Dust Collector CFM Calculator
Count your ports, pick your dust, and see roughly how much air your dust collector has to move. You get a planning number for CFM, the size of the main duct at the collector, and the filter area that airflow calls for.
For illustration and planning only. This calculator gives you a starting estimate. It is not a system design, and it does not account for your duct layout, static pressure or the properties of your dust. Have your numbers checked before you order or buy anything.
1. How many ports of each size?
Count the duct connection at each machine, hood or arm. Leave the others at zero.
2. What are you collecting?
3. How many ports are open at the same time?
4. Air to cloth ratio
CFM for each square foot of filter. A lower ratio means more filter area. Fine or sticky dust calls for a lower number.
Your planning estimate
Enter your ports to see where to start.
Get these numbers checked, freePlanning estimate only. Not a design, a quote or a recommendation to buy. Final airflow, duct sizes, fan and filter area depend on your layout, your static pressure and your dust, and must be confirmed before you order.
If your dust is combustible: airflow is only part of the answer. Where the collector can sit, what protection it carries and how the duct is isolated come from NFPA 660 and from testing on your own dust. NFPA 660 requires a dust hazard analysis (DHA), reviewed at least every five years and whenever your process changes. Have your dust checked for combustibility. Our KSt values page lists general values for common materials as a starting point. You can request the DHA intake form at DHA Report, and read our NFPA 660 guide.
How the calculator works
Each port is treated as a round duct. The air a duct carries is its area multiplied by the air speed inside it:
CFM = duct area (sq ft) × velocity (FPM)
- Velocity comes from what you collect. Light fume moves at a lower speed, and heavy chips need a higher one so they do not settle in the duct
- Airflow to plan for is the total of your ports, reduced by the share that are open at once
- Main duct is the largest standard size that still holds that velocity at the collector
- Filter area is your CFM divided by the air to cloth ratio you pick. We start nanofiber cartridges around 2 to 1 and spunbond around 3 to 1, and you can change it to match your media
The full method, with worked examples, is in our guide on how to size a dust collector.
What this calculator does not do
- It does not work out static pressure, so it cannot pick a fan or a motor
- It does not size the branches and transitions along your duct run. See industrial ductwork
- It does not size open hoods. An arm or hood away from the source needs more air than its duct size suggests, which our fume arm sizing guide and downdraft table sizing guide cover
- It does not decide what your dust requires. If a machine builder publishes a CFM for its port, use that number over this one
Who this is not for
If you have one bench tool or one weld station, you do not need a calculation. A portable fume extractor or a downdraft table arrives sized for the job. If you are setting up a hobby shop, a woodworking retailer will serve you better than we will.
Questions
How do I calculate CFM for a dust collector?
Multiply the area of each duct port in square feet by the duct velocity in feet per minute, then add the ports that run at the same time. A 6 inch port at 4,000 FPM carries about 785 CFM.
How accurate is this calculator?
It gives a planning estimate from port sizes alone. It does not include your duct layout, static pressure or dust properties, so the numbers have to be checked before you order equipment.
What duct velocity should I use?
About 2,500 FPM for weld fume, 3,500 to 4,000 FPM for most dry industrial dust, and 4,500 FPM for chips and heavy material.
Can I size a dust collector by horsepower?
No. Horsepower does not tell you how much air a fan moves against the resistance of your duct and filters. Size by CFM and static pressure.