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How to measure a dust port

Measuring a dust port sounds trivial, but the two things people miss, inner versus outer diameter and the units, are exactly what decide whether a hose will fit.

Inner or outer diameter

A port has two diameters that matter: the outer diameter of its spigot, and the inner diameter of its bore. Which one you need depends on how the hose connects. If the hose slides over the port, you match the port's outer diameter to the hose's inner diameter. If the hose pushes into the port, you match the port's inner diameter to the hose's outer diameter.

So before measuring, decide which way the two parts overlap, then measure the diameter that will actually make contact. Recording just one number without saying whether it is inner or outer is the commonest cause of a wrong-size order.

Measuring accurately

A caliper across the diameter is the accurate tool: measure across the widest part of the round spigot for the outer diameter, and across the bore for the inner. If you only have a tape or ruler, measure the circumference around the port and divide by 3.14 to get the diameter, which is often easier on an awkwardly placed port.

Take the reading in two directions in case the port is slightly oval, and measure the plain round section rather than any moulded lip or flange.

Reading the units

Once you have a number, decide whether the tool is likely imperial or metric. A tool sold in North America is often quoted in inches, such as 2.5 inch, while a European tool is often metric, such as 35 millimetres. Convert your measurement both ways and see which lands on a common nominal size, since a port measuring about 63 millimetres is really a 2.5 inch fitting.

Remember that nominal sizes are not always the exact measured figure, so a port that measures a millimetre off a round number is usually just that nominal size.

Turning the measurement into a match

With the correct diameter, its inner-or-outer sense, and the units settled, you can match the port to a hose or adapter. Feed the number into the finder or compare it against the standard sizes, and choose the hose or stepped adapter that seats firmly against the surface you measured.

Measure it, then find the fit

Open the PortMatch finderPick your tool to confirm its port size and the hose that matches.Find digital caliper measuring tool on eBayaffiliate

Dust port size standardsWhere your number lands Imperial versus metric portsRead the units Tool-port to hose adaptersMatch both ends Makita 24 mm tool portMetric port example

Sources: Festool dust extraction documentation · Rockler dust port measuring reference