Room BTU Tool: What Size AC Does This Room Need?

Sizing a window air conditioner or a mini-split head for one room is the one AC job where a rule of thumb is honestly useful — as long as it accounts for more than square footage. A sunny room with a low ceiling and good insulation is not the same load as a shaded room with a vaulted ceiling and single-pane glass. Enter the room’s size, ceiling height, insulation, and sun exposure, and this tool returns the BTU size to shop for.

The two sizing mistakes are both expensive in opposite directions. Too small, and the unit runs flat-out and never catches up on the hottest afternoon of the year. Too big — the one people make on purpose, “just to be safe” — and the unit cools the room before it dries it, leaving the space cold and clammy and cycling itself to an early death. The right size is the one that runs long, steady cycles on a hot day. If your number lands between two unit sizes, take the smaller one unless the room has an unusual load the tool cannot see: a kitchen’s oven, a wall of west-facing glass, or a room full of people and computers.

This is a single-room sizing tool. Whole-house central systems need a Manual J load calculation — any contractor who sizes one off square footage alone is guessing with your money.