BTU Calculator

Before using the result

Use this calculator to estimate a practical air conditioner size before you compare portable, window, or split systems. The result is a starting point, not a guarantee. Real cooling depends on insulation, outdoor heat, humidity, window leakage, and how often doors are opened.

Start with the ENERGY STAR room AC sizing table, then adjust for your room. This estimates room cooling capacity; portable AC ratings and whole-home systems need separate checks.

After estimating size, compare installation constraints with theair conditioner type finder and estimate running cost with theelectricity cost calculator.

Method and limits

The base capacity, sun, extra-person and kitchen adjustments followENERGY STAR room AC sizing guidance(checked October 6, 2026). The chart assumes an 8 ft ceiling. At shared area boundaries, this calculator uses the lower band; consider room conditions before buying. Our extra 10% per foot above 8 ft is a planning assumption, not an ENERGY STAR formula. Heat-producing equipment is not measured here; the kitchen toggle adds a fixed planning allowance.

For example, a 200 sq ft room starts at 6,000 BTU/h and a 500 sq ft room starts at 12,000 BTU/h before adjustments. Read the product's capacity rating and installation requirements; BTU/h is cooling output, not electrical input watts.

How to interpret the BTU result

A BTU estimate should be treated as a buying range, not a promise that one exact number will cool every version of the room. A shaded bedroom with good insulation can often use the lower end of a range, while an upper-floor room with afternoon sun, poor curtains, old windows, or electronics running all day may need the higher end. Humidity also matters because comfort depends on moisture removal as well as air temperature.

Avoid automatically choosing the largest unit you can afford. Oversized air conditioners can cool the air quickly but cycle off before removing enough humidity, which can leave the room cold and clammy. Undersized units have the opposite problem: they run for long periods, use more electricity than expected, and still may not reach the setpoint during heat waves.

Match the BTU result to the air conditioner type. Portable units often need more caution because hose heat, window leakage, and negative pressure can reduce effective cooling. Window and split systems usually perform closer to their rated capacity when installed correctly. If the estimate sits between two common sizes, decide based on sun exposure, noise sensitivity, insulation, and whether the room is used for sleep.

For safety, also check the electrical circuit, plug type, manufacturer clearance rules, and drainage requirements before buying. A correct BTU number does not override building rules, landlord restrictions, or professional installation requirements.

If the room is part of an open-plan space, measure the connected area that air can move through, not only the corner where the unit will sit. Doorways, stairwells, open kitchens, and large glass walls can turn a simple bedroom estimate into a larger cooling problem. When the room cannot be isolated, expect slower cooling and higher running cost.

Recheck the number when your use case changes. A guest room, nursery, home office, or studio with new equipment can need a different cooling strategy even if the floor area is unchanged.

Use the result as a planning number, then verify the actual product manual, installation clearance, drainage path, and circuit requirements.