How to Calculate CFM from Room Volume and ACH

Q: Can this replace a ventilation design?

No. It is a volume-flow conversion; ventilation design also requires applicable requirements, an identified air stream, equipment, distribution, and pressure review.

Q: Can I calculate from a known room volume?

Yes. Multiply the known volume by ACH and divide by 60 for CFM when the volume is in cubic feet, or multiply cubic metres by ACH for cubic metres per hour.

Direct answer

For a room volume in cubic feet, multiply the volume by the target ACH and divide by 60 to obtain CFM. For a volume in cubic metres, multiply the volume by ACH to obtain m³/h. The ACH must come from a documented project, process, standard, or code basis; the formula does not choose it.

CFM = room volume (ft³) × ACH ÷ 60
m³/h = room volume (m³) × ACH

Choose the room input you actually have

Available information	Volume step	Airflow step
Length, width, and height in feet	Multiply to get ft³	ft³ × ACH ÷ 60 = CFM
Known volume in ft³	No geometry step	ft³ × ACH ÷ 60 = CFM
Length, width, and height in metres	Multiply to get m³	m³ × ACH = m³/h
Known volume in m³	No geometry step	m³ × ACH = m³/h

Step-by-step workflow

Define the room boundary. Open-plan or connected spaces need a documented volume boundary.
Measure inside length, width, and finished ceiling height, or use a reliable known volume.
Identify the air stream: total supply, outdoor air, exhaust, return, recirculated/filtered air, or an explicitly defined clean-air quantity.
Use the ACH basis that belongs to that same purpose.
Apply the formula in one consistent unit system and record the room volume used.
Review ventilation compliance, room pressure, distribution, load, fan, and duct decisions separately.

Checked US customary example

A 20 ft by 15 ft room with an 8 ft ceiling has 20 × 15 × 8 = 2,400 ft³. At a documented 6 ACH, airflow is 2,400 × 6 ÷ 60 = 240 CFM.

Checked SI example

A known room volume of 68 m³ at 6 ACH requires 68 × 6 = 408 m³/h. Dividing by 1.69901082 gives approximately 240.14 CFM.

Do not choose supply or return by habit

Use the flow that matches the stated objective. Total supply airflow can include outdoor and recirculated air. Outdoor-air ACH uses only outdoor airflow. Exhaust ACH uses exhaust airflow. Return airflow describes air routed back toward the air handler and may differ from room supply because of exhaust, transfer paths, leakage, infiltration, or exfiltration.

If the question is whether a room is positively or negatively pressurized, a single ACH conversion is insufficient. Compare all measured or scheduled paths and verify the pressure relationship under the applicable test method.

Common calculation errors

Error	Correction
Using floor area instead of volume	Include ceiling height or enter the known volume.
Mixing ft³, m³, minutes, and hours	Use the matching formula and convert only after the result is established.
Calling total supply “outdoor air”	Use only the outdoor-air portion for an outdoor-air calculation.
Adding supply and exhaust ACH	Do not double-count opposing room flows; use the method tied to the objective.
Treating nominal ACH as perfect replacement	Mixing, short-circuiting, distribution, and treatment effectiveness require separate evaluation.

FAQ

Can this replace a ventilation design?

No. It is a volume-flow conversion. Ventilation design also requires the applicable requirements, air-stream definition, equipment, distribution, pressure relationships, and qualified review.

Can I calculate from a known room volume?

Yes. Use the volume directly rather than reconstructing dimensions. The CFM Calculator supports both input paths.

How do I calculate ACH from known CFM?

Use ACH = CFM × 60 ÷ room volume in ft³, or use the Air Changes per Hour Calculator.

How to calculate CFM