Air Conditioner BTU Calculator
Size an air conditioner for a room. Enter the dimensions, sun exposure, climate, insulation, occupancy, and whether it's a kitchen to get a recommended BTU rating and tonnage.
Example: with Room length 15 ft · Room width 12 ft · Ceiling height 8 ft · Climate Moderate → Recommended cooling: 3,600 BTU.
- Approx. tonnage0.30 tons
- Nearest half-ton unit0.5 tons
- Room area180 sq ft
Computed by the calculator below using its default values. Change any input to see your own numbers.
3D space · drag to rotate
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Check it outSizing rules of thumb
A common guideline is about 20 BTU per square foot at a standard 8 ft ceiling, adjusted for conditions: sunny rooms need more, heavily shaded rooms less, hot climates and poor insulation raise the load, and each person beyond two adds roughly 600 BTU. Kitchens add roughly 4,000 BTU for appliance and cooking heat. Oversizing isn't better — an oversized unit cools fast but won't dehumidify well, leaving the room clammy.
How it’s calculated
Base load = length × width × (ceiling height ÷ 8 ft) × 20 BTU/sq ft, which scales the standard 20 BTU/sq ft rule for taller or shorter-than-standard ceilings. That base is then multiplied by the sun-exposure factor (0.9–1.1), the climate factor (0.9 mild, 1.0 moderate, 1.15 hot), and the insulation factor (1.15 poor, 1.0 average, 0.9 good). Finally it adds 600 BTU per occupant beyond two, and 4,000 BTU flat if the room is a kitchen, then rounds to the nearest 100 BTU. Tonnage is BTU ÷ 12,000, shown both precisely and rounded to the nearest 0.5 ton since AC units are typically sold in half-ton increments. This is a rule-of-thumb estimate, not a substitute for a professional Manual J load calculation.
Results update as you type and are estimates, not professional advice — verify important decisions with a qualified professional.
Common mistakes
- Oversizing the unit, which cools fast but leaves the room clammy.
- Ignoring sun exposure, ceiling height, and occupancy.
- Forgetting the kitchen bump or a hot-climate adjustment, which can leave a unit undersized.
Frequently asked questions
Why not just buy the biggest unit?
An oversized AC short-cycles and removes less humidity, so the room feels cold and damp. Right-sizing is more comfortable and efficient.
What's a 'ton' of cooling?
One ton equals 12,000 BTU per hour. Central systems are usually rated in tons; window units in BTU.
Do ceilings and climate matter?
Yes — that's why both are separate inputs above. Taller ceilings scale up the base BTU estimate, and choosing a hotter climate or poorer insulation raises the multiplier applied to it.
Why does a kitchen need extra BTU?
Ovens, stovetops, and other appliances add heat that a bedroom or office doesn't have, so the calculator adds a flat ~4,000 BTU when you mark a room as a kitchen.