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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

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.

How do I use Dust collection CFM step by step without mistakes?Follow the order given in the usage section: Combined CFM, branch-line diameter, and duct-friction estimates. Pick a dust collector size and branch layout. The step-by-step part matters less than the unit check that sits inside it. Confirm every unit matches the thing you are actually trying to measure before you press calculate - that single check catches more wrong answers than any other habit, and it costs about ten seconds.
What are the common mistakes when using Dust collection CFM?Most common mistakes come down to four habits: entering 20 when the field expects 0.2 because the field is a percentage; leaving a field blank and accepting the default without checking what it stands for; changing one input and reading a result that was calculated before the change; and quoting the output as an exact figure when the page describes it as a reference estimate. Press calculate again after any edit, and treat the number as a reference point rather than a final answer.
How do I read the result from Dust collection CFM?Read the final figure first, then come back to this section for the conditions behind it: Total CFM is the sum of all open gates — what the collector must supply. Branch velocity tells you if your duct diameter is moving air fast enough. Friction loss estimates static pressure drop; if too high, your collector can't push the air. Keep two things apart: the number itself, which comes from the values you typed, and the assumptions it depends on, which decide whether that number is meaningful. The page sets out what a higher or lower result means for your case, so check that direction as well as the magnitude.