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A 1.5-horsepower dust collector wearing a 1,200 CFM sticker will never move 1,200 cubic feet of air per minute in a real shop. It will not do it through 20 feet of 4-inch duct, it will not do it through a flex hose squeezed onto a 2-1/2-inch tool port, and it will do even less once the filter loads with fine sanding dust. That number describes a bare impeller spinning in open air — useful for ranking blower housings against each other, close to useless for predicting what happens at the hood of your planer.
Sizing dust collection is not a hunt for the biggest number on the box. It comes down to three interacting variables: how much air each machine needs, how much resistance the path imposes on that air, and how fast the air must travel to keep debris suspended. Get any one of them wrong and you get the classic hobby-shop result — a collector that roars convincingly, a floor that stays clean right at the hose, and a fresh layer of fine dust on every horizontal surface in the room.
What follows is the arithmetic and the field knowledge behind those variables, with no product pitches. Whether you are hooking a benchtop planer to a shop vac or laying out a central main for four machines, the same method applies.
What CFM Actually Measures (and What It Misses)
CFM is a volumetric flow rate: cubic feet of air passing a point each minute. It says nothing about how hard the blower pulls, and by itself it says nothing about whether the air is moving fast enough to carry a chip up a vertical duct. Airflow and airspeed are different measurements, and confusing them is the root of most undersized or misbalanced systems.
The link between them is pipe area. Flow equals velocity times cross-sectional area, so the same CFM through a smaller opening means faster-moving air and far more friction. A 4-inch duct offers about 0.087 square feet of area; pushing 350 CFM through it produces roughly 4,000 feet per minute. A 6-inch duct offers 0.196 square feet, so the same 350 CFM crawls along at about 1,790 FPM — too slow to reliably carry chips.
That is the whole argument for main sizing in one paragraph. The accepted velocity floor for mixed wood dust in horizontal runs is 3,500 to 4,000 feet per minute; vertical runs want 4,000 or slightly more. Drop below roughly 2,500 FPM and chips start settling, building drifts that narrow the duct until it clogs outright.
Static Pressure: The Half of the Story Ratings Leave Out
Static pressure — measured in inches of water gauge, or w.g. — is the resistance the system presents to airflow. Every filter, hose, elbow, blast gate, and foot of duct adds to it. A blower’s output falls as static pressure rises; that relationship is the fan curve.
Here is the trap. A machine rated at 1,200 CFM at zero static pressure might deliver 400 CFM at 6 inches of water gauge. A typical small-shop run — cyclone, 20 feet of duct, two elbows, a wye, and a pleated filter with a few hours on it — can easily sit at 5 to 7 inches w.g. at the working branch. Two blowers with identical free-air numbers can behave completely differently on that run: the one with a steep fan curve holds flow, and the one with a flat curve collapses.
Why Manufacturer CFM Ratings Mislead
Most published CFM figures are taken at the impeller inlet with no filter, no bag, no hose, and the motor drawing peak current. Some are calculated rather than measured. Others are captured at zero static pressure, a condition that cannot exist once you attach anything to the machine.
When you compare collectors, look for a published fan curve or a table of actual CFM at several static pressure values. A manufacturer who publishes real-world airflow numbers is telling you something. One who publishes only a headline figure is telling you to guess. The broader safety picture matters here too: a collector whose flow collapses under load is not just a performance problem, it is a fine-dust problem.
The Real Cost of Hose Length and Flexible Duct
Corrugated flex hose is the biggest hidden tax in most ducted systems. Per foot, flexible duct has roughly two to three times the friction of smooth-wall pipe, and the effect compounds with every ridge. A 10-foot flex run can cost more pressure than 30 feet of smooth pipe of the same diameter. That does not make flex optional, only something to ration.
Keep flexible hose to the final 3 or 4 feet between the rigid duct and the machine, where it absorbs vibration and allows movement. Run smooth pipe everywhere else. Use wye fittings instead of tees wherever a branch joins a main; a tee forces the airstream to turn hard into the branch and drops velocity, which is exactly where chips fall out of suspension and start a clog.
How Tool Port Size Caps Your Airflow
The machine port is frequently the narrowest point in the entire system, and no downstream blower can pull more air through it than it will admit. A random-orbit sander with a 1-1/4-inch port realistically flows 60 to 100 CFM. A tool with a 2-1/2-inch port tops out near 150 to 200 CFM. A 4-inch cabinet port can pass 350 to 400 CFM.
This is why upgrading the collector while leaving every port at 2-1/2 inches disappoints. The blower works hard, the filter collects, and the hood is starved. For meaningful capture, the port, hose, and branch must pass the airflow — and some tools, like benchtop planers, benefit from staying at 4 inches with high velocity rather than opening to 6.
Calculating Airflow Needs, Machine by Machine
Use these figures as starting targets measured at the tool, not at the blower. They assume a single operator running one machine at a time, which is the correct assumption for nearly every home shop.
Table Saws
A cabinet saw wants 350 to 500 CFM split between the cabinet port and an overarm guard port. Capture at the blade guard does more for visible debris and airborne dust than the cabinet alone, which mostly handles what falls through the table. The practical setups that make this work are documented here.
Planers and Jointers
Planers produce the largest chip volume in the shop. A 13-inch benchtop planer moves 450 to 600 CFM worth of chips and shavings; a 15-inch floor model wants 600 to 800 CFM through a 4- or 5-inch hood. Jointers run 350 to 500 CFM. These are chip-evacuation numbers, and high velocity at the hood matters more than absolute volume.
Sanders and Routers
Sanders make the finest, most respirable dust and deliver it through tiny ports, so 150 to 200 CFM is the realistic ceiling. Router tables with fence and cabinet collection want 300 to 400 CFM. MDF changes the stakes — the dust is finer and more hazardous, and cartridge and mask choices start to matter as much as the collector.
Miter Saws
Miter saws are the hardest common tool to capture at the source because the dust leaves the blade in a wide, fast spray. Expect 300 to 500 CFM at the tool, and expect the hood design and blade choice to decide the outcome. The fixes that work in practice are covered in this guide to miter saw dust collection.
4-Inch vs 6-Inch Mains: Where Most Shops Get It Wrong
A 4-inch main carrying 4,000 FPM moves about 350 CFM. That is plenty for a single machine with a 4-inch port, and it is the reason so many small shops run 4-inch duct successfully. It is also the reason those same shops cannot add a second machine or a long run without losing capture at the tool.
A 6-inch main at the same velocity moves about 785 CFM. If your collector can genuinely deliver 600 to 800 CFM at working pressure, a 6-inch main is the right call, with 4-inch drops to individual machines. If the blower cannot support that volume, a 6-inch main is worse than a 4-inch main — velocity falls below the transport threshold, and the duct fills with drifts. Size the main to the blower’s realistic flow, not to the tool’s wish list. The duct-sizing math is worth working through before you buy a single fitting.
Shop Vac, Dust Collector, or Cyclone?
These three machines solve different problems, and the labels are less useful than the pressure and flow characteristics behind them.
A shop vac generates high static pressure — 50 to 90 inches of water lift — with low volume, typically 100 to 150 CFM. That combination is ideal for small ports, sanders, and cleanup, and poor for planers and long ducted runs, where volume is the limiting factor.
A single-stage dust collector reverses the profile: high volume, low pressure, typically 600 to 1,200 CFM in marketing terms and 350 to 500 CFM usable through a 4-inch duct. Chips pass through the impeller, which accelerates wear, and fine dust loads the filter until flow falls off.
A cyclone separates debris before it reaches the filter, maintaining flow as the drum fills and extending filter life dramatically. The tradeoff is cost, height, and the need to get the ducting right. For a shop processing more than a few hundred board feet a month, the separation is worth it — proven setups and shop-built separators both show the pattern.
Blast Gates, Bends, and Invisible Losses
Every fitting has a price expressed in equivalent feet of straight duct. A tight 90-degree elbow can cost the same as 5 to 10 feet of pipe; two 45s in sequence usually cost less than one 90 and are worth the extra glue joint. Long-radius elbows beat short ones every time.
For a one-person shop, keep every unused blast gate closed; open gates on dead runs leak airflow and lower velocity at the machine you are using. Route the active branch as straight as possible and put the gate at the branch rather than at the bench.
Filter Media and the Airflow Tax
Filter choice is a performance decision as much as a health one. A 30-micron bag passes the dangerous fine fraction back into the room and presents modest resistance. A pleated canister rated at 1 micron captures far more but adds static pressure, especially as it loads. HEPA media raises the cost again.
Nothing about a filter stays constant. As the pleats fill, resistance climbs and flow drops, which is why a collector feels strong after a cleaning and weak a week later. PTFE-coated media releases dust more completely and holds flow longer between cleanings. Whatever you choose, ambient filtration should handle the dust that escapes source capture, because some always does.
Sizing a System for a One-Person Shop
Start with a list of machines, their port sizes, and the CFM each wants at the tool. Note the longest run from the collector to the farthest machine. Because one person runs one machine at a time, size the main for the largest single demand, not the sum — a real advantage over multi-operator shops.
Pick a blower whose published curve still delivers the needed flow at the estimated system static pressure, not its free-air rating. Keep the main at 6 inches if usable flow exceeds about 600 CFM; otherwise stay at 4 inches and shorten the runs. Use rigid pipe for all but the last few feet, wye into branches, and fit long-radius elbows. Add a filter that captures at least the 1-micron range and an ambient unit for the escape fraction.
A worked example: a 2 HP cyclone with a 6-inch main, 25 feet to the farthest drop, two long-radius elbows, a short 4-inch drop, and a pleated canister. Assume 5 to 6 inches w.g. at the tool. Expect 350 to 450 CFM at the hood — enough for a table saw, marginal for a 15-inch planer, and a reminder that big machines want short runs and big ports.
Measuring What You Actually Get
You do not have to guess. A handheld anemometer held at the end of the hose gives you a velocity in feet per minute. Multiply by the duct area in square feet: 0.087 for a 4-inch hose, 0.196 for a 6-inch. A reading of 3,600 FPM in a 4-inch hose works out to roughly 315 CFM.
For a permanent check, install a Magnehelic-style gauge across the filter or at the inlet. Record the baseline static pressure with a clean filter, then watch it over time. When it climbs 1 to 2 inches above baseline, the filter is loaded and your airflow has fallen with it. That gauge turns filter maintenance from a guess into a schedule.
Frequently Asked Questions
Is 1,200 CFM enough for my shop?
Not the way the number is measured. A 1,200 CFM rating is free-air at the impeller; through a realistic ducted run with a loaded filter, expect 350 to 500 CFM at the tool. That is enough for one 4-inch-port machine at a time, and not enough for long runs or 6-inch mains.
Can I run 6-inch duct on a 1.5 HP collector?
Only if the blower can genuinely deliver roughly 600 CFM or more at working pressure. Many 1.5 HP units cannot, and a 6-inch main then drops velocity below transport speed, causing chips to settle in the duct. A short 4-inch run is often the better answer for a small blower.
Why does suction drop as the bag fills?
Two reasons. A full bag raises resistance downstream of the impeller, and a loaded filter raises it on the pressure side. Both push the blower up its fan curve, where flow is lower. Cleaning or replacing the media restores most of the lost performance.
Is a shop vac enough for a table saw?
For the cabinet port, no — a shop vac’s 100 to 150 CFM cannot keep up with a table saw’s debris volume. For an overarm guard or a jobsite saw with a 2-1/2-inch port, it can help. Think of it as a supplement to a real collector, not a replacement.
Do I need a cyclone in a hobby shop?
Not strictly, but the case gets stronger with volume. If you mill rough lumber regularly or fill bags quickly, a cyclone pays for itself in filter life and stable airflow. A few board feet a weekend is fine on a single-stage collector with good filtration.
How do I know what CFM I am actually getting?
Measure it. An inexpensive handheld anemometer at the hose end plus the duct area gives you a working number, and a static pressure gauge across the filter tracks performance over time. Both cost less than the mistake of buying the wrong duct diameter.