Ventilation Calculator: Air Changes per Hour and CO2 Level

The air changes per hour, air for each person and carbon dioxide level in a room, and the airflow needed for a target.

Calculated from standard textbook formulas, with every step shown. The formulas are fixed, so the working does not need checking: what matters is the numbers you type in. Use these calculators only if you are competent to choose the inputs (loads, material strength, dimensions, supports) and check your units and assumptions. For learning and checking your own working: design and sign-off are for a qualified engineer. Drafting Central does not store the numbers you type.

How this calculator works

Ventilation replaces stale air with outdoor air. The air changes per hour show how many times the whole volume of the room is replaced each hour: the airflow in litres per second times 3600, divided by the volume in litres.

The people in the room breathe out carbon dioxide, so its level rises until the fresh air carries away as much as is breathed out. The steady level is the outdoor level plus the carbon dioxide breathed out each second, divided by the airflow. A level above about 1000 ppm usually means too little fresh air for the number of people.

The calculator also gives the airflow needed for a target number of air changes and for the air required for each person. Both targets are inputs, because they depend on the standard and the use of the room.

Formulas used

  • ACH = \frac{3600\,Q}{1000\,V}
  • C = C_o + \frac{N\,G}{Q}\times 10^6
  • Q_t = \frac{ACH_t\,1000\,V}{3600}

Worked example

With the default numbers in the calculator above:

  1. Air changes per hour: 7.20 per hour
  2. Outdoor air for each person: 30.00 L/s
  3. Steady carbon dioxide level in the room: 593.33 ppm
  4. Airflow needed for the target air changes: 250.00 L/s
  5. Airflow needed for the people: 100.00 L/s

Change the numbers in the calculator to see the full working, every step, for your own case.

A second worked example

A different case, with these numbers: Outdoor air flow supplied to the room 200 L/s; Number of people in the room 20.

  1. Air changes per hour: 4.80 per hour
  2. Outdoor air for each person: 10.00 L/s
  3. Steady carbon dioxide level in the room: 940.00 ppm
  4. Airflow needed for the target air changes: 250.00 L/s
  5. Airflow needed for the people: 200.00 L/s

Common questions

What is a good number of air changes per hour?

It depends on the use of the room. It comes from the ventilation standard or the design brief, so it is an input.

Does CO2 show that the air is safe?

It shows whether there is enough fresh air for the people. It does not measure fumes, dust or other contaminants from the work.

Is room ventilation enough for fumes?

Not usually. Where a process makes fumes or dust, remove them at the source with local exhaust ventilation.

Related calculators

Got a question about this? Ask an engineering question and get a plain-language answer with references.

Further reading: AS 1668.2 (ventilation design for indoor air contaminant control, licensed); Managing the work environment and facilities Code of Practice (Queensland).

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