Air Conditioner Size Calculator Formula

Understand the math behind the air conditioner size calculator. Each variable explained with a worked example.

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Formulas Used

Recommended BTU

recommended_btu = (base_btu + occupant_adj) * sun_exposure

In Tons

recommended_tons = (base_btu + occupant_adj) * sun_exposure / 12000

Variables

VariableDescriptionDefault
room_sqftRoom Area(sq ft)350
sun_exposureSun Exposure1
occupantsUsual Occupants2
base_btuDerived value= room_sqft * 20calculated
occupant_adjDerived value= (occupants > 2) ? (occupants - 2) * 600 : 0calculated

How It Works

How to Size an Air Conditioner

The general rule is 20 BTU per square foot. Adjust for sun exposure and occupancy.

Formula

Base BTU = Room Sq Ft x 20

Adjustments: +600 BTU per person over 2, +10% for sunny rooms, -10% for shaded.

1 ton = 12,000 BTU.

Worked Example

350 sq ft room, average sun, 2 occupants.

room_sqft = 350sun_exposure = 1occupants = 2
  1. 01Base BTU = 350 x 20 = 7,000
  2. 02Occupant adjustment = 0 (2 or fewer)
  3. 03Sun factor = 1.0 (average)
  4. 04Recommended BTU = 7,000 x 1.0 = 7,000 BTU
  5. 05In tons = 7,000 / 12,000 = 0.58 tons

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