How to use this calculator
- Measure the tool port. Enter the actual round port or adapter diameter on the woodworking machine.
- Choose a transport velocity. Use a screening value such as 4000 ft/min, then adjust if your shop standard or duct design requires another value.
- Set open branches. Enter how many same-size branches are expected to be open at the same time.
- Check the main duct. Use the main duct velocity result to see whether the chosen main diameter still carries dust after the airflow allowance.
How it works
Dust-collection airflow starts with continuity: airflow is area times velocity. For a round tool port, the calculator finds A = pi x D^2 / 4 and then uses Q_branch = A_port x v_transport to estimate the CFM required at one open branch.
If more than one same-size branch is open, branch CFM is multiplied by the active-branch count. The airflow allowance is then applied: Q_collector = Q_branch x N x (1 + allowance / 100) The main duct velocity is recalculated from that collector airflow and the entered main duct area.
After the airflow screen, use the duct size calculator for general duct velocity checks and the static pressure calculator to budget pressure losses. For the woodworking material side, move into the cut list calculator or plywood sheet calculator.
Worked example
Verified against the live calculator
A table saw has one 4 in dust port, one branch is open, and
the target transport velocity is 4000 ft/min. The port area
is pi x (4/12)^2 / 4 = 0.0873 ft^2, so branch airflow is
0.0873 x 4000 = 349 CFM. With a 15% allowance,
the collector airflow screen is about 401 CFM. In a
4 in main duct, that allowance raises the checked velocity to
about 4600 ft/min.
Frequently asked questions
How do I calculate dust collector CFM from duct diameter?
Multiply the duct or port area by the target transport velocity. A round 4 in port at 4000 ft/min needs about 349 CFM before any allowance for filter loading, separator loss or leakage.
What velocity should a woodworking dust collector use?
For an early shop-duct screen, 3500 to 5000 ft/min is a practical transport-velocity range. Lower velocities can let dust settle in horizontal ductwork; higher velocities raise noise and static-pressure loss.
Why does a larger main duct sometimes show low velocity?
Velocity is airflow divided by area. If a 4 in tool port feeds a much larger main duct without enough extra CFM, the larger duct area can slow the air below the transport-velocity screen.
Does this size the dust collector fan or cyclone?
No. This calculator estimates required airflow and duct velocity. Final collector selection needs static pressure from duct length, elbows, flex hose, separator, filter condition, blast gates and the fan curve.
Should I count every tool branch at once?
Count only the branches expected to be open together. If blast gates keep one machine open at a time, use one active branch; shared sanding tables or multiple open drops need the higher count.
Method & assumptions
- The tool port and main duct are treated as round openings with the entered diameters.
- Each active branch is assumed to be the same size as the entered tool port.
- Transport velocity is an airflow-screening value, not a guarantee of chip capture at the cutter or sanding surface.
- The calculator does not model static pressure, fan curves, filter loading, cyclone or separator loss, fitting equivalent lengths, flex hose, leakage, hood design or blast-gate behavior.
- Use measured collector performance and manufacturer fan data before buying equipment or committing to a final duct layout.