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Construction & HomehvacUnited States only

BTU Calculator

Cooling load in BTU and tons. Oversizing an air conditioner is a real fault, not a safety margin: an oversized unit cools quickly, short-cycles and never runs long enough to remove humidity.

Also called: air conditioner size calculator, cooling load calculator.

m²
m
Climate
Sun exposure
W
Cooling required
5,041

5,041 BTU/hr, which is 0.42 tons or 1.48 kW. The room base is 2,700 BTU with 1,200 for occupants, 682 for equipment and a 10% climate and exposure adjustment. An oversized unit short-cycles and cannot dehumidify. Matching the load beats exceeding it.

In tons of refrigeration
0.42
In kilowatts
1.48
Room base load
2,700
Occupant load
1,200
Equipment load
682
Climate adjustment
10
On oversizing
An oversized unit short-cycles and cannot dehumidify. Matching the load beats exceeding it.
On regulation
This is a sizing estimate. The applicable wiring or building regulation governs and overrides any figure here.
How this is calculated
Method and background

How this is calculated

The base load scales with room volume rather than floor area, since a high ceiling is more air to cool. Occupants add roughly 600 BTU each and equipment converts at 3.412 BTU per watt, since electrical equipment ends up as heat. Climate and sun exposure adjust the total. The reason to avoid rounding up generously is that an oversized unit reaches the set temperature fast, switches off, and leaves the room cold and clammy, because dehumidification needs sustained run time.

volume-based base load plus internal gains, then adjusted for climate and exposure
V
Room volume
n
Occupants

Worked examples

Each of these is asserted on every build. If a change to the engine ever moved one of these answers, the build would fail before the page could print it.

a 20 m² room in a hot climate

Room area
20 m²
Ceiling height
3 m
Climate
Hot
Sun exposure
Normal
Regular occupants
2
Heat-producing equipment
200 W

Cooling required5,041

60 m3 x 45; 2 x 600

Open this example

a temperate shaded room needs less

Room area
20 m²
Ceiling height
3 m
Climate
Temperate
Sun exposure
Shaded
Regular occupants
2
Heat-producing equipment
200 W

Cooling required4,124

boundary: the smallest adjustment

Open this example

Method and limits

What it assumes

  • Reasonable insulation and normal window area.

What it deliberately does not model

  • A proper load calculation accounts for wall construction, glazing, orientation and infiltration.
  • Poor insulation or large west-facing glazing can change this substantially.

Formula version 1.0.0 · definition 1.0.0 · United States · Report a problem with this calculator

Frequently asked questions

Should I round up to the next size?
Generally no. An oversized unit short-cycles and fails to dehumidify, leaving the room cold and damp. Matching the load is the goal, not exceeding it.
What is a ton of cooling?
12,000 BTU per hour, originally the rate of cooling from melting a ton of ice in a day. It has nothing to do with the weight of the unit.