FreeToolStation

BTU Calculator

Size heating or cooling for a room, in BTU and kilowatts.

Home & Energy Calculators Free, no sign-up Runs in your browser
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BTU per hour
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In kilowatts
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Floor area
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Room volume
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Nearest standard unit
ContributionBTU/h
An oversized air conditioner is worse than a slightly undersized one: it cools the air quickly, shuts off before it has removed the humidity, and leaves the room cold and clammy. Sizing up "to be safe" is the wrong instinct here.

About the BTU Calculator

BTU Calculator sizes heating or cooling for a room from its volume rather than its floor area, then adjusts for insulation, orientation, glazing and occupancy. Volume is the better basis because a room with a high ceiling holds more air to heat or cool, and floor-area rules of thumb miss that entirely.

The adjustments matter more than the base figure in difficult rooms. A south-facing room with large windows can need fifteen to twenty per cent more cooling than the same room facing north. A kitchen needs several thousand BTU more because of the cooking. Poor insulation can add half again to a heating requirement.

For cooling in particular, resist sizing up. An oversized air conditioner reaches the thermostat temperature quickly and switches off before it has had time to dehumidify, leaving a room that is cold and damp -- the classic symptom of an over-specified unit. Slightly under is better than substantially over.

The page shows its working, not just its answer. Where there is a formula it is printed with your own numbers substituted in, so you can check the result rather than take it on trust -- which is the difference between a calculator you can use for something that matters and one you cannot.

Everything runs inside your browser. What you type is never uploaded, never logged and never stored, and the calculator keeps working if your connection drops after the page has loaded.

Looking for something this does not cover? FreeToolsZone.org is more free browser-based tools, organised by job, and it is worth a look if btu calculator is only part of what you are working on.

How to use the BTU Calculator

  1. Choose sizing for and insulationThe fields below change to match what you pick, so only the figures that matter to your case are asked for.
  2. Enter room length (m), room width (m) and ceiling height (m)Decimals are fine, and thousands separators are accepted and ignored.
  3. Read the btu per hour, in kilowatts and floor areaThe figures update as you type. The Calculate button is there if you would rather press something, but nothing waits for it.
  4. Copy or check the resultCopy the summary with the copy button, or open the table to check it row by row.

Features

  • Shows btu per hour, in kilowatts, floor area, room volume and nearest standard unit together, so you are not running it twice
  • A results table you can read across
  • Updates as you type, with no button to press and no page reload
  • Copy the result in one click
  • Free, no sign-up, and it runs entirely in your browser

Who uses this tool

The btu calculator is used by writers, developers, students, marketers and anyone else who needs the job done once without installing software. Common cases include:

  • Sizing an air conditioner for a bedroom or office
  • Working out radiator output for a room
  • Allowing for a south-facing room with large windows
  • Converting a BTU rating into kilowatts

Frequently asked questions

What is a BTU?

A British thermal unit, the energy to raise a pound of water by one degree Fahrenheit. BTU per hour is a rate of heating or cooling; 3,412 BTU/h is one kilowatt.

Is a bigger air conditioner better?

No. An oversized unit cools fast, shuts off early and fails to dehumidify, leaving the room clammy. Match the size to the load.

Does ceiling height really matter?

Yes. The load follows the volume of air, so a 3 m ceiling needs about 25 per cent more than a 2.4 m one for the same floor area.

If the btu calculator is not quite what you need, these other free tools solve closely related problems, and there are 17 more in home & energy calculators.

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Checked and rebuilt on 2026-10-14. The method this calculator uses is described in the section above, and corrections go through the contact page.