Air conditioner size per m² and m³: how to calculate kW and BTU/h

Short answer: the air conditioner capacity you need is the room’s highest hourly cooling load in kW. A rule of thumb per m² or m³ only gives a range: the quick estimate in the AC calculator multiplies the floor area by 100 W/m² (20 m² room → 2 kW), while the hour-by-hour calculation for the same room gives 1.38 to 2.01 kW depending on the window, sun, people and insulation. 1 kW = 3412 BTU/h.

Rule of thumb: W per m² and W per m³

Capacity = area × W/m² or volume × W/m³. It is a quick estimate for a typical room and does not see what actually heats a room in summer. The quick estimate in the AC calculator uses 100 W/m² (you can change it): for a 5 × 4 m room, 2.6 m high (20 m², 52 m³) that is 2 kW. A higher ceiling increases the volume but not the sun through the window or the heat from people and appliances.

How the AC calculator works out capacity

For every hour of the design day the detailed mode adds up: conduction through walls and roof (including solar heating of the outer surface), conduction and solar radiation through windows by orientation, people (sensible and latent), lighting, appliances and outdoor air (ventilation and infiltration). The required capacity is the largest hourly total, not the sum of each item’s maximum. It is a simplified steady-state hourly model based on ASHRAE data (clear sky, sol-air temperature) – not a full ASHRAE RTS or Manual J calculation, but it covers the same main items.

Same room, different capacity

Illustrative calculation with the AC calculator: room 5 × 4 × 2.6 m, one outside wall with a 2 × 1.5 m window, 2 people, 60 W lighting and 150 W appliances; July, 45.8° N, 33 °C outside, 26 °C inside. Each row changes one item only.

Variant	Peak hour	kW	BTU/h	W/m²	W/m³
Window facing south (base)	12	1.38	4695	69	26
Window facing west	16	1.75	5977	88	34
4 people instead of 2	12	1.61	5480	80	31
Roof above the room (top floor)	12	1.57	5355	78	30
Ceiling 3.2 m instead of 2.6 m	12	1.41	4805	70	22
Old wall U 1.2, window U 2.8, g 0.75	12	2.01	6859	101	39

Why the per-m³ rule misleads

For the same floor area W/m² ranges from 69 to 101 and W/m³ from 22 to 39. A higher ceiling barely changes the capacity (1.41 instead of 1.38 kW) but lowers W/m³ from 26 to 22. The west window adds 376 W because the afternoon sun hits the glass when it is also hottest outside. Use the per-m² or per-m³ rule only to check the order of magnitude; to choose a unit, calculate the peak.

kW, BTU/h and unit size

1 kW = 3412 BTU/h and 1 BTU/h = 0.293 W. Units are often labelled by BTU/h class: 9000 BTU/h ≈ 2.64 kW, 12000 BTU/h ≈ 3.52 kW. The other way round, 2.5 kW = 8530 BTU/h, 3.5 kW = 11942 BTU/h, 5 kW = 17061 BTU/h. Choose a unit whose cooling capacity at your conditions equals the calculated peak or is slightly larger; a heavily oversized unit cycles on and off more often and dehumidifies less.

NIST SP 811, Appendix B.8: Factors for units listed alphabetically

Common mistakes

Sizing only per m² for a room with large west or south glazing.
Volume as the main criterion – a high ceiling does not mean proportionally more capacity.
Forgetting people, computers and kitchen appliances.
Adding the peaks of all rooms as if they happened in the same hour.
Comparing with the rated capacity regardless of the conditions it is stated for.

Frequently asked questions

How many watts of cooling per m²?

As a range the calculator’s quick estimate uses 100 W/m². The detailed calculation for a 20 m² room gave 69 to 101 W/m² depending on the window, people and insulation.

How many BTU for a 20 m² room?

In the examples 4695 to 6859 BTU/h (1.38–2.01 kW). A 9000 BTU/h class unit (≈ 2.64 kW) covers all the variants shown.

Can AC capacity be calculated per m³?

Only roughly. In the example W/m³ ranged from 22 to 39 for the same room, and a higher ceiling hardly changed the required capacity.

Open the calculator: Air conditioner size