Dust Collection CFM Calculator

The only CFM on this page is dust extraction volume measured at the machine port — the air a collector pulls through a hood on a saw, planer or sander. It is not compressor output or air-supply CFM, and the two numbers are not interchangeable. A collector badged 1,200 CFM is quoting the impeller with nothing attached; put twenty feet of 4in duct, two elbows and a length of flex in front of it and the figure at the port is a fraction of that. Enter your run below and get the derated number.

Machine on the end of the run

Table saw, jointer, router table — wants about 550 CFM at the port.

Collector rated airflow (CFM)

The badge number, measured at the impeller with nothing attached.

Rated max static pressure (in. w.g.)

From the spec sheet. 8–10 is typical for a 1.5–2 HP single-stage.

Duct diameter (in)

The smallest diameter anywhere in the run — that is the one that governs.

Straight smooth duct (ft)

Metal or PVC pipe only. Flex goes in its own box.

90° elbows (count)

Count a wye as one elbow and an open blast gate as none.

Flex hose (ft)

Including the last few feet at the machine.

Filter resistance (in. w.g.)

2 for a clean cartridge, 3–4 for a loaded bag.

Airflow at the machine port

426 CFM

35% of the badge figure

Duct velocity

4878 fpm

Over the 3,500 fpm chip-carrying floor

System static

7.9 in.

50 ft equivalent duct

124 CFM short of the 550 CFM this machine wants. Fastest fixes, in order of effect: cut the flex, step the duct up one size, delete an elbow.

This figure is dust extraction volume measured at the machine port — not compressor or air-supply CFM, which is a different number entirely. Duct friction uses the standard round-duct equation, flex counts at 3× smooth pipe, and the collector is modelled on a square-law fan curve built from its rated free-air flow and maximum static pressure. The rest of the shop numbers live in our calculator library.

Equivalent length of common fittings (feet of smooth duct)
Fitting	3in duct	4in duct	5in duct	6in duct
90° elbow	4.5 ft	6 ft	7.5 ft	9 ft
45° elbow	2.3 ft	3 ft	3.8 ft	4.5 ft
1 ft of flex hose	3 ft	3 ft	3 ft	3 ft
Minimum velocity, branch	4,000 fpm — below this chips drop out and pack the duct
Minimum velocity, main	3,500 fpm