A dust collector that is too small leaves fine dust floating in your shop air — and one with undersized ducting never reaches its rated CFM at the tool. Enter your shop size and your hungriest tool below to find the collector size you actually need and whether your duct can keep up.
Your shop & tools
How the math works
Two separate airflows matter: the air your whole shop needs to turn over, and the air your hungriest tool needs at its port. The collector must satisfy the bigger of the two, plus a safety buffer:
Shop volume = length × width × ceiling height (cu ft)
Ambient CFM = volume × air changes per hour ÷ 60
Recommended collector CFM = max(tool CFM, ambient CFM) × 1.25 (rounded up to the nearest 50)
Then the duct has to move that air fast enough to carry chips without them settling in the pipe:
Duct area (sq ft) = π × (diameter ÷ 2)² ÷ 144
Duct velocity (FPM) = tool CFM ÷ duct area
Worked example: a 20 × 12 × 8 ft shop is 1,920 cu ft. At 8 air changes per hour the ambient need is 1,920 × 8 ÷ 60 = 256 CFM. A table saw needs 450 CFM, so the collector must deliver max(450, 256) × 1.25 = 563 CFM, rounded up to 600 CFM. With a 4″ duct (area 0.0873 sq ft), velocity = 450 ÷ 0.0873 = 5,155 FPM — comfortably above the 4,000 FPM needed to keep chips moving, so the duct is good.
CFM needed per tool
Use these typical ranges when sizing a collector or a branch duct for each machine:
| Tool | CFM range |
|---|---|
| Table saw | 350–450 |
| Thickness planer | 400–600 |
| Random orbit sander | 350–500 |
| Router | ~250 |
| Jointer | 350–450 |
| Bandsaw | 300–400 |
Tips before you buy
- Size for your hungriest tool, not your average one. A planer throws far more chips than a router — if the collector cannot feed the planer, it cannot feed the shop.
- Keep duct runs short and straight. Every 90° elbow costs roughly the same airflow as several feet of straight duct. Use 45° bends and sweep elbows where you can.
- Seal every joint. Leaks at blast gates and duct joints quietly steal CFM — foil tape is cheap insurance.
- Plan the whole shop layout first. Our guide to small garage workshop setup ideas shows how to place machines so duct runs stay short.
Frequently asked questions
What is the difference between CFM and static pressure?
CFM is how much air the collector moves; static pressure is how hard it can push that air through duct, bends and a filter. A collector with high CFM but low static pressure stalls on a long duct run. Check both numbers — CFM for volume, static pressure for the duct system it has to fight.
How many air changes per hour does a woodshop need?
6 to 10 air changes per hour is the usual target for a small hobby shop — 8 is a good default. Go higher if you sand a lot or work with MDF, which makes finer, more hazardous dust.
What duct size should I use for dust collection?
4″ duct is the minimum for most single-tool runs, and it works well up to about 450 CFM. For a planer or a long main run, step up to 6″ — the bigger area drops the velocity less than you would expect, and the lower resistance more than makes up for it.
Why is my dust collector underperforming?
The usual suspects, in order: a clogged or undersized filter, leaks at blast gates and joints, too many tight 90° elbows, a duct run that is too long or too small, and a collector rated at free-air CFM that was never going to deliver that number through real ducting. Fix the filter and the leaks first — they are the cheapest wins.