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Fresh Air Requirement Calculator

Work out fresh air requirement instantly with clear inputs, formula shown and shareable results.

About Fresh Air Requirement Calculator

Fresh air demand scales with occupancy. Multiplying head count by the per-person rate gives total outdoor airflow, and dividing the sedentary carbon dioxide generation rate of about 0.0106 CFM per person by the ventilation rate per person predicts the steady-state indoor concentration above the outdoor baseline of roughly 420 ppm.

Formulas

Fresh air

CFM = occupants x CFM per person; L/s = CFM x 0.4719

Steady-state CO₂

ppm = outdoor ppm + 10600 / (CFM per person)

Worked example

With the default values below, Fresh Air Requirement Calculator returns Fresh air required of 600. Change any input above and every figure here is recalculated in your browser.

Inputs used

Occupantsminimum 1
40
Fresh air per personminimum 1 CFM
15 CFM
Room volumeminimum 1 ft³
12,000 ft³

Results

Fresh air required
600 CFM
Equivalent metric flow
283.2 L/s
Fresh air changes per hour
3.00 ACH
Steady-state CO₂ estimate
1,127 ppm

Step by step

  1. 1Values used
  2. 2Fresh air
  3. 3Steady-state CO₂
  4. 4Fresh air required= 600 CFM
  5. 5Equivalent metric flow= 283.2 L/s
  6. 6Fresh air changes per hour= 3.00 ACH
  7. 7Steady-state CO₂ estimate= 1,127 ppm

Assumptions and limits

  • The result is derived only from the 3 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 2 decimal places, 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 Fresh Air Requirement figure worked out?

CFM = occupants x CFM per person; L/s = CFM x 0.4719. Fresh air demand scales with occupancy.

Why is 15 CFM per person common?

It is the classic office rate that holds carbon dioxide near 1100 ppm, the level long used as a proxy for acceptable bio-effluent dilution.

What values do I need to enter?

This calculator takes 3 inputs: Occupants, Fresh air per person, Room volume. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.

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