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Air Conditioner Sizing Calculator

Work out air conditioner sizing instantly with clear inputs, formula shown and shareable results.

About Air Conditioner Sizing Calculator

Air conditioner selection starts from an area-based load, then adds the gains the area rate ignores: about 600 BTU/h for every occupant beyond two and roughly 900 BTU/h for each sun-exposed window. The raw load is rounded up to the nearest half ton because that is how equipment is actually catalogued, and 400 CFM per ton gives the matching indoor airflow.

Formulas

Sizing load

BTU/h = area x rate + max(occupants - 2, 0) x 600 + windows x 900

Nominal size

tons = ceil(BTU/h / 12000 x 2) / 2; airflow = tons x 400 CFM

Worked example

With the default values below, Air Conditioner Sizing Calculator returns Recommended capacity of 30,000. Change any input above and every figure here is recalculated in your browser.

Inputs used

Floor areaminimum 1 ft²
800 ft²
Climate base rateone of Mild climate — 20 BTU/h per ft², Moderate climate — 25 BTU/h per ft², Hot climate — 30 BTU/h per ft²
Moderate climate — 25 BTU/h per ft²
Regular occupantsminimum 1
4
Sun-exposed windowsminimum 0
6

Results

Recommended capacity
30,000 BTU/h
Nominal size (rounded to 0.5 ton)
2.5 tons
Calculated load
26,600 BTU/h
Indoor unit airflow
1,000 CFM

Step by step

  1. 1Values used
  2. 2Sizing load
  3. 3Nominal size
  4. 4Recommended capacity= 30,000 BTU/h
  5. 5Nominal size (rounded to 0.5 ton)= 2.5 tons
  6. 6Calculated load= 26,600 BTU/h
  7. 7Indoor unit airflow= 1,000 CFM

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 1 decimal place, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is the Air Conditioner Sizing figure worked out?

BTU/h = area x rate + max(occupants - 2, 0) x 600 + windows x 900. Air conditioner selection starts from an area-based load, then adds the gains the area rate ignores: about 600 BTU/h for every occupant beyond two and roughly 900 BTU/h for each sun-exposed window.

Why 400 CFM per ton?

It is the standard design airflow for comfort cooling. Lower airflow increases dehumidification but risks coil freezing; higher airflow reduces latent capacity.

What values do I need to enter?

This calculator takes 4 inputs: Floor area, Climate base rate, Regular occupants, Sun-exposed windows. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.

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