Air Extraction System Designer: CFM, Ducts & Fan Sizing
Describe your extraction job and route to generate a proposed build, hose and duct sizes, airflow allowance, calculated fan pressure and component shopping list. No CFM or engineering knowledge needed to start; refine the concept with supplier data in the optional engineering view.
Quick answer. CFM measures air volume per minute; duct velocity measures how fast it travels. A 6-inch duct carrying 785 CFM averages about 4,000 ft/min. Your collector must deliver the required airflow against the resistance of the complete installed system. See the method and references
Tell us the job. Get a build concept.
What do you want to extract?
You do not need to know CFM, hose size or fan pressure to start. Describe the job and route; we will propose those for you.
You will receive a proposed layout, hose and duct sizes, airflow allowance, calculated pressure and component shopping list. Exact machine and coating requirements still need supplier confirmation.
Dust collector CFM chart by duct diameter
Calculated airflow through a round duct at three example velocities. These columns are comparison inputs, not recommended velocities or guaranteed collector capacities.
| Inside diameter | 3,500 ft/min | 4,000 ft/min | 4,500 ft/min |
|---|---|---|---|
| 3 in | 172 CFM | 196 CFM | 221 CFM |
| 4 in | 305 CFM | 349 CFM | 393 CFM |
| 5 in | 477 CFM | 545 CFM | 614 CFM |
| 6 in | 687 CFM | 785 CFM | 884 CFM |
| 7 in | 935 CFM | 1,069 CFM | 1,203 CFM |
| 8 in | 1,222 CFM | 1,396 CFM | 1,571 CFM |
For example, a 6-inch duct at 4,000 ft/min corresponds to about 785 CFM. That arithmetic does not establish capture at the tool or show that a collector can deliver 785 CFM through your system.
Use the designer’s velocity window and component-loss breakdown to compare another diameter at your required airflow. For component guidance, see Oneida Air Systems’ ductwork reference.
Start at the tool or hood
Get its airflow requirement and identify which tools run together. A connection diameter by itself does not tell you how much air the hood needs.
Account for the complete run
Enter hose and duct lengths, fitting loss coefficients, and filter/separator losses at the chosen flow. Obtain those values from the equipment documentation.
Check the operating point
Compare the chosen CFM and required total pressure with the matching fan curve. Two-pickup estimates assume balanced branches; verify the installed split and all operating configurations.
Keep the result with the work
Print the inputs and result for your equipment or ductwork supplier. Keep the fan curve and actual duct layout with the calculation so the operating assumptions are available for review.
How it works
Set source flow from the machine specification or enclosed-opening area × specified face velocity. Calculate straight-run and fitting losses, use the highest-loss balanced pickup branch, then add common-path losses. Match the required total pressure and flow to a fan curve without extrapolation.
- Example flows and loss coefficients are illustrative. Replace them with supplier data; no port-size-to-capture-flow conversion is assumed.
- One or two balanced pickups feed a single uniform-bore common path. Operating-point estimates hold geometry and coefficients fixed and scale component losses quadratically. Total pressure includes discharge loss; static curves require outlet conversion.
- Low-pressure round-duct planning only. No calculation certifies capture, filtration or exposure control. Paint systems need a coating-compatible, supplier-reviewed design.
Read airflow and pressure together
Formula and units
- Duct area (ft²) = π × (inside diameter in inches ÷ 24)²
- CFM = area × velocity in ft/min
- Straight duct loss: ΔP = f × (L ÷ D) × ρ × v² ÷ 2
- Fitting loss: ΔP = K × ρ × v² ÷ 2
Pressure equations use meters, m/s and kg/m³ and return pascals; divide by 249.09 for inches of water. The engineering view adds the highest-loss balanced pickup branch to the common path, filter, separator and discharge losses. f is the Darcy friction factor; K is a fitting loss coefficient.
| Inside diameter | Average velocity | What changes |
|---|---|---|
| 4 in | 6,875 ft/min | Faster air in a smaller passage |
| 5 in | 4,400 ft/min | Intermediate area and velocity |
| 6 in | 3,056 ft/min | Slower air in a larger passage |
Oneida describes 3,500–4,500 ft/min as a typical dust-collection velocity range [1]. It provides context for the CFM chart above, not a universal pass/fail limit. This builder’s concept velocity window is a separate planning assumption.
Comparing a longer CNC hose
Suppose supplier data calls for 600 CFM. At that same flow, increasing a 5-inch duct to 6 inches changes average velocity from about 4,400 to 3,056 ft/min. A wider duct can reduce resistance, but also reduces transport speed at fixed flow. Recalculate the whole route and check the fan curve before selecting the size. This is an illustrative comparison, not a measured installation.
What your result means
- Airflow allowance
- The goal-based builder proposes a starting allowance. Replace it with the machine or hood supplier’s requirement before purchasing equipment.
- Fan duty
- Keep the CFM and pressure paired. The designer reports total pressure; a static-pressure curve needs the appropriate outlet conversion.
- A changed route
- Longer flex hose, elbows and loaded filters increase resistance. Oneida’s component guidance explains these effects [2]; enter losses appropriate to the chosen flow.
Assumptions and limits
The calculation does not test dust capture or exposure. An apparently suitable velocity cannot confirm hood performance, branch balancing or filtration. Paint and solvent exhaust require a process-specific compatible system; wood-dust equipment is not interchangeable with it.
Sources and content notes
- Oneida Air Systems: airflow versus pipe diameter
Typical velocity context and the relationship between duct area, airflow and velocity.
- Oneida Air Systems: static pressure in dust collection
Why ductwork, flexible hose and fittings affect the operating point. Its published component values are not automatically this planner’s inputs.
Maintained by WoodWorkCalc · Reference content updated . Equations and worked values were checked against the calculator implementation. Scenarios are illustrative; they are not field-test records or independent technical certification.
Continue planning
A desktop CNC example: 120 CFM through a 2½-inch path
| Input or step | Example value |
|---|---|
| Required airflow (illustrative) | 120 CFM |
| Port / hose / common duct inside diameter | 2.5 / 2.5 / 2.5 inches |
| Pickup hose / common duct lengths | 6 / 2 feet |
| Pickup / common Darcy friction factors | 0.06 / 0.025 |
| Bends / K per bend | One per section / 0.5 |
| Hood K / adapter-junction K / discharge K | 0.5 / 0.5 / 1 |
| Separator / loaded filter / other loss | 1 / 1 / 0 in. water |
| Air density | 1.2 kg/m³ |
| Average hose velocity | 3,520 ft/min |
| Required fan total pressure | 5.83 in. water at 120 CFM |
| With pickup hose shortened to 3 feet | 5.16 in. water at 120 CFM |
These reproduce the Desktop CNC example. They demonstrate component interactions, not a validated CNC requirement. The tool maker must establish required airflow; supplier data must replace assumed losses. Shortening this hose reduces modeled pressure demand by about 0.67 in. water.
Common questions
Can port size tell me required CFM?
No. Source capture depends on the tool, hood and process. Use its specified airflow; diameter then determines velocity and contributes to resistance.
What fan rating should I buy against?
Airflow and pressure at the same operating point, using the correct total/static pressure basis and complete installed configuration. Peak horsepower, free-air CFM and sealed suction alone are not sufficient.
How are two simultaneous pickups handled?
Their flows add in the common duct. The higher-loss branch sets branch pressure; the other must be balanced. Shared losses are added once. Actual unbalanced flows can differ from the estimates.
Does a numerical match mean effective extraction?
No. Confirm source capture, installed airflow, suitable filtration and material compatibility. Paint exhaust additionally needs a supplier-reviewed booth, fan/electrics, makeup air and discharge.
Learn the method in the Academy
Follow the matching lesson or workshop reference before applying the result.
Use this in your project
Continue with the next measurement or cost, then keep the assumptions with your quote.
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