Air Conditioner Room Size Calculator

Construction

The Air Conditioner Room Size Calculator estimates the cooling capacity (in BTU/hr) a room needs from its floor area and a BTU-per-square-foot climate factor. As a rule of thumb, rooms need roughly 20 BTU/hr per square foot, adjusted up for hot or sunny rooms.

Formula

BTU = Room Size (sq ft) × BTU per sq ft
  • The cooling load is estimated as floor area multiplied by a BTU-per-square-foot factor.
  • A common baseline is about 20 BTU/hr per square foot for a standard room.
  • Increase the factor for rooms that are very sunny, have high ceilings, or are heavily occupied; decrease it for shaded, well-insulated rooms.
  • This is a quick rule-of-thumb estimate, not a substitute for a full Manual J load calculation.

Example Calculation

Inputs
  • Room Size (sq ft): 200
  • BTU per sq ft: 20

A 200 sq ft room at 20 BTU/hr per square foot needs about 200 × 20 = 4,000 BTU/hr of cooling — roughly a small window unit.

Frequently asked questions

How many BTU do I need per square foot?
A common rule of thumb is about 20 BTU/hr per square foot, adjusted for climate, sun exposure and ceiling height.
How do I find my room size?
Multiply the room's length by its width in feet to get the floor area in square feet.
Should I size up or down?
Slightly undersizing is usually better than oversizing; an oversized AC cools quickly but cycles off before removing humidity.
What raises the BTU-per-square-foot factor?
Lots of sun, high ceilings, many occupants, and kitchens all increase the cooling load per square foot.
Is this an exact sizing method?
No. It is a quick estimate. For precise sizing, use a full Manual J load calculation.