Duct Velocity Calculator
Calculate the mean air velocity in a round or rectangular HVAC duct from its airflow (CFM or metric) and cross-section, reported in FPM and m/s and checked against common SMACNA/ASHRAE duct velocity guidelines. A rectangular duct also gets its circular equivalent diameter.
Input
Duct
Output
| Property | Value |
|---|---|
| No data yet | |
| Duct Role | Range (FPM) | Range (m/s) | This Duct |
|---|---|---|---|
| No data yet | |||
Common SMACNA/ASHRAE rule-of-thumb velocity ranges by duct role, for reference.
More ways to use this tool
REST API
curl -X POST https://api.iotools.cloud/v1/tool/duct-velocity-calculator \
-H "Authorization: Bearer YOUR_API_KEY" \
-H "Content-Type: application/json" \
-d '{
"ductShape": "round",
"airflow": "1000",
"flowUnit": "cfm",
"diameter": "12",
"lengthUnit": "in"
}'Swap in your own key from your account. The tool's fields are the body — no wrapper.
Ask an AI agent
Use the IOTools `duct-velocity-calculator` tool (Duct Velocity Calculator) on this input:
YOUR_INPUT_HEREPaste this at any agent connected to the IOTools MCP server, then add your input.
Embed widget
<iframe
src="https://iotools.cloud/embed/duct-velocity-calculator/"
width="100%" height="520" frameborder="0" scrolling="no" loading="lazy"
title="Duct Velocity Calculator — iotools.cloud"
sandbox="allow-scripts allow-forms allow-same-origin allow-downloads allow-popups allow-popups-to-escape-sandbox"
allow="clipboard-write"
style="width:100%;border:1px solid #e5e7eb;border-radius:12px;overflow:hidden"></iframe>
<script src="https://iotools.cloud/embed.js" async></script>Drop this into your own page — free, no key required, just a link back.
| Cost per API/MCP call | From 5 credits |
|---|---|
| Need more credits? | View pricing |
Also available with
Guides
Calculate the mean air velocity in a round or rectangular HVAC duct from its airflow and cross-section, using the continuity equation V = Q/A. Results are reported in FPM (feet per minute) — the unit duct-sizing charts actually use — and m/s, and checked against common SMACNA/ASHRAE duct velocity guidelines by duct role.
How to use it
- Pick the duct shape — round or rectangular.
- Enter the airflow (Q) and pick its unit (CFM or a metric flow unit).
- Enter the duct's dimensions — a diameter for a round duct, or width and height for a rectangular one — and pick their unit.
Results update as you type: the velocity in FPM and m/s, the airflow echoed back in both CFM and m³/s, the duct's cross-sectional area, and — for a rectangular duct — its circular equivalent diameter.
The formula
V = Q / A, where Q is the volumetric airflow and A is the duct's cross-sectional area (πd²/4 for round, width × height for rectangular).
Circular equivalent diameter
A rectangular duct doesn't have a single "diameter" to look up on a round-duct friction-loss chart, so this calculator also reports its circular equivalent diameter — the round-duct diameter that would carry the same airflow at the same friction loss per unit length, using the standard ASHRAE/SMACNA formula:
De = 1.30 × (a × b)^0.625 / (a + b)^0.25
where a and b are the rectangular duct's two side lengths.
Why FPM, not ft/s
HVAC duct sizing is conventionally done in feet per minute (FPM), not feet per second — every SMACNA/ASHRAE duct velocity table and most duct-sizing slide rules use FPM. This calculator reports FPM first, with m/s alongside it for anyone working in metric.
Duct velocity guidelines
The guideline table checks your computed velocity against common rule-of-thumb ranges by duct role — a residential branch duct runs much slower than a commercial high-velocity main, mainly to control noise and pressure drop. These are general design guidelines, not a substitute for a full duct-sizing calculation that also accounts for friction loss and available static pressure.
Limits
This is a mean-velocity check only — it doesn't compute friction loss, static pressure, or noise (NC) levels, and it doesn't account for duct liner or insulation thickness reducing the clear airway.
Privacy
Everything runs in your browser — no dimensions or airflow data are sent anywhere.
Related tools
For the equivalent calculation in liquid piping, use the Pipe Flow Velocity Calculator. To size the cooling/heating load a duct system needs to move, use the HVAC BTU Load Calculator.