How to Measure Room
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BTU / AC Sizing Calculator

Get a recommended BTU rating for your room based on its size.

BTU / AC Sizing Calculator

Room square footage in, recommended BTU rating out.

Recommended capacity

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How the estimate is calculated

The calculator above starts from the standard Energy Star / DOE rule-of-thumb sizing chart, which maps room square footage to a base BTU rating (for example, 300 sq ft calls for roughly 8,000 BTU). From there, it adjusts up or down for a few factors that meaningfully affect real-world cooling load:

  • Sun exposure: +10% for a very sunny room, -10% for a heavily shaded one.
  • Occupants: +600 BTU for each person beyond the first two regularly in the room, since bodies generate heat.
  • Kitchen: +4,000 BTU flat, to account for cooking appliance heat load.

This gives a reasonable starting estimate for shopping and budgeting. It doesn't account for everything a full load calculation would — insulation quality, window count and glazing, local climate, and ceiling height all affect the real number too. For a room where getting the size exactly right matters (a large or unusual space, or a significant purchase), an HVAC professional's load calculation is worth the cost of a mistake either direction.

Frequently Asked Questions

What happens if an AC unit is too small for a room?

It runs constantly without ever quite reaching the target temperature, since it can't remove heat as fast as the room gains it — this also means it cycles less, which reduces how much moisture it pulls out of the air, leaving the room feeling clammy even at a lower temperature.

What happens if an AC unit is too big for a room?

It cools the air quickly and shuts off before it's run long enough to remove much humidity, leading to a room that's cold but still feels damp. Oversizing also means more frequent on/off cycling, which is harder on the unit and less energy-efficient than a properly sized one running longer, steadier cycles.

Does BTU sizing change for a room with high ceilings?

Yes — the standard sizing chart assumes a typical 8-foot ceiling. A room with noticeably higher ceilings has more air volume to cool for the same floor area, so it generally needs a higher BTU rating than the square-footage chart alone would suggest.

Why does a kitchen need a higher BTU rating?

Cooking appliances (the oven, stovetop, and often a few other heat-generating devices) add a substantial, fairly constant heat load beyond what the room's square footage alone accounts for. The standard adjustment is an extra 4,000 BTU for a room that's used as a kitchen.

Is a bigger BTU rating always better?

No — oversizing has real downsides (see above: poor humidity control, more on/off cycling, wasted energy and cost). The goal is matching the unit to the room, not maximizing capacity, which is why this calculator returns a specific recommended range rather than "go as large as you can."

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