Wondering what size air conditioner a room needs? Enter the floor area and ceiling height, choose how well the room is insulated and how sunny it is, and this tool estimates the cooling capacity in BTU per hour and the heating power in watts. It is a quick starting estimate — not a substitute for a professional heat-load survey.
Enter the room area to estimate the size.
A rough volume-based estimate for guidance only. It does not replace a professional heat-load survey, which accounts for windows, orientation, climate and ventilation.
How it works
The estimate is volume-based: floor area × ceiling height gives the room’s cubic metres, which are multiplied by a watts-per-cubic-metre factor. That factor rises with poorer insulation and more sun — from about 30 W/m³ for a well-insulated, shaded room to 45 W/m³ for a sunny, poorly insulated one for cooling.
Each person in the room adds roughly 100 W of heat that the air conditioner must also remove, so occupants are added to the cooling load. The result is converted to BTU per hour (1 watt ≈ 3.412 BTU/h) and rounded up to the next 1000, since AC units are sold in fixed sizes like 9,000, 12,000 and 18,000 BTU.
The heating figure uses a similar volumetric factor to estimate the radiator or heat-pump power a room needs to stay warm. Both numbers are deliberately simple rules of thumb: real sizing also depends on window area and orientation, climate, floor level and air changes, which a technician measures on site.
Practical examples
A 20 m² living room
With a 2.6 m ceiling and average insulation, the volume is 52 m³. Cooling comes to about 1,820 W (≈ 6,210 BTU/h) with no one in the room — so a common 7,000–9,000 BTU unit covers it with margin.
Accounting for people
Add three occupants to that same room and the cooling load rises by 300 W to 2,120 W — a reminder that a busy office or full living room needs more capacity than an empty one.
A sunny top-floor bedroom
For a 16 m² room set to “poorly insulated / sunny”, the higher 45 W/m³ factor pushes the estimate up noticeably versus the same room shaded and well-insulated — which is why the same floor area can need a bigger unit.
Frequently asked questions
What is a BTU and why does AC use it?
BTU (British Thermal Unit) per hour measures how much heat an air conditioner can move in an hour. Air conditioners are rated and sold in BTU/h, so sizing a room means matching its heat load to a unit’s BTU rating.
How many BTU do I need per square metre?
As a very rough guide, plan for roughly 300–450 BTU/h per m² of floor for a standard-height room, more for sunny or poorly insulated spaces. This tool works from volume rather than area so it can account for ceiling height too.
What if I pick a unit that is too big or too small?
Too small and it runs constantly without cooling the room; too big and it cools quickly but short-cycles, wasting energy and leaving the air humid. Sizing close to the actual load — then rounding up to the next standard unit — is the goal.
How do I convert between watts and BTU?
One watt is about 3.412 BTU per hour, so a 2,600 W (2.6 kW) unit is roughly 9,000 BTU/h. The tool shows both so you can match whichever figure the product uses.
Does insulation really change the answer?
Yes, significantly. A well-insulated, shaded room loses and gains far less heat than a sunny, poorly insulated one, which is why the tool lets you pick a condition that raises or lowers the per-cubic-metre factor.
Why add watts for occupants?
People give off body heat — roughly 100 W each at rest — plus heat from lights and electronics. A room that is often full needs the air conditioner to remove that extra load on top of the fabric heat gain.
Is the heating estimate for a radiator or a heat pump?
Either — it estimates the heat output in watts (and kW) the room needs, which you can match to a radiator’s rated output or a heat pump’s heating capacity. It does not size boilers or account for pipe losses.
How accurate is this?
It is a ballpark first estimate. It ignores window size and orientation, local climate, roof exposure and ventilation, all of which a proper survey includes. Treat it as a sanity check, not a purchase decision on its own.
Does it store what I enter?
No. The calculation runs in your browser and nothing is uploaded or saved.
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