What is a typical supply duct velocity?

Residential supply ducts typically run at 600–900 FPM; main trunks up to 1200 FPM. Commercial systems may use 1000–2000 FPM. Higher velocities increase noise and static pressure loss.

CFM stands for cubic feet per minute — the volume of air moved per minute. Your HVAC unit's rating and Manual J load calculation determine the required CFM for each room or zone.

How is round duct diameter calculated?

Area = CFM / Velocity (sq ft). Diameter = 2 × sqrt(Area / π), then converted to inches and rounded up to the nearest standard size.

What is hydraulic diameter for rectangular ducts?

Hydraulic diameter = (2 × W × H) / (W + H). It is used to compare rectangular ducts to equivalent round ducts for pressure-drop calculations.

Should I round up or down to a standard size?

Always round up to the next standard size (e.g., 6, 7, 8, 9, 10, 12, 14, 16, 18, 20 inches for round) to ensure the duct can carry the required airflow without exceeding target velocity.

Does this account for static pressure loss?

This calculator sizes ducts by velocity only. For detailed pressure loss calculations including fittings and friction, use the equal-friction method or a full Manual D calculation.

Duct Size Calculator

Enter airflow in CFM and target velocity to get the correct round or rectangular duct dimensions for your HVAC system.

Duct Parameters

Airflow (CFM)

Design Velocity (FPM)

Typical residential supply: 600–900 FPM. Main trunk: up to 1200 FPM.

Duct Type

Round Rectangular

Enter airflow and velocity, then click Calculate.

Recommended Duct Size

Required Area

—

sq in

Actual Area

—

sq in (std size)

Actual Velocity

—

FPM

Hydraulic Dia.

—

inches

Airflow (CFM)

—

cubic ft/min

Velocity Rating

—

Velocity Reference Guide

Under 600 FPM — very low velocity, oversized duct

600–900 FPM — ideal residential supply / branch

900–1200 FPM — acceptable main trunk

1200–1800 FPM — commercial, higher noise risk

Over 1800 FPM — excessive noise and pressure loss

Summary

The Duct Size Calculator helps HVAC engineers, contractors, and DIYers determine the correct duct dimensions for a given airflow rate (CFM) and design velocity. It supports both round ducts (giving diameter) and rectangular ducts (giving height/width pairs with equivalent hydraulic diameter). All calculations follow ASHRAE duct design fundamentals.

How it works

Enter the required airflow in CFM (cubic feet per minute).
Set the target air velocity in feet per minute (FPM) — typical range is 600–1500 FPM for residential supply.
Choose round or rectangular duct type.
For rectangular ducts, optionally lock one dimension (height) to get the required width.
The tool computes the required cross-sectional area, then derives duct dimensions and the equivalent hydraulic diameter.

Use cases

Size new HVAC supply or return duct runs during design.
Verify that existing duct is adequate for a new HVAC unit.
Balance airflow velocity to reduce noise and static pressure loss.
Convert between round and rectangular ducts with equal capacity.
Check code compliance for residential and light-commercial HVAC.
Teach HVAC fundamentals — understand the area-velocity-CFM relationship.

Frequently Asked Questions

Last updated: 2026-09-30 · Reviewed by Nham Vu