Free BTU Calculator (+ Guide)

What is a BTU calculator?

A BTU calculator works out the cooling an air conditioner needs for a room, in British thermal units an hour.

This one uses ENERGY STAR's chart for room air conditioners, with its adjustments for sun, people and kitchens.

How it works

The calculator works it out on your device, and your numbers stay there.

ENERGY STAR's chart

ENERGY STAR lists the capacity a room needs by its floor area, from 5,000 BTU for 100 to 150 square feet to 34,000 for 2,000 to 2,500.

The calculator shows the full chart, with your room's row marked.

The adjustments

ENERGY STAR's adjustments come after the chart:

• Heavily shaded: 10% less
• Very sunny: 10% more
• More than 2 people regularly in the room: 600 BTU for each extra person
• A kitchen: 4,000 BTU more

Placement matters too: near a corner, look for a unit that can send the air the right way.

A worked example

Here's the example the calculator loads: a 16 by 20 foot room, very sunny, with 3 people in it.

That's 320 square feet, so 8,000 BTU on the chart. Sun adds 800 and the third person 600, for 9,400 BTU an hour, about 2.75 kW.

Mistakes to avoid

These are the slips that come up most.

• Buying the biggest unit, which cools the room before it removes the humidity, ENERGY STAR says
• Measuring a room open to the next without counting both
• Forgetting the extra 4,000 BTU for a kitchen
• Ignoring a west-facing window's afternoon sun

Who is the BTU calculator for?

It's for anyone buying a window or portable air conditioner, and these tools help around it.

• Electricity cost calculator – for what it costs to run
• Square footage calculator – for an odd-shaped room
• Flooring calculator – for the room's floor while you're measuring

Measure once, size it right, and the room stays cool without the unit fighting itself.