Sizing Blast Gates for Dust Collection Systems | Duct-Size Rule

Match your blast gate’s diameter to your ductwork for dust collection — full gates for high-CFM tools, half gates for low-demand or balancing.

The foundation of sizing blast gates for dust collection systems is simple: match the gate’s nominal diameter to your ductwork’s nominal diameter. A 6-inch duct calls for a 6-inch gate, and that relationship holds across every size. But diameter matching alone won’t guarantee good performance. You also need to choose between full and half gates, account for your system’s CFM and static pressure, and install everything without air leaks that rob suction. This article walks through each decision so your dust collection works the first time.

The One Rule For Sizing Blast Gates

Sizing a blast gate is a diameter-matching exercise. The gate’s nominal diameter must equal the inner diameter of the duct it connects to — a 4-inch gate on 4-inch pipe, an 8-inch gate on 8-inch pipe. This preserves the air velocity needed to keep dust suspended in the airflow. The gate itself, when fully open, should present no more restriction than a straight section of pipe of the same diameter.

The clearance rule is equally firm: manual gates need at least 18 inches of open space around them for access and maintenance. Pneumatic gates need more — enough for airline connections and electrical controls. Plan those locations before cutting any ductwork.

Full vs. Half Blast Gates — Which One Do You Need?

Full blast gates provide 100 percent open area when the slide is retracted, meaning zero additional restriction beyond the pipe itself. The choice depends on the tool’s CFM appetite. High-CFM tools — table saws, planers, drum sanders — need full gates to maintain capture velocity at the hood. Low-demand tools such as small sanders or scroll saws can get by with a half gate, and half gates are sometimes used to balance airflow in branched systems where one leg tends to hog all the suction.

If you are building a new system from scratch, use full gates everywhere. Half gates are a tuning tool for existing layouts, not a first-choice component.

What Size Blast Gate Does Your System Need?

Determine the size by starting with each machine’s CFM requirement at the planned duct length and static pressure. The gate size matches the branch duct size, which must be large enough to carry that machine’s CFM at the target 3,500–4,000 FPM. The table below summarizes the key parameters that govern the sizing decision.

How To Install A Blast Gate Without Leaks

Installation leaks are the most common reason a correctly sized gate underperforms. Map every gate location on paper before cutting any duct to confirm operator access while machines are running. When cutting duct for a full gate, make clean, square cuts — sloppy joints create air leaks that reduce efficiency and raise noise. For half gates, use a reciprocating saw with a fine-tooth blade to minimize burrs that bind the slide mechanism.

Mount the gate with the slide plate oriented to match the material flow direction. Position it so gravity helps clear debris rather than packing it against the slide. Use all provided mounting hardware — missing bolts cause vibration that loosens the seal over time. When connecting flexible ductwork, avoid overtightening hose clamps, which can deform the gate housing and bind the slide. Oneida Air’s cast aluminum gates seal in one direction only — the plate must face toward the collector, and tightening the thumbscrew toward the collector improves the seal.

Common Mistakes That Ruin Dust Collection Performance

The single worst error is leaving multiple gates open simultaneously. Every open gate is a vacuum leak that drops suction at every other tool. Only one gate should be open at a time unless you have a properly designed blast-gate system with balanced branches. Placement also matters. While some argue gate location is irrelevant if joints are sealed, dust collection expert Bill Pentz recommends placing gates as close to the main or lateral lines as possible to reduce the length of duct that stays under negative pressure — and therefore the number of potential leak points.

Knob orientation is a detail that catches first-time builders. Install the gate so the knob faces away from the suction side. If the knob is on the suction side, the slide binds against the opposite side of the housing, creating an air gap. Avoid using screws inside the pipe — any protrusion into the airstream creates a catch point where dust accumulates. Flexible pipe causes greater efficiency loss than rigid pipe; use the largest possible rigid duct and keep flex sections as short as possible. Fit the gate thickness carefully in DIY builds — a gate that is too snug prevents the closed gate from sealing against the vacuum side.

Blast Gate Options For Every Shop

Once you know the size, you can choose a gate type that matches your workflow. Manual gates are the most common and most affordable. Automatic smart gates open and close in response to tool startup, which eliminates the “left open” problem entirely — but they cost more and require wiring or pneumatic lines. The table below compares the major options available today.

For a detailed comparison of these and other models with hands-on testing notes, check out our tested roundup of the best blast gates.

Final Checklist — Sizing Your Blast Gate

Before you cut pipe or order parts, run through this sequence to confirm your sizing decisions:

• Confirm duct diameter. Measure the inner diameter of the branch duct — that number is the gate size you need.
• Match gate type to tool CFM. High-CFM tools get full gates; low-demand tools may accept half gates if you are balancing an existing system.
• Plan gate locations on paper first. Mark every gate position with at least 18 inches of clearance, and verify operator access with the machine running.
• Verify CFM at the hood. The gate is correctly sized only if the target velocity of 3,500–4,000 FPM is achievable at the tool’s actual duct length.
• Test with one gate open. Close all other gates and measure airflow at the open gate. If suction is weak, check for leaks, undersized duct, or an undersized collector.

Follow these steps and your blast gates will deliver the full suction your dust collector can produce — no guessing, no second tries.