Dust Collector CFM Calculator — Duct Velocity & Fan HP

Work the chain in order. First, find the hungriest single machine on the duct run — a table saw needs 350–450 CFM at its hood, a planer 400–800, a jointer 350–500. Second, add the hood entry and duct losses as static pressure, not extra CFM. Third, verify the duct diameter carries that CFM at 4,000–4,500 FPM transport velocity. A one-branch hobby shop with a 6-inch main therefore lands at a 750–900 CFM, 1.5–2 HP class collector; the classic 1,100 CFM 2-bag unit is a comfortable fit, while a 650 CFM 'shop dust collector' is a Shop-Vac upgrade, not a system.

Table saws land at 350–450 CFM through a standard under-blade shroud — the hood is close-coupled and the sawdust is fine. Planers are the hungriest common machine at 400–800 CFM because the cutterhead throws a dense, high-volume chip stream; the 800 end applies to wide (13-inch-plus) planers with an open hood. Jointers run 350–500, drum sanders 400–600 (they make the finest dust, so filtration quality matters as much as CFM), bandsaws 300–400, and router tables 300–450 depending on shroud tightness.

That band is the transport velocity for wood dust and chips. Below about 4,000 FPM the airstream no longer carries particles — they settle along the duct floor, eventually choking the run and firing like kindling if ever ignited. Above about 4,500 FPM every foot of duct adds friction the fan must pay for in horsepower and noise, with zero cleaning benefit. Velocity is pure arithmetic: FPM = CFM ÷ (π × d² ÷ 4 ÷ 144). A 4-inch duct at 350 CFM runs 4,010 FPM — perfect; the same 350 CFM in a 6-inch duct falls to 1,783 FPM and starts dumping chips on day one.

Through the air-power equation: Air HP = (CFM × static pressure in inches w.c.) ÷ 6,356. The 6,356 constant converts CFM-and-inches-w.c. into horsepower. Real brake horsepower divides by total fan efficiency — use 0.6 for small bag collectors and 0.65–0.7 for well-built cyclones. Example: 1,000 CFM against 8 in. w.c. gives 1,000 × 8 ÷ 6,356 = 1.26 air HP; ÷ 0.6 = 2.1 BHP. That is exactly why the classic '2 HP' rating pairs with 1,000-ish ducted CFM — and why any spec quoting 1,500+ CFM from 2 HP is reporting free-air flow, not flow against real resistance.

Match duct to the CFM the fan actually delivers, working backward from the velocity band: 4-inch duct carries 350–400 CFM in band, 5-inch carries 550–610, 6-inch carries 780–880, and 7-inch carries 1,060–1,200. The classic mistake is stepping up duct size 'to reduce friction' without checking velocity — a 6-inch main on a 650 CFM blower drops to ~2,100 FPM and dust settles. Rule of thumb: small-shop mains are 5–6 inch; keep branches one size under the main, and never let a 4-inch gate feed the full output of a big collector without a reducer strategy.

Air-to-cloth (A/C) is CFM per square foot of filter media. Wood-dust baghouses are rated around 5–6 : 1 — at higher ratios, dust embeds in the fabric, pressure drop spikes, and the fan starves. To calculate required cloth: CFM ÷ 6 = minimum sq ft. A 780 CFM system wants ~130 sq ft of cloth. Traditional 2-bag collectors carry only ~30–40 sq ft (they survive because most heavy chips drop in the bag bottom and only fines reach the fabric), which is precisely the case for upgrading to a ~1 : 1 canister cartridge — 75+ sq ft in the same footprint, lower pressure drop, finer capture.

Judge both by ducted CFM at a stated static pressure, never by the free-air headline. A cyclone trades ~1–2 in. w.c. of extra resistance for the benefit of dropping chips before air hits the filter, keeping pressure drop stable as the bin fills — so a cyclone's ducted CFM holds steadier over time. A bag collector starts cheaper but its resistance climbs as the bag loads, and ducted CFM falls with it. For the same 2 HP motor, expect a bag unit near 800–1,000 ducted CFM and a good cyclone near 900–1,100 with steadier numbers across a session.

Budget by component, then sum: hood entry 0.5–1.0 in. w.c. per branch; straight duct roughly 0.1 in. w.c. per 10 ft at band velocity in 6-inch pipe; each 90° elbow ~0.25–0.5; flex hose counts double its smooth-pipe length; filter 1.5–3 when clean (baghouse climbs toward 6 as it loads, canisters stay lower); cyclones add 1.5–2.5 of their own. A typical one-branch hobby system totals 6–8 in. w.c.; that total is the number the HP calculation consumes — guessing it low is how '2 HP' collectors disappoint.

In a well-built shop, no — blast gates exist so one branch feeds the fan at a time, and opening a second gate simply steals air from the first (both branches then fall out of the transport band, which is worse than either alone). Commercial shops with true simultaneous collection design each branch to stay in-band with all its gates open, which multiplies CFM demand fast and is why those shops run 5–10 HP systems. Small-shop strategy: one gate open, generous margins on the single branch, and a fine filter so the fan's CFM holds.

This page sizes the demand side: machine CFM, duct velocity, static budget, and the resulting HP. The full fan side — affinity laws, RPM-to-CFM conversion, reading a fan curve's operating point against a system curve — is on the fan performance calculator, which pairs with this page the way a spec sheet pairs with a shopping list: this page tells you what to buy it for, that one tells you how the fan will actually behave once it is installed.