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Router Bit Shank Size Guide: 1/4 vs 1/2 Inch, 6mm, 8mm and 12mm
Router bit shank size must match the collet in your router, router table or CNC spindle. Common sizes include 1/4 inch, 1/2 inch, 6 mm, 8 mm and 12 mm. These sizes are not interchangeable simply because they look close. The correct choice starts with machine compatibility, then considers cutter size, rigidity and the type of cut.
Quick answer
Use only the shank size that your collet is designed to hold. A 1/4-inch shank is not the same as 6 mm, and a 1/2-inch shank is not the same as 12 mm. Never force a bit into a collet or rely on partial tightening. If your machine supports more than one collet size, choose the cutter and shank combination appropriate for the profile, cut depth and setup.
1/4-inch router bit shanks
One-quarter-inch shanks are common on compact routers and smaller cutters. They can be practical for light trimming, shallow grooves, small edge profiles and other controlled cuts. Check the cutter diameter and cutting length rather than assuming every profile is suitable for a small router. Use multiple passes when the tool, material or cut depth calls for a lighter approach.
1/2-inch router bit shanks
One-half-inch shanks are used with routers and tables that accept a matching 1/2-inch collet. The larger shank can offer more rigidity for suitable cutters and demanding profiles, but it does not remove the need for correct speed, feed, workholding and pass depth. Large-diameter bits may require a router table and a controlled setup.
6 mm, 8 mm and 12 mm shanks
Metric shanks are common on many routers and CNC machines. Six-millimeter bits may serve lighter profiles, while 8 mm and 12 mm options are available for machines with matching collets and for a range of cutter sizes. Always confirm the exact unit marked for the bit and collet. Similar-looking imperial and metric dimensions are still different sizes.
1/4 inch vs 6 mm
A 1/4-inch shank measures 6.35 mm, so it is larger than a 6 mm shank. Substituting one for the other can prevent proper clamping. Use a genuine 1/4-inch collet for 1/4-inch bits and a 6 mm collet for 6 mm bits.
1/2 inch vs 12 mm
A 1/2-inch shank measures 12.7 mm, making it larger than a 12 mm shank. These sizes also require their own matching collets. Check the machine manual and the markings on the collet and cutter before installation.
How to choose the right shank size
- Confirm the router or spindle model and the exact collet sizes it supports.
- Select the cutter profile and dimensions required for the project.
- Check that the full shank, cutting diameter and cutting length suit the machine and setup.
- Insert and tighten the bit according to the machine and collet instructions.
- Make a test cut in matching scrap material before routing finished work.
Related router bit options
Compare the available sizes in Router Bits by Shank Size, or browse the full woodworking router bits collection by profile and application. For setup guidance, see the router bit speed chart and router bit types and uses guide.
Router bit shank size FAQ
Can I use a 6 mm bit in a 1/4-inch collet?
No. The diameters are different. Use the exact collet size specified for the shank.
Is a 1/2-inch shank always better?
No. The correct choice depends on the router, collet, cutter profile and task. A larger shank is useful only when the machine and cutter are designed for it.
Where is the shank size marked?
Check the cutter specification, packaging and collet markings. If the size cannot be confirmed, do not install the bit.
