How to size an air conditioner for your room
Measure the room and its heat exposure to estimate a planning range in BTU/h.
Back to calculator ↑Measure the whole space
Measure internal width and length in metres, or enter a known floor area. Record floor-to-ceiling height and the number of regular occupants. For an open-plan room, measure the connected space that will actually be cooled; a doorway or partition can change how air reaches the far end.
Choose shaded, some sun or strong direct sun from the room’s usual conditions. “Directly under the roof” includes a one-storey house with the room ceiling under the roof. Roof exposure, large glazing, open connections and poor insulation trigger a site-check flag; the calculator does not invent a numerical heat-load adjustment for them.
How the BTU range is made
The tool starts at 700–800 BTU/h per m². It adds a 10% Thailand planning adjustment, changes that by −10% for shade or +15% for strong sun, and adds 10% for every 0.3 m above a 2.4 m ceiling. Each regular occupant beyond two adds 600 BTU/h; a kitchen adds 4,000 BTU/h. The bounds are rounded to the nearest 100 BTU/h.
For a 4 × 5 m room, 2.4 m ceiling, two people, some sun and no kitchen, area is 20 m². The displayed range is 20 × 700 × 1.10 = 15,400 to 20 × 800 × 1.10 = 17,600 BTU/h. If the ceiling is directly under a roof, including in a one-storey house, the tool flags it for assessment but does not silently change that range.
One larger unit or two smaller units
The one-unit and two-unit views compare combined nominal capacity with the same room range. Two 9,000 BTU/h units total 18,000 BTU/h on their labels, but that arithmetic cannot establish whether air reaches each part of the room or whether piping, drainage and electrical supply work at both positions.
A long room, divided zones or obstructing walls may benefit from two air outlets, while a simple enclosed room may be served by one suitably placed unit. Ask an installer to compare airflow, independent control, noise, electrical work and installed cost. Bigger is not automatically better: excessive capacity can cause short cycling and weaker humidity removal.
Check the installed offer
Compare the exact model code, rated BTU/h, efficiency label and warranty. Confirm whether a quoted price covers the unit only or installation too; ask about included pipe length, wiring, breaker, condensate drain, service area and removal of an old unit.
For an existing installation, the tool multiplies the rating per unit by the selected count of one or two and compares the total with the new range. It does not measure an older unit’s actual output, leakage or air distribution, so use that comparison as a discussion prompt rather than a diagnosis.
Common questions
Does a one-storey house count as directly under the roof?
Yes, when the room ceiling sits directly below the roof. The tool flags a site check without applying an unsupported blanket BTU increase.
Can two smaller units replace one larger one?
Combined rated BTU is only a first comparison. Check placement, airflow and installation cost for both layouts.
Why not simply buy the highest BTU rating?
Excessive capacity may cycle frequently and remove humidity less effectively. Confirm the size against the actual room.
Sources and calculation guidance
Check these against your chosen product’s label and instructions.