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BTU Calculator

Estimate how many BTUs of cooling a room needs and which air conditioner size to buy, plus a heating estimate. The calculation starts from the common 20 BTU per square foot rule and adjusts for ceiling height, insulation, sun, people and kitchens.

Units
Room
ft
ft
ft
Conditions
Enter your values to see the result.
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How to use the BTU Calculator

  1. Choose feet or meters and enter the room length, width and ceiling height.
  2. Enter how many people usually use the room.
  3. Pick insulation and sun exposure, and tick the box if the room is a kitchen.
  4. Choose your climate for the heating estimate.
  5. Read the cooling BTU, tons and suggested AC size, and check the breakdown table.

How the BTU estimate works

  1. Base load = floor area (sq ft) × 20 BTU/h.
  2. Ceiling height: multiply by ceiling height ÷ 8 ft, because a taller room has more air to cool.
  3. Insulation: × 1.2 for poor insulation, × 0.9 for good insulation (rule-of-thumb adjustments).
  4. Sun: × 0.9 for a heavily shaded room, × 1.1 for a very sunny room.
  5. People: + 600 BTU/h for each regular occupant beyond two.
  6. Kitchen: + 4,000 BTU/h.
  7. Tons = BTU/h ÷ 12,000; kW = BTU/h × 0.000293.

Example: a 16 × 14 ft bedroom (224 sq ft) with an 8 ft ceiling and average conditions needs 224 × 20 = 4,480 BTU/h, so a 5,000 BTU unit is enough. Make it very sunny and used by 3 people and the load becomes 4,480 × 1.1 + 600 = 5,528 BTU/h, which suits a 6,000 BTU unit.

ENERGY STAR room AC sizing chart

Area to be cooled (sq ft)Capacity needed (BTU/h)
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

The chart assumes a standard 8 ft ceiling and average conditions; apply the shade, sun, occupant and kitchen adjustments above.

Frequently asked questions

How many BTU do I need per square foot?

A common starting point for room air conditioners is about 20 BTU per hour per square foot with an 8 ft ceiling. ENERGY STAR's sizing chart follows a similar curve, for example about 6,000 BTU for 150–250 sq ft and 12,000 BTU for 450–550 sq ft.

What adjustments do sizing guides recommend?

ENERGY STAR suggests reducing capacity by 10% for a heavily shaded room, increasing it by 10% for a very sunny room, adding 600 BTU for each person beyond two who regularly uses the room, and adding 4,000 BTU if the unit is used in a kitchen.

How many BTU is one ton of air conditioning?

One ton of cooling equals 12,000 BTU per hour, the heat needed to melt one short ton of ice in 24 hours. A 24,000 BTU unit is a 2-ton unit.

Is a bigger air conditioner better?

No. An oversized unit cools the room quickly and shuts off before it removes enough moisture, leaving the air cold and clammy, and it cycles on and off more often. Choose a size close to the calculated load.

How is the heating estimate calculated?

Heating uses a rule of thumb of about 30 BTU per square foot in hot climates up to about 60 in very cold climates, adjusted for ceiling height and insulation. Real heat loss depends on windows, air leaks and the outdoor design temperature.

Is this a substitute for a Manual J load calculation?

No. This is a quick estimate for a single room. Whole-house HVAC systems should be sized with a professional load calculation such as ACCA Manual J, which accounts for windows, orientation, insulation values, ducts and local climate data.

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