Room Cooling Load (BTU) Estimator

Understanding the room cooling load estimator — concept and principle

This estimator gives a quick first figure for the air-conditioner capacity a single room needs, in BTU per hour and in kilowatts. It is meant for choosing a window, portable or split unit for a bedroom, living room or small office, before you compare models or ask an installer for a proper load calculation.

For this 30 m² room only the floor area counts: 6,459 BTU/h ≈ 1.89 kW.

How it is calculated

Base load: floor area (m²) × 215.3 BTU/h. This is the common rule of 20 BTU/h per square foot converted to metric (1 ft² = 0.0929 m², so 20 ÷ 0.0929 ≈ 215.3).
Sun exposure: the base is multiplied by 0.9 for a heavily shaded room, 1.0 for normal, or 1.1 for a very sunny room.
Occupants: +600 BTU/h for every person beyond two.
Kitchen: +4,000 BTU/h if the unit will cool a kitchen (enter 1), otherwise 0.
kW: BTU/h ÷ 3,412.14, because 1 kW of heat equals 3,412.14 BTU/h.

The principle

An air conditioner must remove heat as fast as it enters the room. Heat arrives through walls, windows and the roof (roughly proportional to floor area for ordinary rooms), from sunlight through glass, from people (a resting adult gives off on the order of 100 W, part of it as moisture) and from appliances such as cookers. The rule of thumb packs typical values of all of these into a few simple adjustments, following ENERGY STAR room air-conditioner sizing guidance.

Worked example

Inputs: 30 m², 2 occupants, normal sun, no kitchen.
Base: 30 × 215.3 = 6,459 BTU/h; sun factor 1.0 leaves 6,459.
Occupants: two people are already included, so +0.
Kitchen: 0. Total 6,459 BTU/h.
In kilowatts: 6,459 ÷ 3,412.14 = 1.892947 kW of cooling.

Each point is the calculator re-run with only “Floor area (m²)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Floor area (m²)	Cooling capacity (BTU/h)
7.5	1,614.75
15	3,229.5
22.5	4,844.25
30	6,459
37.5	8,073.75
45	9,688.5
60	12,918

How to read the result

Pick a unit whose rated cooling capacity is at or slightly above the estimate. 12,000 BTU/h equals one refrigeration ton (about 3.5 kW). The kW figure is the cooling output, not the electricity drawn; power consumption is roughly cooling output divided by the unit's efficiency ratio (EER or COP).

Common mistakes

Comparing the cooling kW with the electrical input kW on the label.
Choosing a much larger unit "to be safe". An oversized unit cycles on and off quickly and removes less humidity, leaving the room cold and clammy.
Ignoring high ceilings, large west-facing windows or poor insulation, which the rule of thumb does not capture.

Keep in mind: This is a rule of thumb for ordinary rooms. Whole homes, rooms with large glazing, server rooms or hot climates need a full load calculation (for example ACCA Manual J or an equivalent local method) by a qualified HVAC professional.

Frequently asked questions

How many BTU per square metre should I use?

About 215 BTU/h per m² (20 BTU/h per ft²) is the usual starting value for a room with standard ceilings and normal exposure.

How do I convert BTU/h to kW?

Divide by 3,412.14. For example 9,000 BTU/h ÷ 3,412.14 ≈ 2.64 kW.

Does a very sunny room really need only 10% more?

That is the guidance figure for typical rooms. Large unshaded windows facing the afternoon sun can need considerably more, which a detailed calculation would show.

Sources: ENERGY STAR (U.S. EPA), Room Air Conditioner sizing guidance · ASHRAE Handbook — Fundamentals, chapter on residential cooling and heating load calculations · ACCA Manual J, Residential Load Calculation

Checked 8 Nov 2026. Standard textbook formulas in SI units, computed in your browser. For learning and estimates — real designs must follow your local codes and be checked by a qualified engineer.

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