BTUtally

Free calculator

AC BTU calculator (room size)

A 180 sq ft (16.7 m²) room with an 8 ft ceiling, average sun and two people needs 6,000 BTU/h on the ENERGY STAR chart: 0.50 tons, 1.76 kW, a 6,000 BTU room unit. Type your own room, sun, people and kitchen to see yours in BTU/h, tons and kW.

Result

Cooling needed
6,000 BTU/h
Room unit class
6,000 BTU room unit
Tons
0.50 tons
Kilowatts
1.76 kW
Chart band
150-250 sq ft: 6,000 BTU/h
Adjustments
none

180 sq ft (16.7 m²) falls in the chart band 150-250 sq ft (ENERGY STAR, Room Air Conditioners, cooling capacity chart). Sizing estimates from published charts and rules of thumb, each listed with its source and the day it was read: an installer's load calculation (Manual J for a US home, a room-by-room heat loss calculation in the UK) is the final word.

Sizing the whole home? Pro puts every room on one printable report.

A room list with cooling BTU per room and the totals, the heating load, the boiler kW and the equipment classes, with the sources. Printed or exported to Excel.

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From $4.99 a month, or $19 once

How it works

The floor area (length x width) is looked up in the ENERGY STAR room air conditioner sizing chart, which gives a capacity in BTU per hour for each area band from 100 sq ft (5,000 BTU) to 2,500 sq ft (34,000 BTU). The chart's own adjustments are then applied: a heavily shaded room takes 10 percent off, a very sunny room adds 10 percent, each person beyond two adds 600 BTU, and a kitchen adds 4,000 BTU. Tons = BTU/h / 12,000; kW = BTU/h / 3,412.

With the starting room (180 sq ft), the chart band 150-250 sq ft gives 6,000 BTU/h. Make it a very sunny kitchen and it becomes 10,600 BTU/h (x1.1, + 4,000). A 400 sq ft living room with four people needs 11,200 BTU/h: the 400-450 sq ft band plus 1,200 BTU for the two extra people.

ENERGY STAR: "The capacities are calculated based on an 8-foot ceiling." It gives no figure for a taller room, so for one the chart figure is scaled by height / 8 ft (a 10 ft ceiling adds 25 percent); that is our assumption, not ENERGY STAR's, and it is labelled as such in the Sources list. Rooms below 100 sq ft get the smallest band; rooms above 2,500 sq ft are beyond a room unit, so the tool says so and points to the heat pump calculator for a central system. The unit class is the next chart capacity at or above your figure, not a product recommendation.

Every number is yours to change; the starting values only save typing. All calculations run in your browser and the inputs stay there, so the Pro report can pick up your rooms. Sizing estimates from published charts and rules of thumb, each listed with its source and the day it was read: an installer's load calculation (Manual J for a US home, a room-by-room heat loss calculation in the UK) is the final word.

Sources of the US starting values

  • Area to BTU chart: ENERGY STAR (U.S. EPA), Room Air Conditioners, cooling capacity chart (undated page; step "2. Find the correct cooling capacity for your room size"): Area To Be Cooled (square feet) -> Capacity Needed (BTUs per hour): "100 up to 150" -> "5,000"; "150 up to 250" -> "6,000"; "250 up to 300" -> "7,000"; "300 up to 350" -> "8,000"; "350 up to 400" -> "9,000"; "400 up to 450" -> "10,000"; "450 up to 550" -> "12,000"; "550 up to 700" -> "14,000"; "700 up to 1,000" -> "18,000"; "1,000 up to 1,200" -> "21,000"; "1,200 up to 1,400" -> "23,000"; "1,400 up to 1,500" -> "24,000"; "1,500 up to 2,000" -> "30,000"; "2,000 up to 2,500" -> "34,000" — https://www.energystar.gov/products/room_air_conditioners, read on 2026-09-20
  • Heavily shaded room: ENERGY STAR (U.S. EPA), Room Air Conditioners, sizing chart notes (undated page): "If the room is heavily shaded, reduce capacity by 10 percent" — https://www.energystar.gov/products/room_air_conditioners, read on 2026-09-20
  • Very sunny room: ENERGY STAR (U.S. EPA), Room Air Conditioners, sizing chart notes (undated page): "If the room is very sunny, increase capacity by 10 percent" — https://www.energystar.gov/products/room_air_conditioners, read on 2026-09-20
  • More than two people: ENERGY STAR (U.S. EPA), Room Air Conditioners, sizing chart notes (undated page): "If more than two people regularly occupy the room, add 600 BTUs for each additional person" — https://www.energystar.gov/products/room_air_conditioners, read on 2026-09-20
  • Kitchen: ENERGY STAR (U.S. EPA), Room Air Conditioners, sizing chart notes (undated page): "If the unit is used in a kitchen, increase capacity by 4,000 BTUs" — https://www.energystar.gov/products/room_air_conditioners, read on 2026-09-20
  • Ceiling height: ENERGY STAR (U.S. EPA), Room Air Conditioners, cooling capacity chart (undated page; step "2. Find the correct cooling capacity for your room size"): "The capacities are calculated based on an 8-foot ceiling." The page gives no capacity figure or factor for a taller ceiling. BTUtally assumption, not ENERGY STAR's: above 8 ft the chart figure is scaled by height / 8 ft, so a 10 ft ceiling adds 25 percent; below 8 ft the chart figure is used unchanged — https://www.energystar.gov/products/room_air_conditioners, read on 2026-09-20
  • Tons and kW: Wikipedia, British thermal unit (read 2026): "1 ton of cooling, a common unit in North American refrigeration and air conditioning applications, is 12,000 Btu/h (3.52 kW)" — https://en.wikipedia.org/wiki/British_thermal_unit, read on 2026-09-20
  • BTU/h to watts: Wikipedia, British thermal unit (read 2026): "1 W is approximately 3.412142 Btu/h"; the International Table BTU is about 1,055.06 J — https://en.wikipedia.org/wiki/British_thermal_unit, read on 2026-09-20

The same chart serves the UK / metric setting; only the units change (m² is converted to sq ft before the lookup).

Frequently asked questions

How many BTU do I need for a 12 x 12 room?
A 12 x 12 ft room is 144 sq ft, in the chart's "100 up to 150" band: 5,000 BTU/h with average sun and two people (ENERGY STAR, Room Air Conditioners, cooling capacity chart). Very sunny: 5,500 BTU/h; a kitchen: 9,000 BTU/h. Type 12 and 12 in the calculator to see your own adjustments.
How many BTU per square foot for cooling?
The ENERGY STAR chart is not a flat rate: it runs from 40 BTU per sq ft in the smallest rooms to about 15 BTU per sq ft in the largest, because a big room loses proportionally less heat through its walls. The guide on this site has the whole chart as a table.
What does a ton of cooling mean?
One ton of cooling is 12,000 BTU/h (Wikipedia, British thermal unit). So a 6,000 BTU room unit is half a ton, an 18,000 BTU unit is 1.5 tons, and a 3-ton central system is 36,000 BTU/h. The tool shows tons beside BTU/h and kW for every result.
Is a bigger air conditioner better?
The chart sizes the unit to the room and adjusts for sun, people and a kitchen; it does not add a margin on top, and neither does this tool. Pick the chart size for your room, then let the adjustments move it. If your figure sits between two unit sizes, the tool names the next chart capacity up.
Does the ceiling height matter?
ENERGY STAR: "The capacities are calculated based on an 8-foot ceiling." It gives no figure for a taller room. Above 8 ft we scale the chart figure by height / 8 ft, so a 9 ft ceiling adds 12.5 percent and a 10 ft ceiling 25 percent. That scaling is our assumption, not ENERGY STAR's, labelled in the Sources list; an installer's Manual J calculation uses the real volume, glazing and orientation.
Can I use this for a whole house?
Room by room, yes: the Pro report lists every room with its own BTU and adds them up, with a central system class in tons. A single space above 2,500 sq ft is beyond the chart; use the heat pump / furnace calculator for a whole-home figure by climate zone.

Guides

Last updated 2026-10-19.