Dust Collector Duct Size Calculator

Size a dust collection branch by target CFM, duct diameter, conveying velocity, branch length, fittings, duct material, available static pressure, and collector capacity.

🛠 Shop Machine Presets

Choose a common woodworking machine, then adjust duct size, fittings, flex length, material, and collector rating for your actual run.

📐 Duct, Airflow, And Fitting Inputs

Machine / dust source

This sets hood loss and a suggested conveying velocity check.

Target airflow (CFM)

Enter the airflow you want at this machine port or hood.

Duct inside diameter (in)

Use the actual inside diameter, especially for flex hose and thin-wall pipe.

Target conveying velocity (FPM)

Fine dust often needs 3500 FPM; chips usually need about 4000 FPM or more.

Straight branch length (ft)

Measure from the machine branch to the collector inlet or main trunk takeoff.

Flexible hose length (ft)

Flex hose has a much higher loss factor than smooth pipe.

Long-radius 90 elbows

Count full 90-degree turns in the branch path.

45 elbows

Two 45s often flow better than a tight 90 when space allows.

Wye fittings / branch entries

Use wyes or laterals rather than square tees for dust mains.

Blast gates / transitions

Each gate, reducer, hood neck, or abrupt transition adds equivalent length.

Duct material

Material changes friction loss and practical sealing expectations.

Collector rated airflow (CFM)

Use a measured or realistic rating when you have one.

Available static pressure (in. w.c.)

Enter the fan pressure available at the planned airflow from the collector curve.

Filter / separator allowance (in. w.c.)

Cyclones, dirty filters, and small bags can consume a large part of fan pressure.

0.136 Duct area ft²

4070 Actual velocity

55 Equivalent ft

3.8 Pressure need

Recommended Diameter 5.0 in duct at target velocity

Actual Velocity 4070 FPM in entered duct

Total Static Need 4.2 in. w.c. estimated

Collector Margin 2.8 in. w.c. remaining

Estimated Delivered Air 740 CFM after duct losses

Branch Verdict Good velocity and pressure check

Calculation Breakdown

Area and velocity5.0 in duct has 0.136 ft² area; 550 CFM gives 4070 FPM.

Diameter solveRequired diameter from CFM and target velocity is 5.0 in.

Fitting equivalent lengthElbows, wye, gate, flex, and pipe total 55 equivalent ft.

Static pressure estimateDuct, hood, and filter losses total 4.2 in. w.c.

Collector comparisonAvailable static pressure leaves 2.8 in. w.c. margin.

Capacity checkEstimated delivered airflow exceeds the target by 190 CFM.

This branch is close to the usual chip conveying range. Keep flex short, seal joints, and check the collector fan curve when possible.

🧰 Duct Material And Spec Comparison

1.00 Spiral metal loss factor

1.12 Snap-lock pipe factor

1.08 Smooth PVC factor

2.60 Ribbed flex factor

📊 Diameter, CFM, And Velocity Reference

Duct Inside Diameter	CFM At 3500 FPM	CFM At 4000 FPM	Common Shop Use
4 in round duct	305 CFM	349 CFM	Router table, bandsaw, small hood
5 in round duct	477 CFM	545 CFM	Table saw, jointer, upgraded branch
6 in round duct	687 CFM	785 CFM	Planer, drum sander, main branch
7 in round duct	935 CFM	1069 CFM	Large sander or short main run
8 in round duct	1222 CFM	1396 CFM	Main trunk or high-CFM machine

🔀 Fitting Equivalent Length Reference

Fitting Or Run Item	Calculator Allowance	Why It Matters	Better Shop Choice
Long-radius 90 elbow	8 duct diameters	Turns add turbulence and pressure loss	Use a large radius or two 45 elbows
45 elbow	4 duct diameters	Gentler turn with lower separation	Use smooth metal elbows where possible
Wye branch entry	5 duct diameters	Branch angle affects main flow	Use wyes instead of square tees
Blast gate or transition	2 ft each	Edges and size changes disturb airflow	Use smooth reducers and seal leaks
Ribbed flex hose	Material factor plus length	Ribs increase friction sharply	Keep flex to the final machine connection

📋 Machine Airflow Starting Points

Machine	Typical Port	Starting CFM	Velocity Priority
Cabinet table saw	4 in to 5 in	400 to 550 CFM	Keep chips moving through lower cabinet
13 in planer	4 in to 6 in	600 to 800 CFM	High chip volume needs 4000 FPM
8 in jointer	5 in to 6 in	500 to 700 CFM	Short, smooth run helps heavy shavings
Drum sander	6 in or dual 4 in	700 to 1000 CFM	Fine dust needs high capture airflow
Miter saw hood	5 in to 6 in hood	600 to 900 CFM	Large hood capture can need more CFM

🧪 Duct Material Comparison Grid

Material	Loss Factor	Spec Strength	Design Note
Spiral metal duct	1.00	Smooth, rigid, easy to ground bond	Best baseline for permanent mains
Snap-lock metal pipe	1.12	Common shop pipe with seams	Seal longitudinal seams and joints
Smooth PVC / sewer pipe	1.08	Smooth interior and light weight	Support well and address static control
Short clear flex	1.85	Useful at moving machine ports	Use only the length needed for motion
Long ribbed flex	2.60	Convenient but high resistance	Replace long flex runs with smooth pipe

💡 Practical Duct Sizing Tips

Velocity tip: A larger duct can move more air with less loss, but it can also drop velocity below chip conveying speed if the collector cannot supply enough CFM.

Fitting tip: Long flex hose and tight elbows often cost more static pressure than the straight pipe. Shorten flex before upsizing every branch.

Safety note: Dust collector duct calculations are planning estimates, not a substitute for a measured fan curve, airflow test, code review, or combustible dust assessment. Always wear appropriate safety equipment. Never exceed machine, duct, filter, impeller, or collector ratings. Follow the collector manufacturer's instructions for grounding, spark control, filter service, and safe dust disposal.

To choose teh correct duct diameter for a dust collection system, a person must understand the effect that the duct diameter will have upon the ability of that system to effectively move the dust and chips. If a person choose a duct that is too small for the application, the chips will stall within that duct and will not be able to successfully travel through the system. Alternatively, if a person chooses a duct that is too large for that system, the velocity of the air within the system will drop to a velocity that will cause the dust to settle within that duct rather than continuing to the dust collection system.

Each of these factors will have an effect upon the frequency with which the system must be cleaned, as well as the amount of strength that is require of the motor within the system. The calculator utilizes several different inputs to determine the size of the duct that should be created for that system and the amount of air that should move through the system. The target airflow is one of the factors that will determine the amount of air that is to be moved through the system.

How to Choose the Right Duct Size for Dust Collection

The duct diameter will have an impact upon the velocity of the air that passes through the system, as well as the amount of air that is allowed to move. The conveying velocity will allow for the determination of the velocity at which the air will need to move to ensure that the material does not drop out of the system. The branch length and the length of the flex hose that is to be used in the system will have an impact upon the efficiency of the system if those parameters are incorrectly determined; the longer the hose or branch, the more friction will act upon the system.

Similarly, the number of fittings that the user will utilize in the duct will create turbulence in the system, and the calculator will account for that turbulence. Finally, the static pressure that is available from the dust collector will help to ensure that the fan has the correct amount of power to overcome the friction that will act upon the system. Based off the different calculations performed by the calculator, the system will have several different outputs that allow the user to understand the results of the calculations.

One output will be the recommended diameter for the duct system. The calculator can determine the actual velocity of the air within the system, as. The total static pressure that will be demanded of the collector will help to indicate whether that collector has enough capacity to handle the branch of the system that is being tested.

The margin for the system will allow the user to understand if there are any additional units of capacity that can be provided for other branches of the system or if the loading of the filters with dust will overwhelm the system. The calculator will determine the amount of airflow that will be delivered to each branch of the system. Finally, a “verdict” line will allow the user to understand if the system is workable as it is, or if any adjustments must be made to the various parameters of the system.

Many people will begin to design the system starting at the machine and moving towards the collector. However, the collector will have many branches of the system moving through it simultaneously, and the branch that may be acceptable for that machine by itself may not be able to handle the increase in efficiency if all of those other branches are also running at the same time. Thus, it may be beneficial for a person to utilize this calculator and test each branch individually to determine how much capacity remains in the collector.

As with any system using flex hose, it is important to account for the material loss factor. A short length of flex hose may be acceptable, but a longer length of flex hose may lead to a significant loss in static pressure within that system. Some of the common mistakes made by those who are designing a duct system include not appropriately calculating the velocity that will be created by the system, and the number of fittings that will be used.

Many individuals may set the diameter of the duct based upon the size of the port of the machine that is to be connected. However, selecting the diameter of the duct based solely upon the size of that machines port may not provide the necessary velocity to the airflow that is created by the machine. People will typically remember to account for elbows in the system, but may not account for the number of ports in the flex hose.

Similarly, an extra elbow or blast gate in the system will increase the chances of the system failing when the filters are loaded with too much dust. The reference tables that are included in the calculator will show the relationship between the diameter of the duct system, the velocity of the air that will pass through it, and the amount of airflow that will be created by the system. Additionally, the reference tables will include the starting values for CFM for the various machines that may be located in a shop.

Finally, many shops has complex systems due to the different types of machines that move in those shops and the machines that are added to those shops over time. Thus, using this calculator will help a person to determine if the proposed system will effectively move the chips and allow the system to clear those chips rather than simply moving the air throughout the shop and its corners. People would of found that an extra elbow or blast gate in the system will increase the chances of the system failing when the filters are loaded with to much dust.

The system should of been checked for its capacity. The calculation is more important then you think. The furnitures in the shop may also be affected.

Most people dont realize that a small mistake can cause alot of trouble. It is better to be careful than to have a system that dont work. You should check your diameter before you start.

The air flow is more important than the size of the machine. The diameter of the duct will be important. The collector will need to be strong enough.

The system will be tested by the technician. The air will be moved by the fan. The dust will be collected by the system.

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