Dust Collection CFM Calculator

How it works

CFM = velocity (fpm) × duct area (ft²), where area = π ÷ 4 × (diameter in ÷ 12)²

Dust collection airflow is simply velocity times duct area. To keep wood chips suspended and out of the duct floor, you need roughly 4000 feet per minute in branch lines (about 3500 fpm in the main), and that velocity across a given duct size sets the volume of air — the CFM. A 4″ port maxes out around 350 CFM at proper velocity, which is fine for a saw or band saw but starves a planer or sander that throws far more waste; those want a 5″ or 6″ line. Undersizing the duct doesn’t just reduce volume, it lets velocity drop and chips settle and clog. Match the duct to the machine’s appetite, keep runs short and smooth, and minimise 90° elbows, which each cost airflow.

Worked example

A 4″ branch duct at 4000 fpm: area = π ÷ 4 × (4 ÷ 12)² = 0.0873 ft²; CFM = 4000 × 0.0873 ≈ 349 CFM. That comfortably serves a table saw (~350 CFM) but falls short of a drum sander’s ~550 CFM, which needs a 5″ or 6″ line to reach that volume at the same velocity.

Sources: OSHA — Woodworking dust control

Frequently asked questions