BTU For Room Size Calculator

Work out btu for room size instantly with clear inputs, formula shown and shareable results.

About BTU For Room Size Calculator

Room sizing rules of thumb express cooling need as BTU per hour per square foot, with the rate set by how well the envelope resists heat: 20 for a well insulated room, 25 for average construction and 30 for poor insulation or heavy glazing. The result is corrected for ceiling height because the published rates assume 8 ft.

Formulas

Room BTU

BTU/h = length x width x rate x (ceiling height / 8)

rate: 20-30 BTU/h per ft² by insulation quality

Tonnage

tons = BTU/h / 12000

Worked example

With the default values below, BTU For Room Size Calculator returns Cooling capacity needed of 4,200. Change any input above and every figure here is recalculated in your browser.

Inputs used

Room lengthminimum 1 ft: 14 ft
Room widthminimum 1 ft: 12 ft
Ceiling heightminimum 6 ft: 8 ft
Insulation qualityone of Poor — 30 BTU/h per ft², Average — 25 BTU/h per ft², Good — 20 BTU/h per ft²: Average — 25 BTU/h per ft²

Results

Cooling capacity needed: 4,200 BTU/h
Floor area: 168.0 ft²
Nominal tonnage: 0.35 tons
Room volume: 1,344 ft³

Step by step

1Values used
2Room BTU
3Tonnage
4Cooling capacity needed= 4,200 BTU/h
5Floor area= 168.0 ft²
6Nominal tonnage= 0.35 tons
7Room volume= 1,344 ft³

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is the BTU For Room Size figure worked out?

BTU/h = length x width x rate x (ceiling height / 8). Room sizing rules of thumb express cooling need as BTU per hour per square foot, with the rate set by how well the envelope resists heat: 20 for a well insulated room, 25 for average construction and 30 for poor insulation or heavy glazing.

Is a bigger unit always safer?

No. An oversized unit short-cycles, which cools the air without running long enough to remove humidity, leaving the room clammy and the compressor stressed.

What values do I need to enter?

This calculator takes 4 inputs: Room length, Room width, Ceiling height, Insulation quality. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.