Dust Collection Static Pressure Calculator
Enter the airflow, duct size, duct lengths, fittings, separator, filter, and fan efficiency. The calculator estimates the critical-path pressure loss and the fan operating point to check against a fan curve.
Total system static pressure (in. w.g.)9.14
- Duct air velocity (FPM)
- 4,074.37
- Wood-dust transport check
- Near the 4,000 FPM wood-dust transport target
- Smooth duct pressure loss (in. w.g.)
- 1.33
- Flexible hose pressure loss (in. w.g.)
- 0.67
- Fitting pressure loss (in. w.g.)
- 2.64
- Fan selection point
- 800.0 CFM at 9.1 in. w.g.
- Estimated brake horsepower
- 1.92
- Suggested motor size
- 3 hp
Pressure loss breakdown
| Component | Pressure loss |
|---|---|
| Smooth duct, in. w.g. | 1.33 |
| Flexible hose, in. w.g. | 0.67 |
| Fittings, in. w.g. | 2.64 |
| Separator, in. w.g. | 2.5 |
| Filter, in. w.g. | 2 |
Uses 4.4 in. w.g. per 100 ft for equivalent smooth round duct.
Compare the fan selection point with the manufacturer fan curve, not with motor horsepower alone.
How to use this calculator
- Enter the airflow needed at the open machine or machines.
- Enter the inside diameter and critical-path lengths for smooth duct and flexible hose.
- Count the elbows, wye entries, and open blast gates in that same path.
- Enter separator and filter pressure drops from manufacturer data when you have them.
- Compare the reported CFM and static pressure with the dust collector fan curve.
How the static pressure is calculated
This calculator estimates one critical path through a woodshop dust-collection system. That path is usually the longest or most restrictive open branch from a machine hood to the collector inlet, through the separator and filter. It does not automatically solve every branch in a duct network.
The round duct area is A = pi * (d / 12)^2 / 4, where d is the inside diameter in inches. Duct velocity is V = Q / A, with Q in CFM and V in feet per minute. Velocity pressure is VP = (V / 4005)^2 inches of water.
The smooth round-duct friction rate uses the ASHRAE-style relationship R = 0.109136 * Q^1.9 / d^5.02, reported in inches of water per 100 feet. Smooth duct loss is Pd = R * Ls / 100. Flexible hose is treated as a multiple of smooth duct friction, so Ph = R * M * Lf / 100.
Fitting loss is based on velocity pressure. The planning coefficients are 0.75 for a long-radius 90-degree elbow, 0.35 for a 45-degree elbow, 0.50 for a branch-wye entry, and 0.20 for a fully open blast gate. The fitting loss is Pk = Kt * VP. Total static pressure is SP = Pd + Ph + Pk + Ps + Pf, where Ps is separator drop and Pf is filter drop.
What moves the result most
Duct diameter and airflow dominate the result because velocity and friction change quickly as the duct gets smaller or the airflow rises. A few feet of compressed corrugated hose can also matter more than a much longer section of smooth metal duct. Filters and cyclones should be entered from their curves when possible because their pressure drop changes with airflow and dust loading.
The velocity check compares the calculated duct speed with a 4,000 FPM wood-dust transport target. Below that target, chips and dust are more likely to settle in horizontal duct. Far above it, the system may waste pressure in noise, abrasion, and turbulence even though transport is strong.
What this calculator leaves out
The result is a planning estimate, not a balanced duct-system design. It does not model multiple simultaneous branches with different diameters, hood entry losses, transitions, leaks, dirty-filter curves, fan inlet effects, or collector performance outside the published fan curve. Combustible-dust safety, grounding, outdoor discharge, and code compliance also need separate review.
Worked example
Suppose a shop needs 800 CFM through 6-inch duct, with 30 ft of smooth duct, 5 ft of typical flexible hose, two 90-degree elbows, one 45-degree elbow, one branch wye, one open blast gate, a 2.5 in. w.g. separator drop, a 2.0 in. w.g. filter drop, and 60% fan efficiency.
The duct area is 0.196 square feet, so velocity is about 4,074 FPM. The friction rate is 4.44 in. w.g. per 100 ft. Smooth duct contributes 1.33 in. w.g., flexible hose contributes 0.67 in. w.g., and fittings contribute 2.64 in. w.g. Total static pressure is about 9.14 in. w.g., so the fan selection point is 800 CFM at 9.1 in. w.g. Estimated brake horsepower is 1.92 hp, and adding a 15% planning margin suggests the next common motor size of 3 hp.
Common questions
What static pressure should a woodshop dust collector overcome?
Use the static pressure at the airflow you need, not a single generic number. A small direct-connected setup may need only a few inches of water, while a system with long duct, flexible hose, a cyclone, and a filter can need much more. The useful number is the calculated CFM and static pressure pair checked against the fan curve.
How many CFM keeps dust moving in 4-inch, 5-inch, or 6-inch duct?
At a 4,000 FPM transport target, round duct carries about 349 CFM in 4-inch duct, 545 CFM in 5-inch duct, and 785 CFM in 6-inch duct. Those are velocity targets, not guarantees that the collector can overcome the pressure loss at that airflow.
Why does flexible hose reduce dust-collector airflow so much?
Corrugations create turbulence and extra surface drag, especially when the hose is compressed or bent sharply. This calculator treats flexible hose as two to five times the smooth-duct friction rate for planning. Published hose data is better when the manufacturer provides it.
Should I calculate every branch or only the longest branch?
Calculate the most restrictive open path first, which is often but not always the longest branch. A shorter branch with smaller duct, more hose, or tighter fittings can require more static pressure. For a branched system, repeat the calculation for each likely critical path.
Is motor horsepower enough to predict actual airflow?
No. Motor horsepower does not show how much air the blower moves at a specific static pressure. Use the calculated CFM and static pressure on the manufacturer fan curve, then check that the motor size is adequate for that operating point.