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AC / BTU Calculator: What Size Air Conditioner Do I Need?

Enter your room's size and a few details to get the recommended BTU/hr for a room air conditioner.

No tape measure? Multiply length × width in feet.
Recommended size15,100 BTU/hr

How it works

We start from the ENERGY STAR room-AC sizing table, which gives a base BTU/hr rating for your room's area. A heavily shaded room gets a 10% discount on that number, and a very sunny room gets a 10% bump, since sunlight through windows and walls is a major source of heat gain. Each occupant beyond the first two adds 600 BTU/hr for body heat, and a kitchen adds 4,000 BTU/hr for cooking and appliance heat. Rooms over 2,500 sq ft are outside what a single room AC can handle well.

Example

A 400 sq ft, very sunny kitchen with 4 regular occupants: the base rating for 350–400 sq ft is 9,000 BTU/hr. Sunny exposure adds 10% (+900), 2 extra occupants beyond the baseline of 2 add 1200 BTU/hr, and the kitchen adds 4,000 BTU/hr — landing on 15,100 BTU/hr.

Tips

  • When your room falls between two BTU ratings, round up rather than down — a slightly oversized unit on low power beats an undersized one running flat out all day.
  • Ceilings over 8 ft, or a room that opens into another space, both increase the effective volume to cool — size up if either applies.
  • Look for the ENERGY STAR label; a certified unit at the same BTU rating uses meaningfully less electricity over a season.
  • A unit with a programmable timer or smart thermostat can cut running costs without changing the BTU rating you need.
  • If a room runs hot even after sizing up, check for duct leaks, poor insulation, or a sunny attic above it before buying an even bigger unit.

Common mistakes

  • Sizing only by floor area and ignoring sun exposure — this can leave a west-facing room undersized by 10% or more.
  • Forgetting to add BTU for a houseful of regular occupants, not just the people usually in the room.
  • Treating a kitchen like any other room — cooking heat is a real, repeatable load, not a one-time spike.
  • Buying an oversized unit "to be safe," which short-cycles, wastes energy, and leaves humidity in the air.
  • Trying to cool a great room or open floor plan over 2,500 sq ft with a single window unit instead of looking at central or ductless options.

FAQ

Is a bigger BTU rating always better?
No — an oversized AC cools the room too fast without running long enough to remove humidity, which leaves the room feeling damp and clammy. Matching the BTU to the room (not going well over) gives better comfort and lower energy use.
Why does sun exposure change the recommendation?
A heavily shaded room gains less heat from sunlight through windows and walls, so it needs about 10% less cooling capacity. A very sunny room gains more heat, so it needs about 10% more.
Why do extra people and a kitchen add BTU?
The sizing table already assumes a baseline of about 2 occupants. Each additional regular occupant adds body heat (+600 BTU/hr), and a kitchen adds heat from cooking and appliances (+4,000 BTU/hr).
What if my room is bigger than 2,500 sq ft?
A single room air conditioner isn't a realistic option at that size — you're better served by a central or multi-zone system sized by an HVAC professional for the whole space.

Related guides

HVAC Filter Replacement Guide

Keep your system running efficiently.

Seal Window Drafts

Cut cooling loss before you upsize anything.

Best Shop Vacs

Buying guide for cleanup after any install.

Estimates only. The sizing table is based on the ENERGY STAR room AC guidelines — always check the BTU rating against the specific unit you're buying, and consult an HVAC professional for unusual rooms (very high ceilings, poor insulation, large glass walls).