Dust collection CFM
Dust collection fails when branch-line diameter doesn't match CFM, when duct runs are too long, or when the collector is too small for the sum of open gates. This calculator sizes your collector, branch ducting and blast gates so the system actually works.
Calculator
The 3500 FPM rule
Wood dust needs to be carried in moving air. Below ~2,500 FPM, fine dust falls out of the airstream and clogs the duct. Above ~4,000 FPM, you're wearing your ducting thin and wasting CFM. 3,500 FPM is the consensus "right" answer for mixed wood dust — coarse chips stay suspended, fines are captured, ducting lasts.
To hit 3,500 FPM in a 4″ duct, you need ~300 CFM in that branch. Smaller ducts need less CFM but move less air overall — there's a trade-off.
Why duct friction matters
Every foot of duct and every elbow adds resistance. A long run with 6 elbows can drop 1″ of static pressure — easily more than a small collector can produce. The calculator estimates friction using a simplified duct-friction chart. If the number is high, your collector won't push the air no matter what the CFM rating says.
Limitations
- CFM ratings on machines are nominal; real ports restrict flow by 20–40%.
- Collector CFM is free-air; ducted CFM is much less. The reduction depends on the entire run.
- Doesn't model cyclone pre-separators. Adding one reduces duct load significantly.
- Doesn't account for fine dust (MDF, hardwoord) which needs higher velocity to stay suspended.
Frequently asked questions
Do I really need a cyclone?
For a small hobbyist shop (one tool at a time), no — a single-stage collector is fine. For anything over 1,200 CFM, a cyclone pays for itself quickly: less filter clogging, less collector emptying, better fine-dust capture.
Why is my dust collector losing suction?
Three usual causes: (1) full collection bag — empty it; (2) clogged filter — clean or replace; (3) too many open blast gates — close the ones you're not using. The calculator assumes only the gates you need are open.
Should I run metal ducting or plastic?
Metal ducting grounds static electricity (which is a real fire/explosion risk with fine dust); plastic is cheaper and quieter. If you use plastic, ground the run with a copper wire to the collector housing. Most production shops use metal; most hobbyists use plastic and ground it.