Window Air Conditioner Size Calculator

Estimate the BTU rating needed for a window air conditioner from room area, sun exposure, and occupancy.

⏱ Updated: 18 Sep 2026

Calculator

Free window air conditioner size calculator: estimate the BTU rating needed for a room from its area, sun exposure, and occupancy.

Recommended BTU
Room Area (sq ft)

Baseline: 20 BTU per sq ft, a widely published room air conditioner sizing rule of thumb, adjusted for sun exposure, kitchen heat load, and extra occupants beyond two.

Enter a room's length and width, sun exposure, and occupants to estimate the window air conditioner BTU size needed.

How to Use the Window Air Conditioner Size Calculator

Room length and width set the floor area; sun exposure, the kitchen checkbox, and occupant count each nudge the BTU total up or down from there, all updating live as fields change.

Twenty BTU per square foot is the number window-unit manufacturers print on their own sizing charts, and it's a reasonable starting point for an average room with average sun and one or two people in it. Push past those averages and the number needs to move — a west-facing room that bakes in afternoon sun runs hotter than its square footage alone suggests, and a kitchen adds a stove, an oven, and a fridge compressor's worth of extra heat that a bedroom never has to deal with.

BTU = floor area (sq ft) × 20 × (1 + sun adjustment)
    + 4,000 if kitchen
    + 600 × (occupants beyond 2)

Undersizing leaves a unit running flat out without ever quite cooling the room; oversizing cools it fast but shuts off before it's pulled enough humidity out of the air, leaving things feeling clammy despite the low temperature. Matching capacity to the room, not just buying the biggest unit that fits the window, actually matters here.