Airflow Volume Calculator

Calculator Inputs

Use one primary method, then optionally add room or duct data to calculate ACH, equivalent velocity, design flow, and airflow per person.

Primary Calculation Method

Duct Shape

Duct Dimension Unit

Diameter (round)

Width (rectangular)

Height (rectangular)

Air Velocity

Velocity Unit

Direct Airflow

Direct Airflow Unit

Room Length

Room Width

Room Height

Room Dimension Unit

Target ACH

Occupants

Actual Air Temperature

Temperature Unit

Actual Pressure

Pressure Unit

Standard Temperature

Standard Temp Unit

Standard Pressure

Standard Pressure Unit

Design Factor (%)

Reset

Example Data Table

Method	Duct / Room Data	Velocity / ACH	Actual Flow	ACH Result	Mass Flow	Design Flow (+10%)
Duct area × velocity	Round duct, 500 mm diameter; room 12 × 8 × 3 m	8 m/s	5,654.9 m³/h	19.6 ACH	1.89 kg/s	6,220.4 m³/h
Room volume × ACH	Room 12 × 8 × 3 m	6 ACH	1,728.0 m³/h	6.0 ACH	0.58 kg/s	1,900.8 m³/h
Direct known airflow	Rectangular duct 600 × 300 mm; room 10 × 6 × 3 m	2,500 CFM	4,247.3 m³/h	23.6 ACH	1.42 kg/s	4,672.0 m³/h

Formula Used

Duct airflow:
Q = A × V

Round duct area:
A = πD² / 4

Rectangular duct area:
A = W × H

Room volume:
Volume = L × W × H

Air changes per hour:
ACH = Airflow (m³/h) / Room volume

Required airflow from ACH:
Q = Room volume × ACH / 3600

Dry-air density:
ρ = P / (R × T)

Mass flow:
ṁ = ρ × Q

Standardized flow:
Q_std = Q_actual × (P_actual/P_std) × (T_std/T_actual)

Velocity pressure:
VP = 0.5 × ρ × V²

These equations assume dry air and steady flow. For humid air, compressible flow, or system losses, use a more detailed HVAC or fluid model.

How to Use This Calculator

Select the primary calculation method: duct velocity, room ACH, or direct airflow.
Enter duct dimensions if you want airflow from size and velocity or need equivalent velocity results.
Enter room dimensions and target ACH when checking ventilation rate against a room volume.
Add actual temperature and pressure to convert actual flow into standard flow and mass flow.
Set a design factor to estimate a safer fan selection value.
Click Calculate Airflow to show the result above the form.
Review the detailed table and Plotly graph, then export the result as CSV or PDF.

FAQs

1) What does airflow volume mean?

Airflow volume is the quantity of air moving through a duct, fan, or room over time. Common units include m³/s, m³/h, L/s, and CFM.

2) What is the difference between ACFM and SCFM?

ACFM is actual airflow at local temperature and pressure. SCFM corrects that flow to standard reference conditions, making comparison easier across locations, test conditions, and system reports.

3) Why does duct shape matter?

Duct shape changes cross-sectional area and hydraulic diameter. Those values affect airflow, equivalent velocity, friction behavior, and how closely the duct matches a round section.

4) Why should I enter temperature and pressure?

Temperature and pressure change air density. Density affects mass flow, standard flow correction, and velocity pressure, which are useful for engineering checks and fan selection.

5) When should I use the ACH method?

Use the ACH method when designing room ventilation. It helps estimate the airflow needed to replace room air a target number of times per hour.

6) What is the design factor used for?

The design factor adds a percentage margin above the calculated airflow. Engineers often use it to cover uncertainty, future load growth, leakage, or balancing adjustments.

7) Can this calculator size an entire duct system?

It estimates airflow-related metrics well, but it does not replace full duct design. Pressure losses, fittings, filters, coils, noise, and fan curves still need separate evaluation.

8) Why is velocity pressure shown?

Velocity pressure represents the pressure tied to air movement. It is useful for pitot-based measurements, airflow balancing, and checking whether velocity is reasonable for the duct size.