Softonic

BTU Calculator

Step 1 / 3 BTU per hour
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Buy an air conditioner that is too small and the room never gets comfortable; buy one that is too big and it short-cycles, wastes energy and leaves the air clammy. This BTU calculator turns your room dimensions, ceiling height, sun exposure and the number of people in the room into a recommended cooling capacity in BTU per hour and the equivalent AC size in tons, so you can shop with a number instead of a guess.

How to size an air conditioner

  1. 1

    Measure the room

    Enter the length and width in feet. The calculator multiplies them to get the floor area in square feet.

  2. 2

    Add the details

    Set the ceiling height, pick the sun exposure (shaded, normal or sunny) and enter how many people regularly use the room.

  3. 3

    Read the recommendation

    You get the recommended BTU per hour, the matching AC size in tons, and the room area used in the estimate.

The formula

The estimate follows the rule-of-thumb used by HVAC retailers, then adjusts for the factors that matter most:

area      = length × width                      (square feet)
base BTU  = area × 20                            (20 BTU per sq ft)
height    = base × (ceiling height / 8)          (taller rooms hold more air)
sun       = height × 1.1 if sunny, × 0.9 if shaded, × 1.0 if normal
BTU/hr    = sun + max(0, occupants − 2) × 600    (each extra person adds heat)
tons      = BTU/hr ÷ 12,000                       (1 ton of cooling = 12,000 BTU/hr)

The base figure of 20 BTU per square foot assumes a standard 8-foot ceiling and average insulation. The first two occupants are already included in that baseline, so only people beyond two add 600 BTU each.

Worked example

A 15 ft × 12 ft bedroom with a 9 ft ceiling, normal sun and 2 occupants:

  • area = 15 × 12 = 180 sq ft
  • base = 180 × 20 = 3,600 BTU/hr
  • height = 3,600 × (9 / 8) = 4,050 BTU/hr
  • sun (normal) = 4,050 BTU/hr
  • occupants: max(0, 2 − 2) × 600 = 0
  • BTU/hr ≈ 4,100 (rounded to the nearest 100) → 0.34 tons

A 5,000 BTU window unit would comfortably cover this room.

Quick reference by room size

Room area (sq ft) Approx. BTU/hr AC size (tons)
100–150 5,000 0.4
150–250 6,000 0.5
250–350 7,000–8,000 0.6–0.7
350–450 9,000–10,000 0.8
450–550 12,000 1.0
550–700 14,000 1.2
700–1,000 18,000 1.5

Pitfalls to watch for

  • Sun-baked rooms: a room with large west-facing windows can need 10 percent more capacity, which the sunny setting accounts for.
  • Kitchens: add roughly 4,000 BTU on top of the estimate for the heat from cooking appliances.
  • Ceiling height: vaulted or loft ceilings hold far more air; the height adjustment handles this, so measure it accurately.
  • Bigger is not better: an oversized unit cools fast then shuts off before removing humidity, leaving the room cold and damp. Aim for the calculated size, not double it.

Frequently Asked Questions

BTU stands for British Thermal Unit. For cooling it describes how much heat the unit can remove per hour, so a higher BTU rating means more cooling power. Window and portable units are sold by BTU/hr; central systems are usually sold by tons.

One ton of cooling equals 12,000 BTU per hour. So a 24,000 BTU/hr system is a 2-ton unit. The calculator shows both numbers so you can match either a window unit or a central system.

No. An oversized unit cools the air quickly and switches off before it has pulled enough moisture out, leaving the room cold and humid and wearing the compressor with frequent cycling. Matching the calculated capacity gives steadier temperatures and lower running costs.

The room dimensions, ceiling height, sun exposure and occupant count are sent to our server to compute the estimate, and in the step-by-step view they are carried in the page link between steps. None of these values are stored, shared or kept after you leave the page.

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