HVAC

BTU per square foot calculator

It works both ways: ratio from capacity, or capacity from ratio.

Reviewed September 19, 2026FreeNo sign-up≈ 2 minutes

Your inputs

Your inputs are processed on your device and are not sent to a server.

Your result will appear here

There is no figure yet: nothing is calculated until you ask for it.

Enter your measurements.
Check the units and the assumptions.
Press Calculate, or load the example.

Rather see a worked case? Press “Load example” and change any value afterwards.

Next step

Air conditioner size calculator

The per-square-metre rule is for checking; the full estimate is for deciding.

Open the calculator

Where each number comes from Conversion constants are exact; densities and coverage rates come from cited sources; everything else is an assumption you can change. Methodology · Sources and updates

How to use it

Enter the floor area.
Enter either the unit capacity or the ratio per unit area.
The missing figure is calculated, in both unit systems.
Compare the result with the usual range before deciding.

How it is calculated

A division, either way round:

ratio = capacity in BTU/h / area capacity = ratio x area

Per-area rules are good for one thing: spotting an absurd quote. If someone offers a 24,000 BTU/h unit for a 130 sq ft bedroom, the ratio works out at 185 BTU/h per square foot when the domestic norm is 28-42. Something is wrong there.

What these rules cannot do is replace proper sizing. They ignore ceiling height, insulation, glazing, orientation and occupancy, which are precisely the variables that explain why two identical-looking rooms need different units.

Worked example

A 12,000 BTU/h unit in a 215 sq ft room:

Ratio: 12,000 / 215 = 55.7 BTU/h per sq ft
Per square metre: 600 BTU/h per m²
In frigories: 151 frigories/h per m²

55.7 BTU/h per square foot is above the typical domestic range (28-42), which suggests the unit is oversized for that room, unless it has heavy solar gain or poor insulation.

Good to know

Usual domestic range. 28 to 42 BTU/h per square foot, or 300 to 450 per square metre.
This is a guideline estimate. A professional heat-load calculation considers the real build-up of every wall and roof, thermal bridges, infiltration, humidity and hour-by-hour climate data.
Use it to check, not to choose. Treat the ratio as a sanity check on a calculation done another way.
Capacity is not consumption. The BTU/h, frigories or kW on a label measure the heat the unit removes from the room, not the electricity it uses. For that you need the efficiency rating or the rated input power.

Frequently asked questions

How many BTU per square foot do I need?

Between 28 and 42 BTU/h per square foot for a home with a normal ceiling. The low end for well-insulated, shaded rooms; the high end for top floors or west-facing rooms.

How many BTU per square metre?

Between 300 and 450 BTU/h per square metre, the same range in metric units.

What capacity do I need for 300 square feet?

At 35 BTU/h per square foot that is 10,500 BTU/h, so a 12,000 BTU/h unit. It is a starting point: component-based sizing can land well above or below.

Can I rely on the per-area rule?

As a quick check, yes. As the only criterion, no: it ignores height, insulation, glazing and occupancy, which are half the problem.

Related calculators

Air conditioner size calculatorA component estimate: you can see where every watt comes from.Cooling capacity calculatorWith the load per square metre visible, so you can adjust it.BTU to frigoriesFor comparing units advertised in different markets.AC energy consumption calculatorCooling capacity and electricity used are not the same thing.