Duct Size Calculator (Flow Rate × Velocity)

Duct Size Calculator (Flow Rate × Velocity) | Online Free Tool | Elysia Tools

Tool usage guide

Learn when to use this tool, what it supports, and how real users apply it.

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.

shape: rectangular; flowRate: 1000; flowUnit: m3/h; velocity: 4; aspectRatio: 2; decimalPlaces: 2

Outcome

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.

shape: circular; flowRate: 500; flowUnit: CFM; velocity: 5; decimalPlaces: 2

Run this tool

Examples that match this tool

Continue with connected tools and hubs

Learn when to use this tool, what it supports, and how real users apply it.

Key facts

Overview

When to use

How it works

Use cases

Examples

1. Rectangular duct for 1,000 m³/h

2. Circular duct for 500 CFM

FAQ

Web Image Processing Python Samples

Android Image Processing Java Samples

Android Image Processing Kotlin Samples

Web Image Processing Rust Samples

Angular Velocity Converter (rad/s / rpm / deg/s / Hz)

Effect Size Calculator

Fatigue Limit Calculator (Goodman / Gerber / Soderberg)

Half-Life Calculator

HVAC Load, Airflow, Refrigeration, and Thermal Comfort Review