How to Size a Dust Collector: CFM, Duct Velocity and Filter Area

You size a dust collector from the hood back, not from the collector forward. Work out the air each pickup point needs, carry it through the duct at a speed that keeps your dust moving, add up what runs at the same time, and only then pick the fan, the filter area and the collector. Skip a step and you end up with a unit that looks right on paper and leaves dust at the machine.

This guide walks through the six steps with the formulas and worked numbers, so you can check a quote, a spec or your own layout.

Want a quick planning number first? Count your ports in the dust collector CFM calculator, then come back here for the method behind it.

The six steps at a glance

1. List every pickup point and the dust it makes
2. Work out the CFM each hood needs
3. Size each duct to hold transport velocity
4. Add up what actually runs at once
5. Calculate static pressure so the fan can be selected
6. Size the filter area and choose the collector type

Step 1. List every pickup point and its dust

Walk your floor and write down each place dust or fume leaves a process: the saw, the grinder, the weld station, the bag dump, the transfer point. For each one, note what the material is, how fine it is, whether it is dry, sticky or oily, and how hard the process throws it.

Particle size drives more of the design than the material name does. Fine sanding dust and coarse shavings can come off the same board and belong in different collectors. If you already have a system, compare your list with what it was built for. That gap is behind most of the signs a dust collector is undersized.

Step 2. Work out the CFM each hood needs

Airflow at a hood comes from one relationship:

The velocity you need at the point where the dust is released is called capture velocity. The ACGIH Industrial Ventilation manual publishes these ranges:

Use the high end of a range when there are cross drafts, when the material is hazardous, or when production is heavy.

Distance is what costs you

For a plain open hood, the air you need grows with the square of the distance to the source:

Take a 6 inch round opening, which is 0.196 square feet, and a capture velocity of 150 FPM. At 6 inches from the source you need about 400 CFM. Pull that hood back to 12 inches and you need about 1,530 CFM for the same capture. Double the distance, and you need nearly four times the air.

That is why hood position matters more than collector size, and why source capture arms and downdraft tables work: they put the opening close to the work. For weld stations, our fume arm sizing guide has the numbers by arm diameter, and the downdraft table sizing guide does the same for tables. If you are deciding between the two, read downdraft tables versus fume arms.

Machines with a built in dust port are simpler. The machine builder publishes the CFM for that port, so use their number.

Step 3. Size each duct to hold transport velocity

Once the dust is in the duct, the air has to move fast enough to keep it from settling. That speed is transport velocity. For most dry industrial dusts we design horizontal runs at 3,500 to 4,500 FPM. Heavier or damp material needs the high end, and light fume such as weld smoke can run slower. Too slow and dust settles, the duct plugs, and you get the weak branch described in why a dust collector loses suction. Too fast and you pay for it in static pressure, noise and wear.

This table shows what a round duct carries at those velocities.

Read it both ways. A branch that needs 800 CFM belongs in a 6 inch duct, not an 8 inch. Put that 800 CFM in an 8 inch pipe and the air slows to about 2,300 FPM, and dust starts to drop out. Oversized duct is one of the most common dust collection design mistakes.

The main trunk grows as each branch joins it, so velocity stays in range all the way to the collector. That is the work behind any industrial ductwork layout, and it is why Nordfab Quick-Fit duct comes in so many diameters. You can price the pipe and fittings yourself under duct and fittings, and our ductwork cost guide shows what a run adds up to.

Step 4. Add up what actually runs at once

You do not always need the total of every branch. A cabinet shop with ten machines and three operators will never run all ten together. A production line with ten stations will.

• Everything runs together. Add every branch. That total is your system CFM.
• Only some run together. Add the largest group that can run at the same time, and close the rest with blast gates. Keep enough gates open to hold transport velocity in the main trunk.

Be honest about this number. Sizing for four machines and running seven is the fastest way to a system that never performed. If your load swings through the day, a variable frequency drive lets the fan follow it, which our article on saving energy with a VFD explains.

Worked example: four weld stations

Four stations, each with an 8 inch fume arm. An 8 inch arm typically pairs with about 1,000 CFM. All four can weld at once, so the system needs 4,000 CFM. Each arm connects on 8 inch duct, where 1,000 CFM moves at about 2,870 FPM. Weld fume is light, and the ACGIH manual lists 2,000 to 2,500 FPM as the transport range for it, so that branch has room to spare. The trunk then steps up as branches join: 10 inch at 2,000 CFM, 12 inch at 3,000 CFM, and 14 inch to the collector at 4,000 CFM, each moving at roughly 3,700 to 3,800 FPM.

Step 5. Calculate static pressure

CFM is only half of a fan selection. The other half is static pressure, the resistance the fan has to pull against, measured in inches of water gauge. It is the sum of:

• Entry loss at the hood
• Friction along every foot of duct in the longest run
• Losses at each elbow, branch entry and transition
• The pressure drop across the filters when they are loaded, not when they are new
• Anything on the clean side: after filters, silencers, return duct, explosion protection devices

The fan is then selected to deliver your CFM at that static pressure. Two fans with the same horsepower can move very different amounts of air against the same resistance, which is why a collector cannot be chosen on horsepower or on a free air CFM figure. In our own process the collector manufacturer confirms the fan, the motor and the static pressure against the duct layout before anything is ordered.

Step 6. Size the filter area and choose the collector

Filter area comes from the air to cloth ratio: the CFM divided by the square feet of filter media.

As a starting point we design nanofiber cartridges around 2 to 1 and spunbond cartridges around 3 to 1. Fine, sticky or heavy dust calls for a lower ratio, which means more media.

Worked example: 6,000 CFM of fine dry dust

At 2 to 1 on nanofiber media, 6,000 CFM needs 3,000 square feet of filter. A standard nanofiber cartridge for the DustHog SFC carries 255 square feet, so that is 12 cartridges. Run the same airflow through too few cartridges and you get the short filter life covered in how often to change dust collector filters and why your dust collector keeps clogging. When it is time to reorder, you can find dust collector replacement filters by brand and size.

Which type of collector

For a closer comparison, see baghouse versus cartridge, wet versus dry collectors and source capture versus central collection.

If your dust is combustible

Sizing gets you airflow. It does not decide where the collector can sit, what protection it carries or how the duct is isolated. Those come from NFPA 660 and from the properties of your own dust.

Published KSt values are for screening only. Particle size and moisture move the numbers, so protection is designed from lab results on your actual dust. NFPA 660 requires a dust hazard analysis (DHA) for facilities that handle combustible dust, reviewed at least every five years and whenever the process changes. You can request the intake form at DHA Report. Our NFPA 660 guide, the free 18 point NFPA 660 checklist and the combustible dust collectors page cover what that means for the equipment.

Who this guide is not for

If you have one bench tool or a single weld station, you do not need a duct calculation. A DustHog VP-750 portable collector, a DustHog VB-750 downdraft bench or an MFE-750 mobile fume extractor arrives sized for that one job, and you can order it from the store.

If you need a hobby shop collector for a table saw and a planer, we are not the right source and a woodworking retailer will serve you better.

Frequently asked questions

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