Air conditioning size calculator
The calculator needs JavaScript. The worked example, formula and figures below show exactly how it works, so you can do the sum yourself. A guide, not advice. Check your supplier's or maker's figures before ordering.
Short answer
Worked example
| You enter | Value |
|---|---|
| Room length | 4 m |
| Room width | 4 m |
| Ceiling height | 2.4 m |
| Base cooling load | 35 W/m³ |
| People usually in the room | 0 |
| Heat from each person | 100 W |
| Equipment heat | 0 W |
| Sunny window area | 0 m² |
| Unit sizes on sale | 5000, 7000, 9000, 12000 BTU/h |
Cooling: 1,344 W (4,586 BTU/h): a 5,000 BTU/h unit
| Result | Value |
|---|---|
| Cooling needed | 1,344 W (4,586 BTU/h) |
| Cooling needed | 4,586 BTU/h |
| Smallest unit size that covers it | 5,000 BTU/h |
| Room volume | 38.4 m³ |
Show the working
- Volume: 4 × 4 × 2.4 m = 38.4 m³
- Room: 38.4 m³ × 35 W/m³ = 1,344 W
- Cooling: 1,344 W ÷ 1,000 × 3,412.14 = 4,586 BTU/h
- Smallest unit size that covers 4,586 BTU/h: 5,000 BTU/h
How it works
- Room volume (m³) = length × width × height
- Cooling (W) = volume × base load (W/m³) + people × heat per person + equipment W + sunny glazing m² × solar gain W/m²
- BTU/h = W ÷ 1000 × 3,412.14
- Unit size = the smallest size on sale that is at least the cooling needed
A rule-of-thumb estimate, not a cooling load calculation (CIBSE Guide A). South and west-facing glass, a loft room or poor insulation need more.
Split systems contain fluorinated gas: they must be installed by an engineer holding an F-gas qualification. Portable units don't need one.
Our default (adjustable)
These are typical planning figures and our own allowances, not facts about your product. Change any of them in the calculator to match your bag, pack or tin.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Base cooling load | Enter from the bag, pack or tin | Our assumption Not yet checked against the source | Our choice: change it to suit your job Brief v2 H7: W/m³ base load is an assumption the user enters (test AC1 uses 35 W/m³ entered). Researcher to source (e.g. CIBSE Guide A summary). |
| Heat from each person | 100 W per person | Our assumption Not yet checked against the source | Our choice: change it to suit your job Brief v2 H7: 100 W/person. Researcher to source (e.g. CIBSE Guide A). |
| Solar gain through glazing | Enter from the bag, pack or tin | Our assumption Not yet checked against the source | Our choice: change it to suit your job Brief v2 H7: solar W/m² of glazing, Researcher sources (e.g. CIBSE Guide A summary). Depends on orientation. |
| Unit sizes on sale | 5000, 7000, 9000, 12000 BTU/h | Common UK size Not yet checked against the source | From your product Common unit sizes named in brief v2 H7 (5k/7k/9k/12k BTU). Check the maker's range. |
Regulations and standards
- F-gas rules: split systems are fitted by an engineer holding an F-gas qualification
Our checks use the guidance for England unless a badge says otherwise. Wales, Scotland and Northern Ireland have their own rules: check the source for where you live.