Duct Friction Rate Calculator
Calculate the friction loss per 100 ft (or per metre) of a round or rectangular HVAC duct from airflow, size, duct material roughness and air density. Reports in. wg/100 ft and Pa/m, velocity, velocity pressure and the total loss over your duct run.
Input
Duct
Run & Air
1.204 kg/m³ is standard air at 20 °C, sea level. Lower it for altitude or hot air.
Output
| Property | Value |
|---|---|
| No data yet | |
More ways to use this tool
REST API
curl -X POST https://api.iotools.cloud/v1/tool/duct-friction-rate-calculator \
-H "Authorization: Bearer YOUR_API_KEY" \
-H "Content-Type: application/json" \
-d '{
"ductShape": "round",
"airflow": "1000",
"flowUnit": "cfm",
"diameter": "12",
"lengthUnit": "in",
"material": "galvanized",
"runLength": "100",
"runUnit": "ft",
"airDensity": "1.204"
}'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-friction-rate-calculator` tool (Duct Friction Rate 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-friction-rate-calculator/"
width="100%" height="520" frameborder="0" scrolling="no" loading="lazy"
title="Duct Friction Rate 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
Find the friction loss per 100 ft (or per metre) of a round or rectangular HVAC duct. Enter the airflow, duct size and material, and get the friction rate in in. wg/100 ft and Pa/m, plus velocity, velocity pressure and the total loss over your duct run.
How to use it
- Pick the duct shape — round or rectangular.
- Enter the airflow (CFM or a metric unit) and the duct's dimensions.
- Choose the duct material, the run length and, if you're not at sea level, the air density.
How it works
Instead of reading a friction chart, the calculator solves the physics directly. It uses the Darcy–Weisbach equation, Δp/L = f · (1/D) · (ρV²/2), with the Swamee–Jain explicit friction factor, so the duct material's absolute roughness and the air density both change the answer.
A rectangular duct is converted to its circular equivalent diameter (the ASHRAE/SMACNA formula De = 1.30 (ab)^0.625 / (a+b)^0.25), the round duct that has the same friction loss at the same airflow, exactly as you would on a friction chart.
Design guidance
The "equal friction" method sizes ducts to a constant friction rate, commonly 0.08–0.10 in. wg/100 ft for quiet comfort systems. Lower rates mean bigger, more expensive ducts; higher rates save material but raise noise and fan energy.
Notes
- Standard air: 1.204 kg/m³ at 20 °C, sea level; dynamic viscosity 1.81 × 10⁻⁵ Pa·s.
- Results are for straight duct only. Fittings, transitions and dampers add loss on top of this figure.