Dust Collection CFM Calculator
Duct sizing for a shop dust system, including the ceiling nobody expects: a duct can be too WIDE and silt up.
Transport velocity is a convention rather than a sharp cliff — around 4,000 FPM in branches and 3,500 in a main is commonly quoted, and heavier chips want more. This also ignores static pressure, which is the other half of whether a collector actually delivers the airflow you assumed. Nothing is uploaded.
Bigger duct, lower speed
Airflow is area times velocity. Fix the airflow — which is what a machine needs and what a collector delivers — and widening the duct does not increase it. It spreads the same air over more area, so the speed drops. Below roughly 4,000 FPM the chips fall out and the duct silts up from the inside, which is worse than a duct that is merely too small.
| Duct, carrying 600 CFM | Air speed | |
|---|---|---|
| 4" | 6875 FPM | dust stays airborne |
| 5" | 4400 FPM | dust stays airborne |
| 6" | 3056 FPM | silts up |
| 7" | 2245 FPM | silts up |
| 8" | 1719 FPM | silts up |
So there is a largest usable diameter, and at 600 CFM it is 5.24 inches. A four-inch duct is comfortable here and a six-inch is not — the opposite of what "bigger is better" would predict. The ceiling rises with the square root of the airflow, so it takes four times the air to double it.
And flow goes with the square of the diameter
Held at transport velocity instead, a wider duct does carry more — but not in proportion. Going from four inches to six is one and a half times the diameter and 2.25 times the air, because area is quadratic. Which is also why a four-inch hose on a machine designed for six is not a modest compromise: it is under half the air the machine was built to get.
| Duct | Area | Carries at 4,000 FPM |
|---|---|---|
| 2.5" | 0.0341 sq ft | 136 CFM |
| 4" | 0.0873 sq ft | 349 CFM |
| 5" | 0.1364 sq ft | 545 CFM |
| 6" | 0.1963 sq ft | 785 CFM |
| 7" | 0.2673 sq ft | 1069 CFM |
| 8" | 0.3491 sq ft | 1396 CFM |
The two tables are the same equation read in opposite directions, and between them they bracket the problem: the duct has to be wide enough to deliver the airflow and narrow enough to keep it moving. A 2.5-inch shop-vac hose carries under a fifth of what a six-inch duct does, which is why it works on a sander and not on a planer.
How to use
- Enter the airflow the machine needs.
- Enter the duct diameter you are considering.
- Set the minimum transport velocity.
- Read the air speed and whether the dust stays airborne.
Frequently asked questions
Is a bigger duct always better?
No, and this is the part that surprises people. Airflow is area times velocity, so at a fixed airflow a wider duct runs SLOWER — and below about 4,000 FPM the chips drop out and the duct silts up from the inside. At 600 CFM, a 4-inch duct runs at 6,875 FPM and a 6-inch at 3,056. The 4-inch is the one that works.
So how wide can a duct be?
There is a largest usable diameter for any airflow, and at 600 CFM it is 5.24 inches. Past that the run gets worse rather than better. The ceiling rises with the square root of the airflow, so it takes four times the air to double it.
How much more does a 6-inch port carry than a 4-inch?
At the same air speed, 2.25 times — one and a half times the diameter, squared, because area is quadratic. Which is why running a 6-inch machine on a 4-inch hose is not a modest compromise: it gives the machine under half the air it was designed for.
What is transport velocity?
The air speed needed to keep dust and chips suspended rather than settling. Around 4,000 FPM in branch lines and 3,500 in a main is commonly quoted, and heavier chips want more. It is a convention rather than a sharp cliff, but a duct persistently below it will fill.
Why is a silted duct worse than a small one?
Because it gets worse on its own. A duct that is too small starves the machine and stays that way; a duct that is too slow accumulates material, which narrows it, which changes the airflow, and the failure compounds until someone takes it apart.
Does a shop vacuum work for this?
For a sander, yes; for a planer, no. A 2.5-inch hose carries under a fifth of what a 6-inch duct does at the same speed. Shop vacuums move a small volume fast, dust collectors move a large volume slower, and the machines that make chips need volume.
What about static pressure?
Not modelled here, and it is the other half of the problem — every foot of duct, every bend and every filter costs pressure, and a collector rated for a given airflow will not deliver it through a long run. Treat this as duct sizing rather than a full system design.
Does this send anything anywhere?
No. Every figure is computed in your browser, and nothing is uploaded or stored.
🔒 This tool runs entirely in your browser. Nothing you enter is uploaded, logged, or stored.