Calculate the optimal duct diameter for your dust collection system. Proper sizing ensures adequate air velocity to transport chips and dust without settling.
Choosing the correct duct diameter is one of the most critical decisions when designing a dust collection system. Many woodworkers invest in a powerful dust collector only to be disappointed by poor performance, and the culprit is almost always improper ductwork. Undersized ducts restrict airflow, reducing your expensive collector to a fraction of its rated capacity. Oversized ducts waste money and can drop air velocity below the threshold needed to transport dust and chips.
The relationship between duct diameter and airflow follows the laws of fluid dynamics. When air moves through a smaller opening, it must travel faster to maintain the same volume flow rate. This principle is why a garden hose nozzle can spray water much farther than an open hose. In dust collection, we need to maintain a minimum air velocity of 3,500-4,500 feet per minute (FPM) to keep wood chips and shavings suspended and moving toward the collector. Fine dust actually requires higher velocities because it settles more readily than larger chips.
Static pressure is the resistance your duct system creates against airflow, measured in inches of water column (in. WC). Every component in your system adds resistance: each foot of duct, every elbow, every branch connection, and especially flex hose with its corrugated interior. Your dust collector has a rated static pressure capacity, typically 6-12 inches for single-stage collectors and 12-16 inches for cyclone systems. Exceed this capacity, and airflow drops dramatically. This calculator helps you balance duct diameter, system length, and component count to achieve optimal performance.
Your duct system should use 6-inch main trunk lines with 5-inch branches.
For wood chips and shavings, you need 3,500-4,500 FPM (feet per minute). For fine dust, 4,000+ FPM is recommended. Velocity below 3,500 FPM may allow material to settle in the ductwork, causing clogs. Excessively high velocity (above 5,000 FPM) creates noise, increases wear on duct joints, and wastes energy without improving collection efficiency.
The main trunk diameter depends on your CFM requirements and shop size. For small shops with systems under 600 CFM and 2-3 machines, 4-inch trunk can work but limits future expansion. For mid-size shops running 600-900 CFM with 3-4 machines, 5-inch trunk provides good airflow with reasonable cost. For larger shops with 900+ CFM, multiple machines, or long duct runs, 6-inch trunk is recommended. Larger trunk lines reduce static pressure loss per foot and allow for future expansion without replacing the main line. Many experienced woodworkers recommend installing 6-inch trunk even in smaller shops to avoid limitations later.
Both materials work well for dust collection, each with distinct advantages. PVC (Schedule 40 DWV pipe) is significantly cheaper, readily available at hardware stores, easy to cut and assemble with standard fittings, and has extremely smooth interior walls for minimal friction. Metal spiral duct is more durable, handles impacts better, provides natural grounding for static discharge, and looks more professional in exposed installations. The static electricity concern with PVC has been debated for decades, but documented fires are extremely rare in hobbyist shops. If using PVC, ground the system with bare copper wire running inside the duct and attached to fittings. For most home workshops, the decision often comes down to budget and aesthetics.
Limit flex hose to 6 feet or less per machine drop. The corrugated interior of flex hose creates 3-4 times more friction than smooth duct, meaning each foot of flex hose equals 2.5-4 feet of rigid duct in static pressure loss. Excessive flex hose is the most common cause of poor dust collection performance. Use flex hose only for the final connection between the branch line and machine dust port, allowing machines to be moved for maintenance. Replace cheap clear plastic hose with quality wire-reinforced flex hose that maintains its diameter under suction. Never use flex hose as a substitute for permanent ductwork in the main trunk or long branch runs.
Install blast gates at each branch line where it connects to the main trunk, or as close to the trunk as practical while remaining accessible. Blast gates allow you to close off unused branches, directing full suction to the machine currently in use. This is essential because dust collectors are sized to run one or two machines at a time, not the entire shop simultaneously. Self-cleaning blast gates with sliding aluminum blades are recommended over rotating-style gates because chips can jam rotating mechanisms. Position gates within easy reach, typically at chest height or using pull-chains for overhead installations. For benchtop tools, a gate at the tool itself is convenient.
Grounding is recommended as a safety precaution, though the actual risk of static discharge fires in hobbyist dust collection systems is very low. Air moving through ducts generates static electricity, which can theoretically ignite fine dust under specific conditions. For metal duct systems, grounding occurs naturally when the system is connected to a grounded dust collector. For PVC systems, run a bare copper wire (12-14 gauge) through the duct interior, securing it to fittings with sheet metal screws and connecting to an electrical ground. Some woodworkers wrap copper wire around the outside of PVC pipe instead, though interior placement is more effective. At minimum, ensure your dust collector itself is properly grounded through its electrical connection.
Branch lines should match or be one size smaller than the machine dust port, never larger. Most stationary woodworking machines have 4-inch dust ports, so 4-inch branch lines are standard. Some machines like planers and jointers benefit from the full 4-inch connection due to high chip volume. Smaller machines like router tables or bandsaws with 2.5-inch ports should use the same size branch line. When connecting to the main trunk, use wye fittings rather than tee fittings to reduce turbulence and static pressure loss. Enter the main trunk at 45 degrees in the direction of airflow when possible. Never reduce branch diameter to less than the tool port size, as this creates a restriction that hurts collection efficiency.
Several strategies reduce static pressure loss: First, increase duct diameter where possible, especially in the main trunk line. Doubling the diameter reduces friction loss by roughly 75%. Second, minimize flex hose and replace corrugated hose with smooth-bore flex where connections require flexibility. Third, reduce the number of elbows by planning straighter runs. Each 90-degree elbow equals 5-8 feet of straight duct in friction loss. Use two 45-degree elbows instead of one 90-degree when practical. Fourth, use long-radius elbows rather than short-radius or hard elbows. Fifth, enter branch lines into the trunk at 45-degree angles using wye fittings. Sixth, keep total duct runs as short as possible by positioning the collector centrally.
The most frequent mistakes include: using too much flex hose throughout the system instead of rigid duct, undersizing the main trunk to save money (4-inch when 6-inch is needed), failing to use blast gates which spreads suction across all branches, using hard 90-degree elbows instead of sweep elbows, placing the dust collector in a corner requiring long duct runs to every machine, ignoring static pressure calculations and overloading an undersized collector, connecting branch lines with tees instead of wyes, and running flex hose inside walls or ceilings where it cannot be maintained. Many woodworkers also underestimate how much CFM is lost through their duct system and wonder why their collector performs poorly despite its rated capacity.
Consider upgrading your ductwork when: you add a new high-CFM machine like a planer or large table saw, you experience persistent clogs or settling dust in horizontal runs, you upgrade to a more powerful dust collector and want to utilize its full capacity, calculated static pressure exceeds your collector rating, or when you expand your shop and need longer duct runs. Signs your current system is inadequate include visible dust at machine ports during operation, chips accumulating in elbows, the collector bag not fully inflating, and excessive fine dust in the shop air. Often the most cost-effective upgrade is replacing the main trunk with larger diameter duct while keeping existing branch lines, or eliminating flex hose from permanent runs.
Air velocity in dust collection is measured in feet per minute (FPM) and represents how fast air travels through your ductwork. This velocity is calculated by dividing the volumetric flow rate (CFM) by the cross-sectional area of the duct. For example, 800 CFM through a 6-inch duct produces approximately 4,074 FPM, while the same airflow through a 4-inch duct creates 9,167 FPM. Understanding this relationship is crucial because transport velocity, the minimum speed needed to keep particles suspended, varies by material type.
Wood chips and shavings require 3,500-4,000 FPM transport velocity. These larger, lighter particles travel easily in the airstream. Fine sawdust from sanding requires 4,000-4,500 FPM because smaller particles have more surface area relative to their mass and settle more readily. Heavy materials like metal chips or wet sawdust may require velocities above 4,500 FPM. When velocity drops below the transport threshold, material settles in horizontal runs, gradually building up until the duct clogs completely.
Static pressure is the resistance your duct system creates against airflow, measured in inches of water column (in. WC or "WG). Every component in your system adds to total static pressure: straight duct sections, elbows, reducers, branch entries, filters, and collection bins. Your dust collector has a maximum static pressure rating, typically shown on a performance curve alongside CFM output. As static pressure increases, CFM decreases.
A typical 2 HP dust collector might deliver 1,200 CFM at 0 inches static pressure but only 800 CFM at 6 inches static pressure. Add enough ductwork, and that same collector might push only 400 CFM. This is why calculating static pressure is essential before building your system. The calculator above estimates your total static pressure based on duct length, fittings, and flex hose. If the result approaches or exceeds your collector rating, you need to reduce resistance or upgrade to a more powerful collector.
The cross-sectional area of a duct is calculated using the formula: Area = pi times radius squared. For a 6-inch duct, the area is 0.196 square feet. Velocity equals CFM divided by area, so 800 CFM through 0.196 square feet equals 4,074 FPM. Here are the areas for common duct sizes:
When velocity exceeds 5,000 FPM, noise increases significantly, duct joints experience more stress, and energy consumption rises without improving collection. The sweet spot for most systems is 4,000-4,500 FPM in the main trunk. Branch lines can run slightly higher velocity since they carry less total air volume.
Selecting the right duct material affects installation cost, system performance, durability, and maintenance requirements. The three primary options for workshop dust collection are PVC pipe, metal spiral duct, and flexible hose. Each has specific applications where it excels and limitations that make it unsuitable for certain uses.
PVC sewer and drain pipe (Schedule 40 DWV) is the most popular choice for budget-conscious hobbyist dust collection systems. Available at every hardware store and home center, PVC costs roughly 30-50% less than equivalent metal duct. The smooth interior walls provide excellent airflow with minimal friction loss, actually outperforming spiral metal duct in this regard. Standard plumbing fittings allow connections without special tools, though you should use long-sweep elbows rather than short-radius fittings.
The primary concern with PVC is static electricity buildup. Moving air and dust particles generate static charge on the non-conductive plastic surface. While well-documented dust collection fires from static discharge are extremely rare in home shops, the theoretical risk exists. Ground PVC systems by running bare copper wire inside the duct, attaching it to each fitting with sheet metal screws and connecting to electrical ground. Some woodworkers use specialized static-dissipative PVC designed for dust collection applications, which costs more but eliminates grounding concerns.
PVC is best suited for permanent trunk lines and branch runs within walls, ceilings, or underground. It handles the occasional impact reasonably well but can crack from severe blows. UV exposure degrades PVC over time, so cover outdoor sections or use painted duct. Available in 4-inch and 6-inch diameters standard, with reducers and wyes available. Larger sizes require ordering from specialty suppliers.
Galvanized steel spiral duct is the professional standard for industrial dust collection. The helically wound seam creates a rigid, durable tube that handles abuse better than any alternative. Metal naturally conducts electricity, providing inherent grounding when connected to a grounded dust collector. The industrial appearance suits exposed installations where ductwork becomes a visual element of the shop.
Metal duct costs more than PVC, typically 40-60% higher for equivalent diameter. Installation requires sheet metal tools, pop rivets or sheet metal screws, and foil tape or mastic for airtight seams. The spiral seams create slightly more friction than smooth PVC, though the difference is minimal. Available in every diameter from 3 to 12 inches, metal duct accommodates any system size. HVAC supply houses and online retailers stock comprehensive fitting selections.
Metal duct works best for exposed runs where appearance matters, high-traffic areas subject to impacts, and systems requiring maximum durability. The material handles temperature extremes without degradation, making it suitable for unheated shops. Some woodworkers prefer metal for its familiar industrial look and the satisfaction of building a professional-grade system.
Flexible hose serves an essential but limited role in dust collection systems. The corrugated interior that allows flexibility also creates tremendous friction, generating 3-4 times the static pressure loss of smooth duct per foot. Clear plastic flex hose commonly sold at hardware stores is the worst performer, often collapsing under suction and kinking at bends. Quality wire-reinforced flex hose with fabric or rubber covering maintains shape better and lasts longer.
Use flex hose only for the final 3-6 feet connecting branch lines to machine dust ports. This allows machines to be rolled out for blade changes or maintenance without disconnecting rigid ductwork. Limit flex hose length aggressively; every additional foot hurts system performance. Replace damaged or crushed sections immediately, as restrictions multiply static pressure loss.
Specialty smooth-bore flex hose eliminates the corrugation problem but costs significantly more and offers less flexibility. This material works well for longer flexible connections where machine movement requires it. For stationary machines that never move, consider eliminating flex hose entirely with rigid drops and unions that allow disconnection for cleaning.
For most hobbyist shops, the practical approach combines materials: PVC for the main trunk and permanent branch lines where cost savings are significant, metal duct for exposed runs and areas subject to impact, and quality flex hose limited to short final connections. This hybrid approach balances performance, durability, and budget effectively. Whatever materials you choose, prioritize proper sizing and installation over material selection, as an undersized metal system will underperform a properly sized PVC installation every time.
Quality ductwork components make installation easier and improve long-term system performance. These recommended products represent proven options used by woodworkers building professional-grade dust collection systems.
Industrial-grade galvanized steel spiral duct, 5-foot sections. Ideal for main trunk lines requiring durability and professional appearance.
View on AmazonSchedule 40 sewer and drain pipe, 10-foot sections. Cost-effective for concealed installations within walls or above ceilings.
View on AmazonSelf-cleaning aluminum blast gate with smooth slide action. Easy installation with machine ports and branch lines.
View on AmazonHeavy-duty blast gate for main trunk isolation. Allows sectioning larger systems for maintenance or zone control.
View on AmazonLong-radius metal elbow reduces static pressure loss compared to short-radius fittings. Essential for direction changes.
View on AmazonProper branch entry fitting with 45-degree angle for smooth airflow. Superior to tee fittings for dust collection.
View on AmazonSmooth transition from 6-inch trunk to 4-inch branch lines. Tapered design minimizes turbulence and pressure loss.
View on AmazonHeavy-duty flexible hose with wire helix for crush resistance. Use for short machine connections only.
View on AmazonStainless steel clamps sized for 4-inch flex hose connections. Secure grip prevents disconnection under suction.
View on AmazonAs an Amazon Associate, WorkshopCalc earns from qualifying purchases. Product recommendations are based on performance and value for dust collection systems.
Before purchasing any ductwork, sketch your shop layout with all machine locations marked. Position the dust collector centrally to minimize the longest run to any machine. Identify the path for the main trunk line, typically overhead or along a wall. Plan branch drops to each machine, noting where blast gates will be placed. Calculate total duct length, count all fittings, and use the calculator above to verify your system will perform adequately.
Consider future expansion when planning. Running a 6-inch trunk line costs marginally more than 5-inch but provides significant capacity for adding machines later. Stubbing out capped branch lines for planned future machines costs little during initial installation but saves considerable work later.
Run the main trunk as straight as practical, using long-radius elbows for direction changes. Slope horizontal runs slightly toward the dust collector (1/4 inch per foot) to prevent moisture accumulation and encourage chip flow. Support duct every 4-6 feet with hangers that allow slight movement for thermal expansion. Metal duct expands and contracts with temperature changes; rigid mounting creates stress on joints.
Seal all joints with foil tape or duct mastic for metal systems, or PVC cement for plastic systems. Air leaks reduce suction at the intended collection points and can introduce dust into the shop. Test the completed system by holding a tissue at joints to detect leaks, then seal any problem areas.
Connect branch lines to the main trunk using wye fittings, entering at 45 degrees in the direction of airflow. This creates smooth airflow transition with minimal turbulence. Tee fittings work but create more resistance. Install blast gates at the branch entry point or within 2-3 feet. Keep branch runs as short and straight as possible.
For drops to benchtop machines, run the branch line down a wall and connect with flex hose at machine height. For machines requiring mobility, end the branch line above the machine area and drop flex hose from an overhead connection. Use the minimum flex hose length that allows necessary machine movement.
Metal duct systems ground automatically when connected to a grounded dust collector. Verify the collector outlet provides a ground path by checking continuity from the duct to the electrical ground. For PVC systems, thread bare copper wire (12-14 gauge) through the duct interior. Attach the wire to each fitting using sheet metal screws that penetrate to the interior. Connect the wire to electrical ground at the dust collector end.
After installation, test each branch by opening only that blast gate and verifying strong suction at the machine port. Use a piece of paper or plastic bag to confirm airflow. If suction seems weak at distant machines, check for leaks, verify all other blast gates are closed, and consider whether the collector is adequately sized for the system static pressure.
Monitor system performance over the first few weeks of use. Dust accumulation at elbows or low spots indicates velocity is too low. Excessive noise suggests velocity is too high or there are sharp transitions creating turbulence. Clean out any accumulated material and consider adjustments if problems persist.
Design your complete dust collection system with these complementary tools and guides.