Duct Sizing Formula
Calculate the minimum duct diameter needed based on required CFM and target air velocity.
Last updated: 2026-02-27
Duct Diameter Calculator
Calculate optimal duct diameter for your dust collection system based on CFM requirements and target air velocity.
Round Duct Sizing Rectangular Duct Air Velocity CFM Matching
System Parameters
CFM
Cubic feet per minute needed FPM
Optimal range: 3500-4000 FPM Recommended Duct Size
Enter CFM and velocity requirements to calculate optimal duct diameter.
Standard Duct Sizes
2.5 inUp to 150 CFM
3 in150-250 CFM
4 in250-400 CFM
5 in400-650 CFM
6 in650-900 CFM
8 in900-1500 CFM
Proper duct sizing ensures efficient dust collection. Maintain 3500-4000 FPM air velocity for optimal dust transport.
How to Use This Calculator
- 1 Enter Required CFM Input the total CFM needed for your tool or duct branch (use CFM Requirement Calculator if unsure).
- 2 Set Target Velocity Enter target FPM (feet per minute). Optimal: 3500-4000 FPM for wood dust transport.
- 3 Choose Duct Type Select round (most efficient) or rectangular (for tight spaces) duct.
- 4 Review Results Get calculated diameter, nearest standard size, and actual velocity at that size.
Pro Tip: Always round UP to the next standard duct size. A slightly oversized duct is far better than undersized — you can always add a blast gate to control airflow, but an undersized duct causes settling and clogs.
D = sqrt(CFM x 183.3 / Velocity) Air Velocity Guide
Proper air velocity is critical for effective dust transport. Too slow and dust settles; too fast causes excessive noise and wear.
| Velocity (FPM) | Status | Effect |
|---|---|---|
| Below 3000 | Too Low | Dust settles in ducts, clogs form |
| 3000-3500 | Minimum | Light dust transport, marginal |
| 3500-4000 | Optimal | Reliable dust transport, low noise |
| 4000-4500 | Acceptable | Good transport, moderate noise |
| Above 4500 | Too High | Excessive noise, duct wear, wasted energy |
Target 3500-4000 FPM for wood dust. Heavy materials like metal shavings need 4000+ FPM.
Duct Design Best Practices
- Round ducts are most efficient — 40% less friction than rectangular at same area
- Use largest practical size — reduces static pressure and noise
- Never reduce trunk size prematurely — reduce only after last branch connection
- Smooth-wall PVC or metal — flex hose creates 3-5x more friction
- Ground PVC ducts — bare copper wire inside to prevent static buildup
- Seal all joints — use foil tape and sealant, never duct tape
Round vs Rectangular Ducts
Round ducts are preferred for most installations. Rectangular ducts offer advantages in tight spaces.
| Feature | Round | Rectangular |
|---|---|---|
| Efficiency | Most efficient | 10-40% more friction |
| Cost | Lower material cost | Higher fabrication cost |
| Space | Requires more headroom | Fits tight spaces |
| Best For | Main trunk lines | Drops, branches, tight areas |
| Installation | Easy with fittings | Custom fabrication needed |
| Air Distribution | Uniform flow | Non-uniform, corner eddies |
Use round for main runs, rectangular only when space constraints require it.
Glossary of Terms
Volume of air flowing through the duct per minute. Determines the duct diameter needed at a given velocity.
Air velocity inside the duct. Must stay above 3500 FPM to prevent dust settling. Target: 3500-4000 FPM.
For rectangular ducts, the round duct diameter that would provide the same airflow at the same pressure drop.
Width-to-height ratio of rectangular ducts. Higher ratios increase friction. Keep below 3:1 for efficiency.
Frequently Asked Questions
What size duct do I need for 400 CFM?
At 4000 FPM target velocity, 400 CFM needs a 4.3-inch duct — round up to standard 5-inch. At 3500 FPM, you need 4.6 inches — also rounds to 5-inch standard size.
Can I use smaller ducts to save money?
No. Undersized ducts increase velocity beyond optimal range, causing excessive noise, wear, and higher static pressure. This forces your collector to work harder and may not transport dust effectively.
How do I convert round to rectangular duct size?
Match the cross-sectional area. A 6-inch round duct (28.3 sq in) equals approximately an 8x4 inch rectangular duct (32 sq in). Always round up to account for increased friction.
What duct material is best for woodworking?
Smooth-wall PVC (Schedule 40) or spiral metal ductwork. PVC is cheaper and easy to install; metal is more durable. Always ground PVC with bare copper wire to prevent static discharge.