ComfortCalc

BTU Calculator

A room-level version of the BTU rule of thumb — for sizing a single unit, not a whole-home system.

Room area (sq ft)

BTU per sq ft

BTUs needed

5,000 BTU/hr

Use this for sizing a single window AC unit, space heater, or portable unit — for a whole-home system, see the HVAC Sizing Calculator.

What you entered

Room area (sq ft): 200BTU per sq ft: 25

1
Apply the BTU-per-square-foot rule of thumb
200 × 25= 5000 BTU/hr
2
Convert to tons of cooling (1 ton = 12,000 BTU/hr)
5000 ÷ 12,000= 0.4167 tons

Result

BTUs needed: 5,000 BTU/hr

A 200 sq ft room needs roughly 5000 BTU/hr of heating or cooling capacity.

BTU requirement by room size

Heating and cooling load scales with the space. These figures use BTU per square foot, the quick estimate used for sizing a room unit.

Room size	BTU/sq ft	BTU/hr needed	Equivalent tons
500 sq ft	20	10,000	0.83
800 sq ft	20	16,000	1.33
1,000 sq ft	20	20,000	1.67
1,000 sq ft	25	25,000	2.08
1,000 sq ft	30	30,000	2.50
1,500 sq ft	20	30,000	2.50
2,000 sq ft	20	40,000	3.33

Adjust upward for a sunny room, a top floor, a kitchen, or high ceilings, and downward for a heavily shaded or well-sealed space. Rooms with more than two regular occupants add roughly 600 BTU per extra person.

Room-level vs. whole-home sizing

This calculator is scaled for sizing an individual window unit, portable AC, or space heater for one room — for a whole-home central system, use the HVAC Sizing Calculator instead, which works from total home square footage.

Ceiling height and sun exposure matter

A room with high ceilings or lots of direct sun exposure needs more BTU capacity than this simple area-based estimate suggests — treat the result as a starting point and round up for rooms with those factors.

Frequently asked questions

My bedroom is 12x15 ft — what size window AC do I need?

About 7,000-8,000 BTU for a 180 sq ft room. Add 10% if the room gets heavy sun, or reduce 10% if it's heavily shaded. For a room used as a kitchen, add 4,000 BTU.

How many BTU per square foot do I need?

About 20-25 BTU per square foot for cooling in a typical room. Rooms with high ceilings, lots of windows, or heavy sun exposure need 25-30 BTU per square foot. Heavily shaded rooms can get by with 18-20 BTU per square foot.

Can a window AC cool more than one room?

Typically no — window ACs cool the room they're in, and air doesn't circulate well through doorways. For adjacent rooms, a portable AC with a longer reach or a mini-split system is more effective. The BTU rating assumes one enclosed room.