Air Conditioning
Air Conditioning Sizing Calculator (BTU/kW)
Cooling load in BTU and kW for a room
Updated July 12, 2026 · Live
What this tool does
Estimates the cooling load of a room in BTU/h and kW from its floor area, ceiling height, glazing, occupancy and sun exposure.
Enter the room area, ceiling height, glazing area, occupancy and sun exposure; the calculator returns the cooling load in BTU/h and kW.
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Formula Used
Estimates the cooling load of a room in BTU/h and kW from its floor area, ceiling height, glazing, occupancy and sun exposure. It runs entirely in your browser and shows the working, so you can check every step against your own figures.
How the air conditioning sizing calculator works
The fabric load is the floor area times a base rate per m², raised a little for a tall ceiling. Glazing adds roughly 70 W per m² of glass, and each person adds about 100 W of sensible heat. A sun-exposure multiplier scales the total up for a sunny room or down for a shaded one. The result is a cooling load in both BTU/h and kW.
What you enter
Enter the room area, ceiling height, glazing area, occupancy and sun exposure; the calculator returns the cooling load in BTU/h and kW. Every field is editable, and the result recalculates the moment you change a value.
Reading the result
Round the result up to the next standard unit size — an oversized unit short-cycles and dehumidifies poorly, which feels clammy. The base rate is yours to set so the estimate matches your building’s fabric.
Safe by design
This tool sizes the cooling load and running cost only. Refrigerant handling and the electrical connection are regulated work for a qualified F-Gas and electrical engineer.
Assumptions and limits
This is a rule-of-thumb load, not a full heat-gain survey. It does not separate sensible and latent loads, account for kitchen or IT heat sources, or model the climate — all of which a detailed cooling-load calculation includes. Sizing only: refrigerant and electrics are a qualified engineer’s job.
Using it with the rest of BuildMetricLab
This calculator is one step in planning the job. Pair it with the related tools in this category and across BuildMetricLab to go from a single figure to a full costed plan — a sizing figure feeds a cost estimate, a cost estimate feeds a project budget, and each result carries straight into the next tool. Every BuildMetricLab tool runs entirely in your browser — no sign-up, nothing sent to a server, and the formula is shown on the page so the maths can be audited.
Sources & methodology
Estimates the cooling load of a room in BTU/h and kW from its floor area, ceiling height, glazing, occupancy and sun exposure. Every result is calculated from the values you enter, all inputs are editable, and no time-sensitive prices or rates are embedded.
Frequently asked questions
How many BTU do I need per m²?
A typical base rate is around 60-200 W per square meter of floor (about 200-680 BTU/h per m², or roughly 19-63 BTU/h per ft²), with 100-130 W/m² common for an average room. This calculator defaults to 110 W/m² but lets you enter your own base rate, then adjusts for glazing, occupancy and sun exposure.
Can an AC unit be too big?
Yes. An oversized unit cools fast then short-cycles, leaving the air cold but clammy because it never runs long enough to dehumidify.
Does glazing matter much?
A lot in a sunny room — each square meter of glass adds around 70 W in this model, and south-facing glass under sun adds more. The sun exposure multiplier captures some of that additional load.
Is this a full survey?
No — it is a sizing estimate. A detailed heat-gain survey such as ACCA Manual J separates sensible and latent loads and models the local climate in detail.
Spotted something off?
Calculations or display — let us know.