Air Conditioner Size Calculator (BTU)
Enter your room's square footage and conditions to estimate the air conditioner size (BTU/h) you need.
Last updated: September 22, 2026
Frequently asked questions
How many BTUs do I need for a 300 sq ft room?
About 6,000 BTU/h as a baseline (300 × 20), adjusted up or down for sun exposure, extra occupants, or if it's a kitchen.
Why does sun exposure matter for AC sizing?
Rooms with lots of direct sunlight gain significant extra heat through windows, requiring more cooling capacity than a shaded room of the same size - conversely, a heavily shaded room needs somewhat less.
Why do kitchens need extra cooling capacity?
Cooking appliances (stoves, ovens) generate substantial extra heat beyond what's typical for a room that size, so kitchens typically need roughly 4,000 additional BTU/h compared to a similarly sized non-kitchen room.
What happens if I oversize my air conditioner?
An oversized unit cools the room quickly but shuts off before properly removing humidity, leading to a cold but clammy feeling and more frequent on/off cycling that wears out the compressor faster - right-sizing (not just maximizing capacity) matters for comfort and equipment life.
How many BTUs for a 450 sq ft sunny living room with 4 typical occupants?
Baseline is 450 × 20 = 9,000 BTU/h. Sunny exposure adds 10% (9,900 BTU/h), and 2 occupants beyond the standard 2 add 2 × 600 = 1,200 BTU/h, bringing the total to about 11,100 BTU/h, or roughly 0.93 tons.
Does ceiling height affect the BTU estimate this calculator gives?
No - this calculator's 20 BTU/h per sq ft rule of thumb assumes a standard roughly 8 ft ceiling. Rooms with noticeably higher ceilings have more air volume to cool and typically need more capacity than this estimate, so increase the room area input or add a margin if your ceilings are unusually tall.
Is 'tonnage' the same thing as BTU/h for sizing an air conditioner?
They describe the same capacity in different units - one ton of cooling equals 12,000 BTU/h, which is where this calculator's tonnage figure comes from (BTU/h ÷ 12,000). Central air systems are commonly sized in tons, while window and portable units are usually labeled in BTU/h.