Duct Velocity and CFM Table (Rectangular Duct)
CFM from a rectangular duct's size and air velocity, worked for common duct sizes at 500 to 900 FPM, with the same arithmetic as the Equipment Tracker app. No account, nothing to install.
REFERENCE TOOL — NOT A GUARANTEE
This tool does the math for you, but it is not a substitute for your own professional judgment, code compliance check, or manufacturer specifications. Always verify results before doing any work — you are responsible for your own calculations.
Duct Velocity & CFM Calculator
CFM = FPM × Area (ft²), where area is width times height in inches divided by 144. The airflow is rounded to a whole CFM.
ACCA Manual D Target Ranges: Supply Trunk ~700-900 FPM · Branch ~600 FPM · Return ~500-700 FPM
| Duct (W x H in.) | Area (sq ft) | CFM at 500 FPM | CFM at 600 FPM | CFM at 700 FPM | CFM at 800 FPM | CFM at 900 FPM |
|---|---|---|---|---|---|---|
| 6 x 6 | 0.250 | 125 | 150 | 175 | 200 | 225 |
| 8 x 6 | 0.333 | 167 | 200 | 233 | 267 | 300 |
| 10 x 6 | 0.417 | 208 | 250 | 292 | 333 | 375 |
| 8 x 8 | 0.444 | 222 | 267 | 311 | 356 | 400 |
| 10 x 8 | 0.556 | 278 | 333 | 389 | 444 | 500 |
| 12 x 8 | 0.667 | 333 | 400 | 467 | 533 | 600 |
| 12 x 10 | 0.833 | 417 | 500 | 583 | 667 | 750 |
| 14 x 8 | 0.778 | 389 | 467 | 544 | 622 | 700 |
| 14 x 10 | 0.972 | 486 | 583 | 681 | 778 | 875 |
| 16 x 10 | 1.111 | 556 | 667 | 778 | 889 | 1000 |
| 18 x 12 | 1.500 | 750 | 900 | 1050 | 1200 | 1350 |
| 20 x 12 | 1.667 | 833 | 1000 | 1167 | 1333 | 1500 |
| 24 x 12 | 2.000 | 1000 | 1200 | 1400 | 1600 | 1800 |
| 24 x 16 | 2.667 | 1333 | 1600 | 1867 | 2133 | 2400 |
Frequently asked questions
How do I calculate CFM from duct size and velocity?
Multiply the air velocity in feet per minute (FPM) by the duct's cross-section area in square feet. Width times height in inches divided by 144 is the area. The Equipment Tracker app's example, a 12 by 10 inch duct at 700 FPM, is 0.833 square feet and 583 CFM.
What duct velocity should I aim for?
The app lists the ACCA Manual D target ranges as about 700 to 900 FPM for a supply trunk, about 600 FPM for a branch and about 500 to 700 FPM for a return.
How is the answer rounded?
The app rounds the airflow to a whole number of CFM. The area column on this page is shown to three decimals.
Does this cover round duct, friction loss or equivalent diameter?
No. The Equipment Tracker app's calculator takes a width, a height and a velocity and gives the airflow, so only rectangular sizes are shown here.
Does this page store anything or need an account?
No to both. It does the math for you, but it is not a substitute for your own professional judgment, a code compliance check or the manufacturer's specifications.
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Write each unit's airflow readings and service history down once and they are there on the next service call, on your phone, with or without signal. The core of the app is free on the phone, and every first plan, Pro or Team, starts with a 7-day free trial.
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