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BTU Calculator

Heating and cooling BTU needed for a room.

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How the estimate builds up

ComponentCooling BTU/hHeating BTU/h

Common unit sizes

CapacityTonsKilowattsSuits

Cooling capacity needed

—

Capacity

Heating—
Cooling in tons—
Cooling in kW—
Heating in kW—

The room

Floor area—
Volume—

—

Nearest standard air conditioner size

Do not round up generously. An oversized air conditioner cools the air, satisfies the thermostat and shuts off before it has run long enough to remove humidity, leaving the room cold and clammy — and the short cycling wears the compressor. Match the capacity to the load.

Quick Tool Overview

The BTU Calculator estimates the heating and cooling capacity a room needs, adjusting a base rate per square foot for insulation, climate, ceiling height, sun exposure, window area and occupancy — because the base rate alone is why so many units are wrongly sized.

What it does
Estimates heating and cooling BTU for a room
You give it
Room size, insulation, climate, sun and occupancy
You get back
BTU per hour for heating and cooling, plus kW and tons
Best for
Sizing air conditioners, heaters and radiators
Calculators Free, unlimited use No sign-up Runs in your browser

About the BTU Calculator

Estimate the heating or cooling capacity a room needs in BTUs per hour, adjusted for insulation quality, climate, ceiling height, sun exposure, window area and how many people normally occupy it.

A base rate per square foot is only the starting point, and treating it as the answer is why so many units are the wrong size. Oversizing an air conditioner is a specific and common mistake: an oversized unit cools the air quickly, satisfies the thermostat, and shuts off before it has run long enough to remove humidity — so the room ends up cold and clammy, and the compressor wears out faster from short cycling.

Key features of the BTU Calculator

Six adjustment factors

Insulation, climate, ceiling height, sun exposure, windows and occupancy, rather than a flat rate per square foot.

Warns about oversizing

An oversized air conditioner short cycles and leaves the room cold and clammy — explained rather than assumed.

Heating and cooling both

The two loads differ, and each is calculated separately.

kW and tons

BTU converted to the units heaters and air conditioners are actually sold in.

Volume, not just floor area

Ceiling height is factored in, since you condition air rather than carpet.

How to use the BTU Calculator

  1. 1

    Enter the room size

    Floor area, or length and width, plus ceiling height.

  2. 2

    Describe the room

    Insulation quality, climate zone, sun exposure and window area.

  3. 3

    Add occupancy

    Each person adds roughly 600 BTU of cooling load. Kitchens need a further allowance.

  4. 4

    Read the capacity

    Heating and cooling BTUs per hour, with the kilowatt and ton equivalents.

Frequently asked questions

Roughly 20 BTU per square foot for cooling in a moderate climate, and 30 to 60 for heating depending on insulation and climate. Adjustments for sun, ceiling height and occupancy matter substantially.

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