Dust Duct Velocity Calculator
CFM and duct size to velocity verdict.
What it does
Dust collection lives or dies on duct velocity: too slow and chips settle into piles and plugs, too fast and noise plus static pressure waste the motor. Wood dust wants 3500 to 4500 feet per minute. Division checks any run.
The tool above divides CFM by duct area and verdicts against the wood band. A 350 CFM pull through 4-inch duct runs 4,011 fpm: good. Six-inch duct at 200 CFM crawls near 1,000: plugged soon.
Ventilation references with workplace-safety guidance publish the same velocity math with the wood-dust band behind it. Diameter squared rules capacity, so one inch of duct beats hundreds of CFM on paper.
Size ducts to the tool and the runs to reality. Flex hose, sharp elbows, and leaky joints all tax velocity; smooth rigid runs with swept bends and sealed seams hold it. Respirators cover what ducts miss.
How to use
- Enter collector CFM at the tool.
- Enter duct inside diameter.
- Read velocity in fpm.
- Read verdict against the wood band.
- Upgrade duct or CFM when slow.
- Seal joints and sweep elbows.
Formula
- CFM
- Collector airflow in cubic feet per minute.
- diameter
- Duct inside diameter in inches.
Area uses inside diameter in feet squared times pi over four. Band follows wood-dust transport guidance; metal dust and chips shift it. Flex hose, elbows, and leaks cut real velocity below math.
Worked example
Example: 350 CFM, 4 in duct
350 CFM through 4 inch duct.
- Collector pulls 350 CFM through 4 in duct.
- Area: 3.1416 x (4/12)^2 / 4 = 0.0873 sq ft.
- Velocity: 350 / 0.0873 = 4,011 fpm.
- Band is 3500 to 4500 for wood dust.
- Verdict reads good; flex and bends cost more.
Reference table
| Setup | Velocity | Verdict |
|---|---|---|
| 350 CFM, 4 in | 4,011 fpm | Good |
| 800 CFM, 6 in | 4,074 fpm | Good |
| 500 CFM, 6 in | 2,546 fpm | Too slow |
| 1200 CFM, 6 in | 6,112 fpm | Noisy |
Limits
- Branch math only. Trunk design differs.
- Band suits wood dust; metal differs.
- Flex and elbows cut real velocity.
- Respirators still required.
Common mistakes
- Sizing by CFM while ignoring diameter.
- Running long flex everywhere.
- Starving tools with wye splits.
- Skipping respirators anyway.
- Grounding metal duct never.
Try it yourself
Quick checks. Answers sit tucked under each question.
800 CFM through 6 in duct?
Area is 0.196 sq ft. Velocity is 800 / 0.196 = 4,074 fpm: good for wood dust.
200 CFM through 6 in duct?
Velocity is 200 / 0.196 = 1,019 fpm: too slow, dust settles.
Questions
What velocity does dust need?
3500 to 4500 feet per minute for wood dust in branches. Slower drops chips into piles; faster wastes static and screams. Trunk lines run lower by design.
How is velocity figured?
CFM divided by duct area in square feet. Four-inch duct carries 0.087 square feet; six-inch carries 0.196. Diameter squared rules capacity.
What kills duct velocity?
Crushed flex, sharp elbows, long runs, and leaky joints. Smooth rigid duct with swept bends and sealed seams holds the math.
Is fine dust more dangerous?
Yes, and worse: fine dust hangs explosive in air while chips pile into kindling. Metal duct grounded against static plus good velocity manages both.
Where should collection happen?
At the tool with hoods fitted close, since capture beats transport every time. Distant ceiling filters polish air; hoods catch chips at birth.
What safety gear matters?
Respirators during dusty work, hearing protection near collectors, and no loose clothing near spinning steel. Velocity protects lungs; guards protect fingers.
Sources
- ACGIH: industrial ventilation resources
- OSHA: woodworking safety and health
- OSHA: ventilation guidance