Introduction
A cyclone separator (often called a cyclone dust collector) removes chips and dust from an airstream by forcing the air to spin. The swirling motion creates centrifugal forces that push heavier particles toward the wall, where they lose energy and drop into a collection bin. Cleaner air exits through the central outlet (often called the vortex finder) and typically continues to a filter or exhaust.
In small woodworking shops, a cyclone is commonly placed ahead of a cartridge filter to capture most chips and coarse dust, extending filter life and reducing clogging. In larger systems, cyclones are also used for grain handling, metal grinding, and industrial pre-separation. Regardless of application, the cyclone’s diameter is the main scaling dimension: once you choose a diameter that matches your airflow and inlet velocity, the rest of the geometry can be estimated using well-known ratios.
Units matter: this calculator expects airflow in m³/h and inlet velocity in m/s. It converts m³/h to m³/s internally. If you work in CFM or inches of water gauge, you can still use this tool by converting inputs first, then converting the output dimensions back to your preferred units.
How to use the calculator
- Enter airflow in m³/h (cubic meters per hour). If you have m³/s, multiply by 3600 to convert to m³/h.
- Enter desired inlet velocity in m/s. Many shop cyclones target roughly 15–20 m/s at the inlet for good separation.
- Click Calculate to get the estimated cyclone diameter and the main dimensions based on classic Stairmand-style proportions.
- Use the results as a starting point for layout and fabrication. Confirm that your blower can handle the pressure drop and that your available height can accommodate the body + cone.
Tip: If your system is ducted to multiple machines, use the airflow you expect at the cyclone inlet under real operating conditions. Blast gates, duct losses, long flex hose runs, and filter loading can reduce delivered flow compared with a fan’s free-air rating.
Formula and assumptions
This calculator estimates cyclone diameter by relating volumetric flow rate to inlet velocity. Conceptually, if you know how much air you want to move (Q) and how fast you want it to enter the cyclone (V), you can infer an equivalent flow area. The calculator then expresses that area as a diameter D using a circular-area relationship.
- Qh = volumetric airflow rate (m³/h), the value you enter.
- Q = volumetric airflow rate (m³/s). The calculator converts using
Q = Qh / 3600. - V = desired inlet air velocity (m/s).
- D = cyclone body diameter (m).
The diameter is computed as:
What this means in practice: for a fixed airflow, choosing a higher inlet velocity produces a smaller cyclone diameter. That can be helpful when space is tight, but it usually increases pressure drop and noise. Choosing a lower inlet velocity produces a larger cyclone, which can reduce pressure drop but may reduce separation if velocity becomes too low to keep particles moving toward the wall.
Stairmand ratios (what the outputs mean)
After D is found, the calculator applies common Stairmand-style geometric ratios to estimate the main dimensions: body height, cone height, outlet diameter, and inlet dimensions. These ratios are widely used as a practical baseline because they tend to provide good separation with a reasonable pressure drop for many dust-collection scenarios.
The outputs are not a complete fabrication drawing. They are the core dimensions you typically need to sketch a cyclone: the cylinder, the cone, the tangential inlet rectangle, and the outlet tube. Details like the dust outlet diameter at the cone tip, the outlet tube insertion depth, and the exact inlet transition are important too, but they vary by build style and are often tuned during prototyping.
| Component | Ratio to D | Meaning |
|---|---|---|
| Body Height | 4 D | Cylindrical section height above the cone |
| Cone Height | 2 D | Conical section height below the body |
| Outlet Diameter | 0.5 D | Vortex finder / outlet tube diameter |
| Inlet Height | 0.5 D | Rectangular inlet opening height |
| Inlet Width | 0.25 D | Rectangular inlet opening width |
Worked example
Suppose your dust collector delivers 1800 m³/h at the cyclone inlet and you want an inlet velocity of 15 m/s.
The calculator converts airflow to m³/s:
Q = 1800 / 3600 = 0.5 m³/s.
Then it computes:
D = sqrt((4/π) × (Q/V)) = sqrt((4/π) × (0.5/15)) ≈ 0.21 m.
Using the ratios, the estimated dimensions are: body height ≈ 0.85 m, cone height ≈ 0.42 m, outlet diameter ≈ 0.11 m, inlet height ≈ 0.11 m, inlet width ≈ 0.05 m. These are starting dimensions for a Stairmand-like cyclone; your final design may change based on available height, duct transitions, and fabrication constraints.
Build notes: ducting, inlet, and bin
Ducting and inlet transitions
Many shop systems use round ducting, but classic cyclone inlets are rectangular and tangential. The calculator reports a rectangular inlet height and width. If your duct is round, you will typically build a transition piece. A smooth transition helps maintain velocity and reduces turbulence. Avoid abrupt steps, sharp internal corners, or long sections of flex hose right at the inlet; these can increase losses and reduce separation.
If you must use a tight transition, consider increasing the inlet velocity target slightly (within reason) to compensate for added losses. However, remember that increasing velocity can also increase pressure drop, so the fan must be able to sustain the flow.
For layout, it helps to mark the inlet rectangle on the cylinder and ensure it enters tangentially (not radially). A tangential entry is what initiates the swirl. Some builders add an internal ramp or neutral vane to guide the flow; this can improve stability but also complicates fabrication.
Dust bin, sealing, and leaks
Cyclones work best when the collection bin is well sealed. Leaks at the lid, drum rim, or cone-to-bin connection can pull in extra air. That extra air can disturb the downward spiral and may re-entrain dust, sending it back up toward the outlet. A simple gasket and sturdy latches often make a noticeable difference.
Keep the bin from overfilling. When chips pile up into the cone outlet region, the cyclone can short-circuit and carry material into the outlet. A clear drum, a sight window, or a simple level indicator can prevent surprise overfills.
Material choice matters too. Metal is durable and naturally conductive, while plastic drums can accumulate static. If you use plastic, follow best practices for static control and grounding appropriate to your environment and local codes.
Troubleshooting and tuning
If your cyclone is built close to these proportions but performance is disappointing, the cause is often elsewhere in the system. Use the checklist below to diagnose common issues before rebuilding the cyclone.
- Low airflow at the cyclone inlet: Measure or estimate actual flow. Long duct runs, undersized duct, too many elbows, and clogged filters can reduce Q. If Q is lower than expected, inlet velocity drops and separation suffers.
- Inlet turbulence: A poor round-to-rectangular transition, a sharp elbow immediately before the inlet, or internal obstructions can destabilize the swirl. Try adding a short straight section before the inlet or smoothing the transition.
- Outlet tube placement: The outlet tube (vortex finder) should be centered and sealed. If it is off-center or leaky, it can pull dusty air directly. Many designs also rely on a specific insertion depth; if you change it, test and observe dust carryover.
- Bin leaks or poor drop-out: If you see dust swirling in the bin or being lifted back up, improve sealing and ensure the cone outlet is not restricted.
- Very fine dust expectations: Even a well-built cyclone may pass the finest particles. A downstream filter is still recommended for indoor air quality.
A practical tuning approach is to keep the cyclone geometry fixed and focus on achieving the intended airflow and inlet velocity. Once the system is delivering stable flow, evaluate separation by checking how quickly the downstream filter loads and how much dust appears in the filter housing.
Limitations and practical notes
This tool provides first-pass sizing. Real cyclone performance depends on more than geometry:
- Pressure drop: Higher inlet velocity generally improves separation but increases pressure loss. Many Stairmand-type cyclones operate around ~1–1.5 kPa at design conditions, but actual values vary with inlet/outlet details, surface roughness, and flow regime.
- Fan curve and system losses: Duct length, elbows, flex hose, filters, and dirty filters can reduce delivered airflow. If the fan cannot maintain the target flow, inlet velocity drops and separation efficiency decreases.
- Fine dust capture: Cyclones excel at chips and coarse dust. Very fine particles may still pass through and require a downstream filter.
- Inlet shape and transitions: The calculator outputs a rectangular inlet size, but your ducting may be round. Poor transitions can add turbulence and reduce performance.
- Build tolerances and leaks: Air leaks at seams, lids, or the dust bin reduce effective velocity and can re-entrain dust.
- Safety: Dust can be combustible. Use appropriate grounding/static control, avoid ignition sources, and follow local codes and best practices.
If you need a design for compliance, hazardous dust, or industrial duty cycles, consult a qualified engineer or manufacturer data. This page is intended for estimation and educational use.
Additional guidance for builders
The dimensions from this calculator are often used to draft a simple fabrication plan: a cylindrical body (rolled sheet metal or a pipe section), a cone (rolled and riveted/welded), a tangential inlet, and an outlet tube. A smooth interior and a well-sealed collection bin help maintain stable flow.
When space is limited, builders sometimes shorten the cone or body. Be aware that changing proportions can affect separation and pressure drop. If you modify geometry, treat the results as an estimate and consider testing with your actual dust load.
Finally, remember that cyclone sizing is only one part of a dust collection system. Duct diameter, hood design at each machine, and filter selection often dominate real-world performance. Use this calculator to get the cyclone in the right range, then verify the whole system delivers the airflow you need.
If you want to sanity-check results, compare the calculated diameter to commercially available cyclones intended for similar airflow. If your calculated cyclone is dramatically smaller than typical products at the same airflow, your chosen inlet velocity may be too high for your fan. If it is dramatically larger, your inlet velocity may be too low to separate fine dust effectively.