Free Workshop Ventilation Tool

Dust Collection Calculator — CFM, Duct Size, Velocity & Static Pressure

Dust collection fails for one reason: air moving too slowly to carry the chips. This tool gives the CFM for the machines running at once, the duct size that keeps velocity in the 3,500–4,500 fpm transport band, the static pressure of your longest run, and therefore the collector or blower you actually need — not the free-air number on the box.

No signup · Works on mobile · Formulas shown below · Updated 2026-08-24

Dust Collector & Duct Sizing

Duct area = CFM ÷ velocity. Static pressure by Darcy–Weisbach (steel) plus hood, elbows, flex and filter.

Quick preset

Machine 1

Machine 2 (largest other)

Custom machine CFM

CFM

Machines running at once

Material

Design transport velocity (main duct)

fpm

20 m/s ≈ 4,000 fpm

Longest run — rigid duct length

Elbows + wyes on that run

Flexible hose at the machine

Flex costs ~3× rigid duct per foot — keep it short.

Filter / separator / hood pressure drop

in. w.g.

Cyclone 1.5–2.5, cartridge filter 0.5–1.5 (loaded), bag 1.0, hood 0.3–0.8.

Results

Engineering estimate for preliminary sizing. Verify against the fan curve, local codes and your system measurements before purchase. Ask a Longwell engineer to check your duty point free of charge.

CFM requirements by machine

Typical values for fine-dust collection (ACGIH Industrial Ventilation and manufacturer data). Older "chip collection" figures are ~30 % lower; use these for health-grade collection.
Machine	CFM (at the port)	Port / branch
Table saw (cabinet)	350	4 in
Table saw (contractor / open base)	450	4 in
Band saw 14 in	350	4 in
Band saw 18–24 in	550	5 in
Jointer 6–8 in	350	4 in
Jointer 12 in+	550	5 in
Planer 12–15 in	400	4 in
Planer 20 in+	800	6 in
Router table	350	4 in
Shaper	550	5 in
Drum sander 16–25 in	550	5 in
Wide belt sander 37 in	1200	8 in
Disc / belt sander	350	4 in
Miter / radial arm saw	350	4 in
Lathe	350	4 in
CNC router 4×8	800	6 in
Floor sweep	350	4 in
Downdraft sanding table	800	6 in
Metal grinder / belt (spark trap)	400	4 in
Laser cutter fume	400	4 in
Plasma table downdraft 4×4	1500	8 in

Design rules

Duct area A = CFM ÷ velocity (fpm) → D = √(4A/π)
Transport velocity wood 3,500–4,000 fpm (18–20 m/s) · metal 4,000–5,000 · fine dust 3,000–3,500 · vertical runs +10 %
Velocity pressure p_v = (fpm ÷ 4,005)² in. w.g.
Static pressure = duct friction + 3× flex friction + fittings (K × p_v) + hood entry (0.5 p_v) + filter/cyclone
Collector select at design CFM and static pressure; free-air ratings overstate delivery by 40–60 %

Round down the main duct so velocity stays above transport speed; oversize ducts are the #1 cause of clogged horizontals.
Blast gates on every branch; open only the machine in use.
Wyes, not tees; long-radius elbows (R ≥ 1.5 D); flex ≤ 6 ft at the tool only.
Cyclone first for chip-heavy work — it keeps the filter drop flat and the airflow constant.
Grounding: bond metal duct; run a ground wire in plastic duct for wood dust.
Blower type: backward-curved material-handling impeller (radial for heavy chips), motor out of the airstream; spark trap for metal.

Frequently Asked Questions

How many CFM do I need for a dust collector?

Size for the largest machine (or the two you run together): 350 CFM for a 4 in port machine (table saw, jointer, band saw), 550 for 5 in (shaper, drum sander), 800 for 6 in (planer 20 in, CNC), 1,200+ for wide belt sanders. Then check the static pressure — a collector must deliver that CFM at 3–6 in. w.g., not in free air.

What duct size for 800 CFM?

At 4,000 fpm transport velocity 800 CFM needs 0.2 ft² → 6 in round duct (5.8 in calculated). Use 6 in mains and 4 in branches to 350 CFM machines.

What velocity should dust collection ducts have?

3,500–4,000 fpm (18–20 m/s) for wood chips and dust, 4,000–5,000 fpm for metal, 3,000–3,500 for fine sanding dust only. Below transport velocity, material drops out in horizontal runs and blocks the duct.

Why does my 2 HP dust collector not pull enough?

Its "1,500 CFM" is a free-air rating. With a 30 ft run, a few elbows, flex hose and a filter you typically have 4–6 in. w.g. of static pressure, at which the same unit delivers 500–800 CFM. Reduce static (shorter, bigger-radius, less flex) or choose a blower rated at pressure.

Link to or Cite This Tool

This calculator is free to reference in articles, forums, specifications and course material. Please credit it with a link:

<a href="https://www.longwellfans.com/resources/dust-collection-cfm-calculator/">Dust Collection CFM & Duct Velocity Calculator</a> by LONGWELL Fans

APA: LONGWELL Fans. (2026). Dust Collection CFM & Duct Velocity Calculator. https://www.longwellfans.com/resources/dust-collection-cfm-calculator/

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