BTU Calculator

How air conditioner sizing actually works

Cooling capacity is measured in BTU per hour — the amount of heat the unit can move out of a room every hour. The industry rule of thumb is 20 BTU per square foot of floor area, which assumes an eight-foot ceiling, average insulation and a temperate climate. Everything else on this page is an adjustment to that baseline: taller ceilings mean more air volume to cool, direct sun loads the room through the glass, people give off heat continuously, and a kitchen adds appliances that dump several thousand BTU into the space every time someone cooks.

The calculator applies those factors in the order a contractor would: area times 20, scaled by ceiling height relative to eight feet, multiplied by the sun factor, then 600 BTU added for each person beyond two and a flat 4,000 BTU if the room is a kitchen. The climate setting multiplies that running total last — leave it on temperate for a mild summer, +15 percent for a genuinely hot region, +25 percent where hot and humid is the norm from June to September. The final figure is rounded to the nearest 500 BTU because that is the granularity units are actually sold in.

Worked example: a 15 by 12 ft living room

Floor area is 180 sq ft, so the baseline is 3,600 BTU. With a standard 8 ft ceiling, average sun and two occupants, nothing changes — the answer is 3,600, rounded to 3,500 BTU. In practice the smallest window units on the market are 5,000 BTU, so that is what you would buy. Now raise the ceiling to 9 ft (multiplier 1.125, giving 4,050), make the room very sunny (+10 percent, 4,455) and seat four people (+1,200), and the requirement climbs to 5,655 — call it 5,500 to 6,000 BTU. Same room, and the answer moved by more than half again.

Worked example: a 12 by 10 ft kitchen

120 sq ft gives 2,400 BTU on area alone, which sounds trivially small. Add the 4,000 BTU kitchen allowance and the requirement is 6,400, rounded to 6,500. Put that same kitchen in Houston and the very hot setting lifts it to 8,000. That is not a fudge factor: an electric oven at full tilt puts out roughly 2,000 to 4,000 BTU per hour into the room, a hob adds more, and a refrigerator rejects its own condenser heat all day. Kitchens are the one room where the floor-area rule fails badly if you skip the appliance load.

Worked example: a 5.5 by 4 m bedroom, metric

Switch the unit selector to metres and the tool converts internally. 5.5 x 4 m is 18.04 x 13.12 ft, or about 237 sq ft, giving 4,734 BTU. A 2.4 m ceiling is 7.87 ft, slightly under the 8 ft reference, so the figure drops to about 4,660. A heavily shaded north-facing room takes another 10 percent off, landing near 4,194 — rounded to 4,000 BTU, or 0.33 tons. A single small split handles it comfortably.

Why bigger is not better

The most common mistake in residential cooling is buying two sizes up "to be safe". An air conditioner removes moisture only while it is running; a unit that satisfies the thermostat in six minutes never runs long enough for the coil to condense much water. The result is the classic cold, clammy room where 22 °C feels worse than 24 °C would in a properly sized system. Short cycling also hammers the compressor, since start-up is the hardest thing it ever does, and it drags down real-world efficiency well below the sticker rating. Undersizing has an honest failure mode — the room stays a degree or two warm on the hottest days — but oversizing fails in ways people blame on the building instead of the equipment.

Adjustments this calculator does not make

Insulation is the big one. A 1920s house with single glazing and no wall insulation can need 20 to 30 percent more capacity than the rule of thumb suggests; a well-sealed new build with double glazing and a decent air-tightness result can need 10 to 20 percent less. Large west-facing windows deserve their own allowance beyond the sun setting here — afternoon sun through unshaded glass is the single largest load in many rooms. A top-floor room under an uninsulated roof also runs hot, and a room with a door permanently open to a hotter space is effectively larger than its own floor area.

Efficiency ratings and running cost

Capacity tells you whether the room gets cool; EER, SEER and SEER2 in the US, or SEER and the A-to-G energy label in the UK and EU, tell you what it costs to keep it that way. Two 12,000 BTU units can differ by 40 percent in electricity use. Inverter models modulate their output instead of cycling on and off, which is why they hold humidity and temperature steadier and why the sizing penalty for being slightly oversized is smaller with an inverter than with a fixed-speed unit.

Where the rule of thumb stops working

This is a single-room estimate. For a whole-house system, ducted or multi-zone, a proper Manual J load calculation is the right tool: it accounts for wall assemblies, window U-values and shading coefficients, infiltration rate, duct losses and local design temperatures. Contractors who size by square footage alone routinely oversize by 30 to 50 percent. Use this calculator to sanity-check a quote or to pick a window unit or single split, and get a load calculation before committing to anything ducted.