WorldCalcs

BTU Calculator

What size AC or heater do I need? Sizing in BTU per hour.

Reviewed by the WorldCalcs team · Methodology · Last reviewed: June 2026

Recommended cooling capacity

6 000 BTU/hr

(about 0.50 tons)

Estimates only — these are rule-of-thumb figures, not a professional load calculation. For a permanent heating or cooling system, get a Manual J assessment. See our full disclaimer.

Energy Star room air conditioner sizing

Room sizeCapacity
100–150 sq ft5 000 BTU
150–250 sq ft6 000 BTU
250–300 sq ft7 000 BTU
300–350 sq ft8 000 BTU
350–400 sq ft9 000 BTU
400–450 sq ft10 000 BTU
450–550 sq ft12 000 BTU
550–700 sq ft14 000 BTU
700–1 000 sq ft18 000 BTU

What is a BTU calculator?

A BTU calculator estimates the heating or cooling power a room needs, measured in BTU per hour (British Thermal Units). For cooling, it tells you roughly what size window or portable air conditioner suits a room. For heating, it estimates the output a heater needs to keep the room warm. Choosing a unit that is too small leaves the room uncomfortable, while one that is too large cycles on and off and wastes energy, so a sensible estimate is a good starting point.

How it's calculated

Cooling starts from a common guideline of about 20 BTU per square foot of floor area, then adjusts for conditions the way Energy Star recommends: add 10% for a very sunny room or subtract 10% for a heavily shaded one, add 600 BTU for each person beyond two, and add 4,000 BTU for a kitchen, where appliances give off extra heat. Heating uses a climate factor instead — colder regions need more BTU per square foot, typically from about 35 in hot climates up to 60 in very cold ones. These are rules of thumb that assume standard 8-foot ceilings; a professional Manual J load calculation is the accurate method for a permanent system.

Example

A 300 square foot living room with normal sun and two people needs about 300 × 20 = 6,000 BTU of cooling, which is roughly half a ton. Make that room very sunny with four people and a kitchen, and it becomes 6,000 × 1.10 = 6,600, plus 1,200 for the two extra people, plus 4,000 for the kitchen — about 11,800 BTU. To heat the same 300 square foot room in a moderate climate at 45 BTU per square foot, you need about 13,500 BTU.

Related: Square Footage Calculator, Paint Calculator, Home & Garden Calculators.

All calculations happen in your browser. Nothing is sent, stored, or tracked.

Results are estimates and may contain errors — for general information only, not professional advice. Always verify before relying on them. Disclaimer

How to use

Pick cooling or heating, choose square feet or square meters, and enter the room size. For cooling, set the sun exposure, how many people use the room, and whether it's a kitchen. For heating, pick the climate. The result is a recommended capacity in BTU per hour.

Frequently asked questions

How many BTU do I need to cool a room?+

A common guideline is about 20 BTU per square foot, so a 300 square foot room needs roughly 6,000 BTU. Adjust up for sunny rooms, extra people or a kitchen, and down for shaded rooms.

How many BTU per square foot do I need?+

For cooling, about 20 BTU per square foot is the usual starting point. For heating it depends on climate, ranging from about 35 BTU per square foot in hot regions to 60 in very cold ones.

What size air conditioner do I need?+

Match the room size to capacity: roughly 6,000 BTU for up to 250 square feet, 8,000 BTU for around 300–350, 10,000 BTU for 400–450, and 12,000–14,000 BTU for 550–700 square feet. The chart above shows Energy Star's recommended sizes.

How many BTU do I need to heat a room?+

Multiply the floor area by a climate factor — about 35 BTU per square foot in hot climates up to 60 in very cold ones. A 300 square foot room in a moderate climate needs about 13,500 BTU.

How many BTU are in a ton of cooling?+

One ton of air conditioning equals 12,000 BTU per hour. So a 24,000 BTU unit is a 2-ton system. The term comes from the cooling power of a ton of melting ice.

Does a sunny room need more BTU?+

Yes. Energy Star suggests increasing the estimate by about 10% for a room that gets a lot of direct sun, and decreasing it by about 10% for a heavily shaded room.

How many extra BTU does a kitchen need?+

Add about 4,000 BTU for a kitchen, because the stove, oven and refrigerator release extra heat that the air conditioner has to remove.

How many BTU do I need for 500 square feet?+

At about 20 BTU per square foot, a 500 square foot room needs roughly 10,000 BTU for cooling under normal conditions, before adjusting for sun, occupants or a kitchen.

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