kitchens with a stove or oven, about 4,000 BTU/hr is added. These values are higher for high ceilings, mild climates, or thin insulation.

Tips for an accurate estimate:

Accurately measure the floor area and include any adjoining spaces where air circulates (e.g. open kitchens or hallways). Only use the adjustment factors that actually apply to the space; do not apply all of them at once.

For a full house or complex floor plans you should consider this as the starting point and hire an installer to do a Manual J load calculation which will take into account windows, doors and orientation in detail.

This sizing tool is for informational and planning purposes only. It estimates cooling capacity based on standard rules of thumb and does not replace a professional load calculation required for permanent installation.

Frequently asked questions

How much power does an air conditioner need for a room?: The required capacity is calculated by using a guideline of about 20 BTU per hour and square foot. So for a room that measures 300 square feet, you would need approximately 6,000 BTU/hour. This number can be increased depending on the amount of sunlight, people in the room, cooking, ceiling height, and climate. Then choose the next highest standard size.
How many BTUs are needed per square foot?: For typical rooms the rule of thumb is 20 BTUs per hour and square foot assuming an eight-foot ceiling. Rooms with lots of sun or poor insulation may need more, while well-shaded and insulated rooms may require less.
What happens if the air conditioner is too big?: An air conditioner with too much capacity will cool the air quickly and turn off before it has removed moisture. This is called short cycling. The room will end up being cooler and more humid, which can cause the compressor to wear out faster and increase operating costs. It's better to choose an air conditioner with the right capacity for your needs.
How do you convert BTUs to tons?: Divide the number of BTUs per hour by 12,000. One ton of cooling is equal to 12,000 BTUs/hour. A unit with 24,000 BTUs/hour has a capacity of two tons while one with 6,000 BTUs/hour has half the capacity of one ton.
Does the required air conditioning capacity change depending on ceiling height?: Yes. The square footage is based on a standard ceiling height of 8 feet but the higher your ceilings are, the more air you will need to cool. If you select the option to customize your ceiling height, the estimated value will be adjusted accordingly for the increased volume of space.

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Disclaimer: This calculator is provided for general informational and educational purposes only. Our calculators are under active development, and results may be inaccurate, incomplete, or unsuitable for your situation. Always verify the figures independently and seek advice from a qualified professional before relying on them. We make no warranties and accept no liability for any loss or decision arising from use of this tool.

References

ENERGY STAR: Room Air Conditioners
Sizing guidance and the area-to-capacity table for room air conditioners.
U.S. Department of Energy: Room Air Conditioners
Federal energy-saver guidance on sizing and choosing a room unit.