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BTU calculator: air conditioner and heater size

Enter the room's size and a few details about it to see how many BTUs a window or portable air conditioner needs, and what a heater would need in your climate. Cooling sizes follow the ENERGY STAR room air conditioner guide.

What do you need?
Units
Room size

Sizing charts assume 8 ft.

Sun on the room

ENERGY STAR takes off 10% for a heavily shaded room and adds 10% for a very sunny one.

ENERGY STAR adds 600 BTU for each person over two.

Adds 4,000 BTU for the heat from cooking.

Air conditioner size
6,000 BTU
The room needs about 6,000 BTU/h (0.5 tons). That is a common unit size.

How it adds up

ENERGY STAR chart150 to 250 sq ft (room: 180 sq ft)
6,000
Cooling needed (BTU/h)
6,000
Tons (12,000 BTU/h each)
0.5
Kilowatts of cooling
1.76

Common room AC sizes: 5k, 6k, 8k, 10k, 12k, 14k, 15k, 18k, 24k BTU.

Bigger isn’t better. An oversized unit cools the room before it can pull out the humidity, so the air ends up cold and clammy, and it costs more to buy and run.
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How to work out the BTUs a room needs

  1. 1

    Choose Cooling for an air conditioner or Heating for a heater, then enter the room's length and width, or its floor area.

  2. 2

    Enter the ceiling height. The sizing charts assume 8 ft, so taller ceilings get a larger answer.

  3. 3

    For cooling, say how sunny the room is, how many people usually use it, and whether it's a kitchen. For heating, pick your climate.

  4. 4

    Read the BTUs and the next common unit size, with tons and kilowatts, or the watts for an electric heater.

Features

  • ENERGY STAR sizing

    Cooling follows the ENERGY STAR room air conditioner chart and its adjustments for sun, shade, people and kitchens.

  • The size to shop for

    Shows the next common unit size up, so you know whether to look at 8,000, 10,000 or 12,000 BTU models.

  • Watts for heaters

    Heating needs are shown in BTU per hour and in watts, the unit electric space heaters are sold in.

  • Tons and kilowatts

    Every cooling answer is also given in tons (12,000 BTU per hour) and kilowatts, for mini-splits and metric labels.

The ENERGY STAR room air conditioner chart

ENERGY STAR sizes room air conditioners by the floor area they cool. Its chart is calculated for rooms with 8 ft ceilings, and the calculator uses it as the starting point for every cooling answer.

Area to cool (sq ft)Capacity (BTU per hour)
100 to 1505,000
150 to 2506,000
250 to 3007,000
300 to 3508,000
350 to 4009,000
400 to 45010,000
450 to 55012,000
550 to 70014,000
700 to 1,00018,000
1,000 to 1,20021,000
1,200 to 1,40023,000
1,400 to 1,50024,000
1,500 to 2,00030,000
2,000 to 2,50034,000

Each row runs up to, but not including, the next one: a room of exactly 150 sq ft uses the 6,000 BTU row. Then adjust for the room:

  • Heavily shaded: take off 10%.
  • Very sunny: add 10%.
  • More than two people regularly in the room: add 600 BTU for each extra person.
  • Kitchen: add 4,000 BTU for the heat from cooking.

Worked examples

  • Bedroom, 12 × 15 ft (180 sq ft), average sun, two people: 6,000 BTU straight from the chart. Shop for a 6,000 BTU unit.
  • Sunny living room, 20 × 22 ft (440 sq ft), four people: 10,000 BTU, plus 10% for sun (1,000), plus 1,200 for two extra people = 12,200 BTU. The next common size is 14,000 BTU, about 1.17 tons.
  • Kitchen, 12 × 14 ft (168 sq ft): 6,000 BTU for the area plus 4,000 for cooking = 10,000 BTU, or 2.93 kW.

Tall and vaulted ceilings

Because the chart assumes 8 ft ceilings, a taller room has more air to cool than its floor area suggests. ENERGY STAR doesn't give a formula for this, so the calculator scales the chart figure by the extra height: a 10 ft ceiling adds 25%, a 12 ft ceiling 50%. A 16 × 20 ft room (320 sq ft) with a 12 ft ceiling comes to 12,000 BTU instead of 8,000, so look at 12,000 BTU units. For vaulted ceilings, use the average height.

Heating: a rough rule of thumb

There's no ENERGY STAR chart for heaters. The common shortcut is a number of BTUs per square foot that depends on your climate, from about 30 in the warmest parts of the US to 60 in the coldest. It assumes 8 ft ceilings and average insulation.

ClimateBTU per sq ft200 sq ft room
Hot (Florida, the Gulf Coast, south Texas)30 to 356,000 to 7,000 BTU/h
Warm (Southeast, Southwest, most of California)35 to 407,000 to 8,000 BTU/h
Mixed (Mid-Atlantic, Tennessee, Kansas, Pacific Northwest)40 to 458,000 to 9,000 BTU/h
Cold (Midwest, Northeast, Colorado)45 to 509,000 to 10,000 BTU/h
Very cold (Minnesota, northern New England, the Dakotas, Alaska)50 to 6010,000 to 12,000 BTU/h

Treat these as a first guess for a single room or a space heater. Drafty rooms, big windows and poor insulation need more; a well-sealed, insulated room needs less. To size a furnace, boiler or heat pump for a whole house, HVAC contractors use a Manual J load calculation, the method published by the Air Conditioning Contractors of America (ACCA). It accounts for your walls, windows, insulation, air leaks and local design temperatures.

BTU, watts and tons

  • Watts to BTU per hour: multiply by 3.412. A 1,500 W heater gives about 5,118 BTU/h.
  • BTU per hour to tons: divide by 12,000. 18,000 BTU is 1.5 tons.
  • BTU per hour to kilowatts: divide by 3,412. 12,000 BTU is about 3.52 kW, the unit mini-splits are often rated in outside the US.

Working out an odd-shaped room first? Get its floor area from the square footage calculator and choose Floor area here. Finishing a basement or an attic room? The drywall calculator works out the sheets, screws and mud.

Frequently asked questions

How many BTUs do I need for a 12 × 12 room?

A 12 × 12 ft room is 144 square feet, which falls in the 100 to 150 sq ft row of the ENERGY STAR chart: 5,000 BTU, the smallest common size. Add 10% if it gets a lot of sun, and 600 BTU for each regular occupant beyond two.

How many square feet does a 10,000 BTU air conditioner cool?

On the ENERGY STAR chart, 10,000 BTU covers 400 to 450 square feet with 8 ft ceilings and average sun. An 8,000 BTU unit covers 300 to 350, and a 12,000 BTU unit 450 to 550. Shade, sun, people and a kitchen move those numbers.

What is a ton of air conditioning?

One ton of cooling is 12,000 BTU per hour, the heat it takes to melt a ton of ice in a day. A 24,000 BTU unit is a 2-ton unit. Central air systems for houses are usually sized in half-ton steps.

How do I convert BTU to watts?

One watt is about 3.412 BTU per hour, so divide BTU by 3.412 to get watts. A 1,500 W space heater puts out about 5,118 BTU per hour. An air conditioner's cooling capacity in BTU is not the same as the power it draws, which is far lower and is printed on its label.

Is a bigger air conditioner better?

No. ENERGY STAR warns that an oversized unit costs more, wastes energy and cools the room before it has time to remove the humidity, which leaves the air cold and clammy. Pick the size the chart gives, or the next common size up.

How many BTUs do I need to heat a room?

A common rule of thumb is 30 to 60 BTU per square foot, depending on how cold your winters are. A 200 sq ft room in a mixed climate at 40 to 45 BTU per sq ft needs about 8,000 to 9,000 BTU per hour, roughly 2,490 W of electric heat. For a whole-house system, a Manual J load calculation is the real answer.

Why does my portable air conditioner list two BTU numbers?

Portable air conditioners sold in the US are rated with a Department of Energy method that gives a seasonally adjusted cooling capacity (SACC). It is lower than the older ASHRAE figure because it counts the heat a portable unit lets back into the room. Compare portable units by their SACC.

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