BuildersMeasures

Air Flow Calculator

Calculate required airflow from room volume and air changes per hour, or calculate airflow through a duct using its area and air velocity.

Calculate Air Flow

Calculation Method
Enter the target number of complete air changes per hour.
Room method: Airflow = Room Volume × Air Changes Per Hour. Duct method: Airflow = Duct Cross-Sectional Area × Air Velocity.

Air Flow Result

Air Flow 0 CFM
Cubic Feet Per Minute 0 CFM
Cubic Meters Per Hour 0
Liters Per Second 0 L/s
Cubic Meters Per Second 0
Room Volume 0
Air Changes Per Hour 0 ACH
Airflow calculations are planning estimates. Actual HVAC design also depends on duct resistance, pressure, fittings, filters, equipment capacity, room use and applicable design standards.

How the Air Flow Calculator Works

Airflow measures how much air moves through a space or ventilation system over a given period. HVAC professionals commonly express airflow in CFM, cubic meters per hour, liters per second or cubic meters per second.

This calculator uses two common approaches. The room method starts with the room volume and required air changes per hour. The duct method starts with the duct's cross-sectional area and air velocity.

Airflow From Room Volume and ACH

Air changes per hour, or ACH, describes how many times the equivalent volume of air in a room is replaced during one hour.

Airflow (cubic meters per hour) = Room Volume (cubic meters) × ACH

Room volume is calculated by multiplying length, width and height.

Room Volume = Length × Width × Height

For example, a 6 m × 5 m × 2.8 m room has a volume of 84 cubic meters. At 6 ACH, the required airflow is 504 cubic meters per hour.

Airflow From Duct Area and Velocity

For a duct, airflow depends on the cross-sectional area and the average air velocity through the duct.

Airflow (cubic meters per second) = Duct Area (square meters) × Air Velocity (meters per second)

For a round duct, the cross-sectional area is based on its radius.

Round Duct Area = Pi × Radius × Radius

For a rectangular duct:

Rectangular Duct Area = Width × Height

Airflow Unit Conversions

  • 1 cubic meter per second = 1,000 liters per second
  • 1 cubic meter per second = 3,600 cubic meters per hour
  • 1 cubic meter per second is approximately 2,118.88 CFM
  • 1 CFM is approximately 1.699 cubic meters per hour
  • 1 CFM is approximately 0.4719 liters per second

What Is ACH?

ACH stands for air changes per hour. It is used in ventilation planning to describe the rate at which air is replaced within a room.

The appropriate ACH depends on the type of space, occupancy, activity, ventilation strategy and applicable building requirements. A bedroom, office, bathroom, workshop and commercial kitchen might require different ventilation rates.

Important: Do not select an ACH value only because it produces a convenient airflow number. Use the ventilation requirement applicable to the building, room type and project.

Why Duct Size Matters

A duct that is too small for the required airflow might require higher air velocity and create greater pressure losses and noise. A larger duct reduces velocity for the same airflow, although duct sizing also depends on the overall HVAC system design.

The calculator helps establish the basic relationship between airflow, duct area and velocity. Detailed HVAC design requires additional calculations.

Frequently Asked Questions

For room ventilation, airflow in cubic meters per hour equals room volume multiplied by air changes per hour. For ducts, airflow in cubic meters per second equals cross-sectional area multiplied by air velocity.

CFM means cubic feet per minute. It is a common airflow unit used for ventilation, fans, air conditioners and HVAC equipment.

ACH means air changes per hour. It represents the equivalent number of times the air volume of a room is replaced during one hour.

Multiply the duct's cross-sectional area in square meters by the average air velocity in meters per second. The result is airflow in cubic meters per second.

Multiply CFM by approximately 1.699 to convert cubic feet per minute to cubic meters per hour.

No. It provides basic airflow calculations. Full HVAC sizing also considers heating and cooling loads, duct pressure losses, equipment performance, outdoor air requirements and applicable design standards.