Dust collector size: the duct decides, not the motor

350 CFM. That is what a 4 inch duct carries at the pressure a small shop blower actually generates, and it barely moves when you buy a bigger motor. A 3/4 hp blower on 4 inch pipe manages about 300 CFM. A 2 hp collector rated at 1200 CFM is lucky to give 450 CFM through the same pipe. Dust collector size, read off the label, is close to meaningless until the ductwork is settled.

Most numbers here are Bill Pentz's, from his ducting work. Some are air engineering tables he cites, others are readings he took in his own shop with gauges, and this page says which. OSHA covers the health side, quoted rather than extended.

The two standards nobody separates

Chip collection means the sawdust and offcuts you would otherwise sweep up. Almost every stationary tool in a small shop gets good chip collection at 350 CFM, and that requirement has been well understood since the 1920s.

Fine dust collection is a different target. Those particles are invisible without magnification and light enough that any air movement scatters them, so catching them at the source means surrounding the working area of the tool with moving air instead of pulling at a port. Run the arithmetic on that and it lands near 1000 CFM for most stationary machines, close to three times the chip figure.

The dust collector CFM on a box describes neither, because a working system runs at about half the pressure its blower could make, and Pentz puts it harder still: small shop vendors, he says, ship about half the maximum airflow they advertise.

The health case here is two claims, not one. OSHA carries wood dust with an IARC classification of carcinogenic to humans, Group 1, and an NTP classification of known to be a human carcinogen, and adds that western red cedar dust has been shown to cause asthma. It also says, on its overview page, that "The extent of these hazards and the associated wood types have not been clearly established", and ties the classification to no particle size. So the argument that the invisible fraction is the one that matters is Pentz's, not OSHA's, and it rests on other endpoints: work he cites finds every fine dust exposure costs measurable respiratory capacity, building toward asthma, COPD and emphysema.

Where dust collector size finally matters

Once the duct diameter is settled, the motor sets a ceiling. A 1.5 hp collector whose chart says 1100 CFM delivers about 785 CFM on 6 inch duct, about 550 CFM on 5 inch, and about 350 CFM on 4 inch. Which of those you get is decided by the pipe. Whether 785 CFM is on the table at all is decided by the motor.

He gives that 6 inch figure as 785 CFM in four places and as 875 CFM twice, once generally and once as a reading off his own 14 inch impeller. This page follows the 785 he repeats and reasons from.

Notice that 785 CFM is not 1000 CFM.

Pentz is blunt about what a normal blower does with that. Go to 7 inch duct and the 1000 CFM is available, because a wider pipe resists less, but air speed then drops below what keeps a vertical run clear and it plugs. Stay at 6 inch and the runs stay clear at 785 CFM.

Three horsepower is the floor for keeping that 6 inch run clear at all, and the blower tables want 3 hp or a 5 hp cyclone before they show 1000 CFM anywhere. Neither of those buys both at once. Pentz says there were no small shop collectors or cyclones that resolved it, so he stopped shopping horsepower and built an oversized impeller and motor instead, which is what his own cyclones and the Clear Vue units use to move a real 1200 CFM through 6 inch duct with air speed to spare. Anyone whose chain ends at fine dust plus 6 inch has not finished the decision.

Past that there is a wall, and it is why the horsepower chart is a trap. Doubling the air moved requires four times the static pressure and nine times the horsepower. Adding half a horsepower and a larger impeller to that 1.5 hp 1100 CFM blower nets about 100 CFM.

The table saw case, in numbers

A cabinet saw arrives with a 4 inch port in the cabinet. That supports about 350 CFM, and the charts ask 550 CFM for fine dust below the blade, which is where most of a table saw's dust goes. Opening it to a full 5 inch is the first move and rarely the last. Pentz got his own saw there with a larger down drop split into two hoses, an added overhead hood, and then a 4 inch hose inside the cabinet that also had to come up to 5 inch. That 550 CFM is one port's share of a machine total near 1000.

Overhead guards are their own problem. Pentz gauged a 2.5 inch blade guard port at 183 CFM against the 350 CFM his chart wanted for that hood, and replaced the guard with a larger one carrying a 4 inch port. For tools whose port cannot easily be modified, mostly chop, miter and radial arm saws, his answer is not a bigger collector either: it is a shop vacuum on the small port and the dust collector on a separate large hood, both running at once.

Going larger is not automatically better either, which is where this gets counterintuitive. The Wood Whisperer enlarged a table saw cabinet port from 5 inch to 6 inch to match the main trunk, then added an overhead guard with a 3 inch port, and finished with more branch area than the main could feed. His own conclusion afterward was that leaving the saw at 5 inch most likely would have collected better.

The two sources word the rule differently, and the gap decides what you buy. He keeps the summed branches no greater than the main, within reason. Pentz aims to match or slightly exceed it, off a chart: on a 6 inch drop that gives a 4.5 inch cabinet port beside a 4 inch guard, within a percent of the main. He could not affordably get 4.5 inch, so he runs 5 inch, about 14 percent over. This page follows Pentz, because he sizes both branches on purpose where a rule of thumb only says when you have gone too far. The Wood Whisperer's 6 inch and 3 inch ran about 25 percent over, which is the difference between compromising and overshooting.

The pile in the pipe

Air speed is the half of this that is a safety question. Below about 3800 FPM a vertical run starts plugging, and larger chips want up to 4500 FPM. Below about 2800 FPM a horizontal run builds piles rather than carrying material, so the design targets are 4000 FPM vertical and 3000 FPM horizontal.

A 4 inch drop feeding a 6 inch main is the usual way to break this. The 350 CFM that line carries arrives in the 6 inch main at 1783 FPM, in a 7 inch main at 1310 FPM, in an 8 inch at 1003 FPM. The main then fills until the leftover area is tight enough to carry the air, and OSHA says significant accumulations of fine wood dust can be a fire and explosion hazard. When flow comes back, the pile moves at once and takes joints and filters with it.

What the cheap fix actually is

Unless fine dust is the target, nothing above argues for a bigger collector, and the order of returns is roughly the reverse of what gets bought. Open the machine's port. Shorten and straighten the run. Swap ribbed flex hose for smooth wall, several times less resistance per diameter. Then take the trashcan separator out: Pentz gauged one of those lids pulling a real 1100 CFM down to under 450 CFM on a 6 inch test pipe. His shop, not a design figure, and still a loss of over 600 CFM against the 100 CFM half a horsepower buys.

If a separator has to stay, a cyclone costs airflow and buys something back. Pentz reports his own separating material out before it reached the impeller, which matters because a steel screw hitting an impeller can put a spark in the collection bin. He said it worked well, and still glued magnets into his floor sweep to catch nails before they got that far. Separately: most dust collectors are poor vacuums because their air speed is too low for pick up, which is not the spark objection.

Sizing questions decide most shop purchases, which is why they get a page here rather than a paragraph inside a ranked list. The rest of the category board works the same way, and how the evidence is graded explains what a number here has to clear.