Calculator Tools

BTU Calculator

Calculate the BTU air conditioner you need for any room using DOE and ENERGY STAR sizing. Adjust for ceiling height, sun, climate, occupants, and kitchen.

Inputs

Enter the room size and a few details. Defaults follow the DOE and ENERGY STAR guidelines for room air conditioner sizing.

Room size

Length

Width

Unit

Common room presets

Room details

Ceiling height

Unit

The 20 BTU/sq ft rule assumes an 8 ft ceiling. Taller ceilings scale the load linearly with the volume.

Regular occupants

Add 600 BTU/h for each person beyond the first two.

Room is a kitchenCooking heat adds about 4,000 BTU/h. Tick when sizing for a kitchen or any room with a regular cooking load.

Sun exposure

South or west-facing rooms with large windows usually qualify as very sunny. North-facing rooms or rooms shaded by other buildings count as heavily shaded.

Climate

Result

Recommended BTU/h

6,000

Calculated capacity for a 300 sq ft room with the inputs above.

Suggested AC size (next standard)

6,000

Off-the-shelf room AC sizes round up to this capacity. Going one size larger is fine; going smaller leaves the room undercooled.

Floor area
300 sq ft / 27.87 sq m
Length times width of the room you entered.
Base load (20 BTU per sq ft)
6,000 BTU/h
Floor area multiplied by the DOE baseline of 20 BTU/h per sq ft.
Ceiling factor
x1
8 ft / 8 ft. Scales the load by air volume.
Sun factor
x1
Normal sun exposure (no change)
Climate factor
x1
Moderate (x1.0)
Occupant load
0 BTU/h
0 extra occupants x 600 BTU/h.
Equivalent capacity
1.76 kW / 0.5 tons
1 ton of refrigeration = 12,000 BTU/h. 1 kW = 3,412 BTU/h.

Math runs entirely in your browser. Nothing is uploaded.

How the math works

Base load is 20 BTU/h per square foot, the DOE and ENERGY STAR rule of thumb for a properly-insulated room with an 8 ft ceiling and normal sun.
Ceiling factor is ceiling height divided by 8 ft. A 10 ft ceiling scales the load by 1.25 because there is 25% more air volume to cool.
Sun factor is +10% for very sunny rooms and -10% for heavily shaded rooms.
Climate factor ranges from 0.9 in cool climates to 1.2 in hot climates, matching the DOE multipliers for room AC sizing.
Occupants add 600 BTU/h for every regular occupant past the first two. A kitchen adds 4,000 BTU/h to cover the steady cooking heat.
The final figure is rounded up to the closest off-the-shelf AC size (5,000, 6,000, 8,000, 10,000, 12,000, 14,000, 18,000, 24,000, 30,000, 36,000 BTU/h) so the answer maps to a unit you can actually buy.

Room size to BTU quick chart

Baseline values from ENERGY STAR. Adjust up or down using sun, climate, ceiling, and occupant settings above.

Floor area	Suggested BTU/h
100 to 150 sq ft	5,000 BTU/h
150 to 250 sq ft	6,000 BTU/h
250 to 300 sq ft	7,000 BTU/h
300 to 350 sq ft	8,000 BTU/h
350 to 400 sq ft	9,000 BTU/h
400 to 450 sq ft	10,000 BTU/h
450 to 550 sq ft	12,000 BTU/h
550 to 700 sq ft	14,000 BTU/h
700 to 1,000 sq ft	18,000 BTU/h
1,000 to 1,200 sq ft	21,000 BTU/h
1,200 to 1,400 sq ft	23,000 BTU/h
1,400 to 1,500 sq ft	24,000 BTU/h
1,500 to 2,000 sq ft	30,000 BTU/h
2,000 to 2,500 sq ft	34,000 BTU/h

For a room with full afternoon sun, a 10 ft ceiling, more than two occupants, or kitchen cooking heat, use this calculator instead of the flat chart.

How to use

Pick a room size mode: Length x Width for a rectangular room, or Floor area when you already know the square footage.
Enter the room dimensions. You can switch between feet and meters at any time.
Set the ceiling height if it is not the standard 8 ft. Add the number of regular occupants past two and tick Kitchen if the room has a cooking load.
Pick the sun exposure (very sunny, normal, or heavily shaded) and the climate that best matches your area.
Read the Recommended BTU/h figure and the Suggested AC size next to it. The breakdown below shows how each adjustment changed the base load.
Click Copy summary to capture every input, factor, and result in a single block ready to paste into a contractor quote, a shopping list, or a note.

About this tool

BTU Calculator answers the question every shopper asks before buying an air conditioner: what size unit do I need? Enter the room length and width (or the floor area directly) plus a few details about the room, and the tool prints the recommended cooling capacity in BTU per hour, the equivalent in kilowatts and tons of refrigeration, and the next off-the-shelf AC size to look for. The math follows the rule of thumb published by the U.S. Department of Energy and ENERGY STAR for room air conditioner sizing: a base load of 20 BTU per square foot for a properly-insulated room with an 8 foot ceiling and two occupants, then five standard adjustments on top: ceiling height (the load scales linearly with the volume ratio), sun exposure (plus 10 percent for very sunny rooms, minus 10 percent for heavily shaded), climate (multipliers from 0.9 in cool climates to 1.2 in hot climates), extra occupants (plus 600 BTU per regular occupant past the first two), and a kitchen adjustment (plus 4,000 BTU for the steady cooking heat). The recommendation is rounded up to the closest standard AC capacity (5,000, 6,000, 8,000, 10,000, 12,000, 14,000, 18,000, 24,000, 30,000, and 36,000 BTU/h) so the answer maps to a unit you can actually buy on a retailer site. A quick reference chart shows the baseline ENERGY STAR sizing for common room sizes alongside the calculation result. Useful for picking a window AC for a bedroom, sizing a portable AC for a studio, comparing two mini split heads, or sanity-checking a contractor quote for a central system. All math runs in your browser; nothing you type is uploaded.

Free to use. Works in your browser. No signup, no login.

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