A British Thermal Unit (BTU) is the amount of energy needed to raise one pound of water by 1°F. HVAC equipment is rated in BTU/hr — the rate at which it adds or removes heat. Residential units typically range from 5,000 to 60,000 BTU/hr.

How many BTUs do I need per square foot?

A common rule of thumb is about 20 BTU/hr per square foot for cooling. The ENERGY STAR room air conditioner chart lists 6,000 BTU/hr for 150 to 250 sq ft, 10,000 for 400 to 450 sq ft and 14,000 for 550 to 700 sq ft. This calculator adds up heat through the walls, ceiling, floor and windows instead, and shows the ENERGY STAR figure next to its result. Window area moves the cooling load the most, so enter the real value when you know it.

How do I convert BTU/hr to tons of cooling?

Divide BTU/hr by 12,000. One ton of cooling equals 12,000 BTU/hr. A 24,000 BTU/hr unit is a 2-ton unit.

Why are cooling and heating BTUs different?

Cooling load includes solar heat gain through windows and heat from occupants and appliances, which can push the cooling BTU higher than the heating BTU in sunny climates. Heating load depends mainly on the temperature difference between inside and outside and the insulation quality of the building envelope.

Should I buy a unit slightly larger than the calculated BTU?

A modest 10–15% buffer is reasonable, but avoid oversizing significantly. Equipment that is too large short-cycles, which reduces efficiency, increases wear, and causes humidity problems in cooling mode. Size close to the calculated load.

How accurate is this calculator?

This tool uses simplified Manual J-style rules of thumb. It is suitable for pre-sizing and product research but is not a substitute for a full ACCA Manual J calculation performed by a licensed HVAC engineer, which accounts for local design temperatures, infiltration rates, duct losses, and more.

BTU Calculator

Calculate the BTU heating or cooling requirement for a room based on its dimensions, insulation, windows, occupants, and climate.

Room Details

Room Length (ft)

Room Width (ft)

Ceiling Height (ft)

Total Window Area (sq ft)

Insulation Quality

Sun Exposure

Number of Occupants

Climate Zone

BTU Requirement

Cooling (AC)

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BTU/hr

Heating

—

BTU/hr

Cooling Load Breakdown

Component	BTU/hr

Summary

The BTU Calculator estimates how many British Thermal Units per hour (BTU/hr) are needed to heat or cool a room. Enter the room length, width, ceiling height, insulation quality, window size, sun exposure, number of occupants, and climate zone to get a recommended BTU figure. Results help homeowners and contractors select the right air conditioner, heater, or mini-split unit before purchasing equipment.

How it works

Enter the room length and width in feet.
Set the ceiling height — taller ceilings increase the volume of air to condition.
Choose an insulation quality that best describes your walls and attic.
Enter the total window area, or leave it blank to assume windows equal to 10% of the floor area, and select sun exposure.
Set the number of regular occupants — each person adds roughly 600 BTU/hr of heat gain.
Select a climate zone to apply the correct design temperature difference.
Click Calculate to see the recommended BTU/hr for cooling and heating, plus the ENERGY STAR room AC chart figure for comparison.

Use cases

Sizing a window air conditioner or portable AC unit for a bedroom or living room.
Choosing the right mini-split capacity for a home addition or converted garage.
Estimating heater size for a basement, workshop, or sunroom.
Comparing how adding insulation or window film reduces required BTU capacity.
Planning multi-zone ductless systems by calculating each room separately.
Verifying that an existing HVAC unit is not grossly over- or under-sized.
Teaching students the fundamentals of residential heat load estimation.
Generating a quick BTU figure before requesting a contractor quote.

Frequently Asked Questions

Last updated: 2026-05-25 · Reviewed by Nham Vu