Size for the airflow the machine hood needs, not the collector’s peak CFM — duct, filter, and separator losses cut what the fan delivers.
That gap — not the number on the box — decides whether a table saw hood catches dust or throws it back in your face.
Every elbow, hose, blast gate, and loaded filter takes a bite, which is why working out how to size a dust collector system for a woodworking shop starts at the tool and works backward to the fan. Three numbers drive the answer: hood airflow, transport velocity inside the duct, and static pressure at the fan.
Sizing A Dust Collector For A Woodworking Shop: What Decides The Outcome
Hood airflow with the whole system connected is the figure that matters; a peak or free-air rating tells you almost nothing. Collector selection runs off the fan curve at the static pressure your duct, fittings, cyclone, and filters actually create.
Published fan data shows the trade-off plainly. Same two collectors, less airflow as the pipe narrows:
| Duct Size | V-2000 (CFM @ in. WC) | V-3000 (CFM @ in. WC) |
|---|---|---|
| 6-inch | 1,140 @ 2.5 | 1,285 @ 3.0 |
| 5-inch | 913 @ 4.5 | 1,045 @ 5.4 |
| 4-inch | 760 @ 6.3 | 840 @ 7.4 |
| 3-inch | 620 @ 8.1 | 670 @ 9.2 |
Read down a column, not across: airflow falls as static pressure climbs. Those figures belong to those two machines and should never be applied to a different collector.
How Much Airflow Does Each Machine Actually Need?
Enough air at the hood to capture the dust that machine makes — and sanders need the closest look, because they throw the fine particles that linger in the air. A wide port proves nothing on its own; hood shape, leaks, and branch size decide whether capture happens.
List every machine with its port diameter and hood style, then mark which ones run at the same time. Adding every CFM on the spec plates oversizes the collector; sizing for the largest simultaneous demand plus system losses keeps it honest. Two tools running at once is the case that catches small shops out, since the trunk and the fan curve have to carry the combined load.
Dust and chips stay suspended only while air moves through the duct at roughly 3,500 to 4,500 feet per minute (FPM). Oneida Air Systems’ airflow versus pipe diameter figures put a 6-inch duct at about 687 CFM at 3,500 FPM, 785 CFM at 4,000 FPM, and 883 CFM at 4,500 FPM. A smaller duct raises resistance and cuts the flow that reaches the tool.
- Keep the trunk short and straight, with as few elbows and transitions as you can manage.
- Seal every joint with silicone, duct tape, or duct mastic — small leaks cost measurable airflow.
- Run rigid pipe to each machine and save corrugated flex for the last foot or two.
- Close blast gates on every branch you are not using.
Fan curves differ enough between models that spec-sheet math gets old fast. If you would rather start from machines with published numbers, this roundup of woodworking dust collection systems compares tested collectors and duct setups side by side.
Where Airflow Disappears, And How To Check The System
Undersized branches, flex hose, unsealed joints, a loaded filter, and open blast gates account for most weak systems. Woodworking collectors are high-volume, low-static-pressure machines, typically working between 3 and 6 inches of water column (WC).
Total static pressure adds every straight run, elbow, transition, gate, hood, cyclone, and filter, then you compare that total against the fan curve at your required CFM. A filter that has not been cleaned behaves like a smaller collector.
- Measure airflow at the machine, or use the manufacturer’s commissioning method.
- Check for crushed pipe, undersized hoods, and leaks between collector and bin.
- Clean the filter — daily where practical — and measure again.
- Close every gate you are not using and take a final reading.
A sealed system shows it on the gauge: hood airflow holds steady with one gate open and sags the moment a second one opens.
Wood dust is combustible. Particles below 420 microns (0.017 inch) — the fraction a sander throws in volume — can deflagrate at the right concentration, so never run a wood collector on flammable vapors, gases, welding slag, or grinder sparks. Ground the collector and ductwork, and use metal duct, since PVC generates static charge.
Common Questions
Can A Shop Vac Replace A Dust Collector?
No. A shop vac moves a small volume of air at high static pressure, which suits a 1¼-inch hose and spot cleanup. A dust collector moves a large volume at low static pressure, which is what a 4- or 6-inch ducted trunk needs. Put a shop vac on ducted pipe and you get almost no flow at the machine.
What Size Duct Should The Main Trunk Be?
Match the trunk to the airflow the collector can produce at working static pressure. For a small-shop unit delivering roughly 700 to 900 CFM, a 6-inch main keeps air moving inside the 3,500 to 4,500 FPM transport range, then steps down at each branch. Long 4-inch mains strangle flow and clog.
Should The Ductwork Be Metal Or PVC?
Metal. PVC pipe builds static charge as dust rushes through it, and it cannot be grounded the way metal can. Go with metal duct, ground both the collector and the run, and keep flex hose short — corrugated hose carries the most resistance of any section in a typical duct run.
Sources
- Oneida Air Systems. “Airflow vs. Pipe Diameter” Supports the CFM figures for 6-inch duct at 3,500–4,500 FPM.