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Start With the Machine, Not the Collector
The right dust-collection duct diameter depends mainly on the machine’s port size, the type of waste it produces, and the airflow your collector can actually deliver. A large collector connected through undersized pipe will still perform poorly. So will a wide duct connected to a small shop vacuum.
For most small workshops, the practical choices are 2-1/2-inch hose, 4-inch duct, and 5- or 6-inch main duct. Use the smallest diameter that maintains the airflow the tool needs, but avoid long runs of flexible hose and sharp reductions. Every restriction costs airflow.
As a starting point, fine dust collection generally needs about 350 to 450 CFM at a tool connection. Coarse chips from a planer or jointer often need 500 to 800 CFM. These figures are measured at the machine, not at the collector’s advertised free-air rating.
Recommended Duct Diameter by Machine
| Machine | Typical port | Practical duct size | Useful airflow target |
|---|---|---|---|
| Random-orbit sander | 1-1/4 to 2-1/2 inches | 2-1/2-inch hose | 100-200 CFM |
| Track saw or circular saw | 1-1/4 to 2-1/2 inches | 2-1/2-inch hose | 150-250 CFM |
| Router table | 2-1/2 to 4 inches | 2-1/2-inch hose for bit guard; 4-inch for cabinet | 250-450 CFM |
| Table saw | 2-1/2 to 4 inches | 4-inch duct | 350-500 CFM |
| Band saw | 2-1/2 to 4 inches | 4-inch duct | 350-500 CFM |
| Jointer | 4 inches | 4-inch minimum; 5 or 6 inches for larger machines | 450-700 CFM |
| Thickness planer | 4 inches | 4-inch short run; 5 or 6 inches for high-output machines | 500-800 CFM |
| Drum or wide-belt sander | 4 to 6 inches | 5 or 6 inches | 600-1,000+ CFM |
These are working ranges, not guarantees. Check the machine manual first. A tool with a small plastic port may still need more airflow than that port suggests because its internal shroud leaks or fills quickly with chips.
Match the Duct to the Collector
A shop vacuum is designed for high static pressure and relatively low volume. It works well with 1-1/4- or 2-1/2-inch hose on sanders, routers, and portable saws. Connecting a shop vacuum to a 4-inch duct usually reduces velocity so much that chips settle in the pipe.
A dedicated dust collector moves more air at lower static pressure. A typical single-stage collector is suited to 4-inch duct, while a larger blower may justify a 5- or 6-inch main. A 1-1/2 or 2 hp collector can often support one 4-inch machine connection, but its actual performance depends on filter resistance, hose length, fittings, and the collector’s inlet design.
Do not size a main duct from the collector’s horsepower alone. Look for a published airflow curve or a measured CFM figure at a stated static pressure. A “2 hp” label does not tell you whether the system will move 400 CFM through 20 feet of pipe and several elbows.
Size the Main and Branches Separately
Use the largest practical duct for the main run, then reduce near each machine. For a small collector, a 4-inch main is usually adequate. With a stronger collector and long runs, a 5- or 6-inch main reduces friction and preserves airflow. A 6-inch main does not force you to use 6-inch connections at every tool.
Branches should normally match the machine’s intended port. A 4-inch branch to a table saw is sensible; reducing it to 2-1/2 inches for convenience creates a major restriction. Keep reducers gradual when possible, and avoid stacking several adapters together.
Use metal spiral duct or smooth rigid pipe for permanent runs. Smooth-wall dust collection ducting has much less resistance than corrugated flex hose. Reserve flex hose for the final few feet where the machine moves or needs service access. Even a few extra feet of flex can cost more airflow than a much longer section of smooth pipe.
Why Airflow Velocity Matters
The duct must carry chips without allowing them to settle. For ordinary woodworking debris, a velocity around 3,500 to 4,000 feet per minute is a useful design target. Fine sanding dust is more complicated because capture depends on hood shape and leakage, not just pipe velocity.
If the duct is too large for the collector, air velocity drops. This commonly happens when someone connects a small shop vacuum to a 6-inch system. The vacuum may still sound strong at the hose end, but chips will collect in horizontal sections and elbows.
If the duct is too small, velocity may be adequate but the friction loss becomes excessive. The collector runs hard, airflow falls at the machine, and the system becomes noisy. A 2-1/2-inch hose is particularly restrictive for planers and jointers, even if an adapter makes it physically fit.
Use Blast Gates and Keep the Layout Simple
Most small collectors cannot supply full airflow to several open branches. Install a blast gate at each machine and keep every unused branch closed. A gate placed near the main duct is easier to reach, but a gate close to the machine can help isolate leaks and makes troubleshooting simpler.
Use long-radius elbows where space allows. Two 45-degree elbows are usually easier on airflow than one tight 90-degree elbow. Keep the main run as straight as possible, and place the collector where the duct route is short rather than where it is merely convenient.
For a typical one-machine-at-a-time shop, a 4-inch system with smooth pipe, one open blast gate, and a good filter is often the cheaper and better choice. You need a larger main when the run is long, the machine has a high-volume hood, or you plan to run more than one pickup at once.
Check These Common Failure Modes
A system that “doesn’t collect dust” usually has a specific restriction. Check for a clogged filter, a full collection bag, an open unused blast gate, a crushed flex hose, or a reducer that blocks most of the port. A sharp adapter can behave like a smaller duct even when its labeled diameter looks correct.
Also inspect the machine itself. Table saws often leak around the blade opening, cabinet seams, and rear access panel. Planers can eject chips faster than a weak collector can remove them. Sanders need a properly sealed pad or shroud. Improving the hood can produce more benefit than buying a larger blower.
If you are building a new system, buy a set of dust collection blast gates and a high-efficiency dust collector filter before adding complicated duct branches. Clean filtration and controlled airflow usually improve a modest system more than oversized pipe added without a matching collector.
Finally, ground metal ductwork where required by local practice, avoid loose conductive dust around motors, and wear a properly fitted respirator when cutting or sanding. Dust collection reduces airborne particles; it does not make breathing protection or routine shop cleaning unnecessary.