Size a duct cross-section from the air flow rate Q and the design mean velocity v: area A = Q/v. Circular duct diameter D = √(4A/π). Rectangular duct with aspect ratio r = a/b gives b = √(A/r) and a = r·b, plus the ASHRAE equivalent diameter D_eq = 1.30·(a·b)^0.625/(a+b)^0.25. Flow rate in m³/s/m³/h/CFM, velocity in m/s; dimensions reported in mm and inches.
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Key facts
Category
Math & Numbers
Input types
select, number
Output type
json
Sample coverage
4
API ready
Yes
Overview
The Duct Size Calculator determines the required duct cross-sectional area from airflow and design velocity, then calculates dimensions for circular or rectangular ducts. Enter airflow in m³/s, m³/h, or CFM and velocity in m/s to receive dimensions in millimetres and inches.
When to use
Estimate a duct cross-section for a specified airflow and design velocity.
Compare circular and rectangular duct dimensions for HVAC layouts.
Calculate rectangular duct dimensions using a chosen aspect ratio and equivalent diameter.
How it works
1Enter the airflow rate and select m³/s, m³/h, or CFM as the flow unit.
2Enter the design mean air velocity in m/s.
3Choose a circular or rectangular duct shape; for rectangular ducts, set the aspect ratio a/b.
4The calculator finds area using A = Q/v and reports the resulting diameter or width and height in millimetres and inches.
Use cases
HVAC design estimates for supply and return ductwork.
Preliminary sizing of circular ducts from CFM or metric airflow values.
Selecting rectangular duct proportions and reviewing their equivalent diameter.
Examples
1. Rectangular duct for 1,000 m³/h
HVAC designer
Background
A designer needs a rectangular duct for 1,000 m³/h of airflow at a 4 m/s design velocity.
Problem
The duct must be sized with a 2:1 aspect ratio.
How to use
Select Rectangular, enter 1000 as the airflow, choose m³/h, enter 4 m/s, and set the aspect ratio to 2.
The required area is 0.07 m². The calculated rectangular size is 372.68 × 186.34 mm, or 14.67 × 7.34 inches, with an equivalent diameter of about 283.86 mm.
2. Circular duct for 500 CFM
Mechanical contractor
Background
A contractor is checking the approximate circular duct diameter for 500 CFM at a 5 m/s design velocity.
Problem
The airflow must be converted from CFM and used to calculate the required circular cross-section.
How to use
Select Circular, enter 500 as the airflow, choose CFM, and enter 5 m/s as the velocity.
FAQ
What inputs does the calculator require?
It requires an airflow rate and a design velocity. You can also select the duct shape and, for rectangular ducts, an aspect ratio.
Which airflow units are supported?
The calculator supports cubic metres per second (m³/s), cubic metres per hour (m³/h), and cubic feet per minute (CFM).
What velocity unit should I enter?
Enter the design mean air velocity in metres per second (m/s).
How are circular duct dimensions calculated?
The calculator first finds area with A = Q/v, then calculates diameter using D = √(4A/π).
What does the rectangular aspect ratio mean?
It is the long side divided by the short side, expressed as r = a/b. The calculator uses it to determine the rectangular duct width and height.