Router Bit Sizes Explained

Posted by John Rennie on

When it comes to finding router bits in the UK, you’re often presented with a lot of jargon and measurements that may not make immediate sense. It’s not just the endless names (engraving router bits, straight flute router bits, router milling bits, spiral router bits, etc.), but it’s also the seemingly never-ending measurements. Even as a tradesperson, navigating the world of router cutters in the UK is by no means straightforward. It’s for this reason that we’ve put together this handy blog to help you sift through the almost unending selection of router bits to find the best one for your project. Discover more below.

Shank Diameter

First up is the shank diameter, which is measured across the centre of the bit’s shank from any point on the circumference. This is one of the most important aspects to consider when selecting your chosen router bit, and two of the most common sizes you’ll encounter on handheld routers are ½-inch and ¼-inch spiral up-cut router bits. Detailed below are some of the other common shank diameters available in the UK. 

If using a CNC router, then you will be able to get collets for any shank size. For CNC router cutters, most sizes above 1/8” (3.175mm) cutting diameter will be on the same size shank as the cutting diameter. For example, a 4mm cutter would be on a 4mm shank. For sizes 1/8” (3.175mm) and under, most cutting diameters would be on a 1/8” (3.175mm) shank. For instance, a 2mm cutting diameter two-flute up cut router would be on a 1/8” shank. We show the full dimensions and shank size in the description of every product.

¼” V-Groove Router Bits

As previously mentioned, ¼” router bits are one of the most common shank diameters in Britain. This is because the majority of light-duty routers are supplied with ¼” collets. 

½” Router Bits

Much like ¼” bits, router bits of ½” are some of the most common solutions on the British market. In fact, heavy-duty routers are predominantly supplied with ½” collets. 

3/8” Router Drill Bits

Unlike ¼” and ½” bits, 3/8” router bits have recently been deemed as historical sizes, meaning they’re being used much less frequently. 

6mm Router Bits (1/4” Shank Approx.) 

6mm bits are approximately a quarter of an inch; however, they are measured under European sizes, meaning the metric system is used. Most 6mm shank router cutters will not fit in a ¼” collet on handheld routers.

8mm Router Bits (5/16” Shank Approx.)

8mm shank router bits are one of the most popular European sizes; however, they’ve recently been introduced in the UK for use in smaller medium-duty routers. This is because they’re needed for cutting larger diameters safely – when using an 8mm shank, extra care needs to be taken. For instance, a suitable collet must be fitted, and all manufacturer’s recommendations need to be met regarding the depth of cut and cutting speed. 

12mm Router Bit (1/2” Shank Router Bits Approx.)

After 8mm bits, a router bit of 12mm is the most popular European size. 

3mm Router Bits (1/8” Shank Router Bits and Under)

When using a CNC router, you’ll require a smaller shank of 3mm (1/8”) or under. Router bits of this size can’t be used in conjunction with fixed or standard overhead routers. 

Overall Length

After the shank diameter, you need to consider the overall length of the router bit. This is measured from the top of the shank to the bottom of the cutting edges or guide. 

Cutting Diameter

Next comes the cutting diameter, which is the maximum width of the router cutter. As a result, this measurement is taken from the outermost point of the cutting edges and through the centre of the cutting bit. If the router bit doesn’t have a guide beneath its cutting edges, this measurement also indicates the maximum width of the path made by the bit in one pass.

Cutting Edge Length

Following this, the cutting-edge length is the maximum length of the full cutting edge of a router bit, which is measured from the cutting edge’s top to its bottom. 

It’s always wise to use a shorter cutter edge length as the final depth of cut. This is because, although long cutting edges are available, production applications dictate that the length should be restricted to as short as possible to help prevent deflection and vibration.

Despite this, there are instances in which over-length router bits will be required. In this case, you should cut in shallow steps until the full depth of the cut is reached. This will reduce any stress being placed on the cutter during use.

Generally speaking, the edge of the cutting edge should never exceed three times the diameter; however, the shorter, the better. 


Penultimately, you need to assess the radius of your router bit; for bits that produce a curved cut, the radius will be given. This radius determines the size of the curve and a larger radius results in a greater curve. 


Last but not least, you need to evaluate the angle of your router bit; for bits that produce an angled cut, the angle of the cutting edges is given in relation to the horizontal plane. Despite this, the effect of this angle will depend on the type of cutter. 

Pros and Cons of Different Router Bit Sizes

When selecting the right router bit for the job, it’s essential that you weigh up the pros and cons, as detailed below. 

½” and 8mm Bits

The obvious pro of ½” and 8mm bits is that larger shank diameters are inherently stronger. As a result, they’re not as vulnerable to side deflection. What’s more, a larger surface area provides the bit with a better grip in the collet. Therefore, they’re less likely to release in turning. Similarly, a thicker cutter shank results in more material being removed in one pass, which leads to a minimised risk of breaking.  

¼” Bits

The downfall of a ¼” shank is that it will be weaker and more vulnerable to bending and flexing stresses. This is especially true if it’s applied in a heavy-duty router.

The Verdict

Ultimately, you should always use the largest shank diameter possible for a specific cutter. Regardless of which shank sizes you choose, you should always keep your imperial bits separate from your metric shanks and ensure they’re properly marked. This is because they can be of similar sizes, which can result in confusion and improper use. 

Shop Router Bits at Rennie Tool Company

Whether you’re after a 5mm router bit or ¾” router bit (1/4” shank), Rennie Tool Company is here to help. We stock a variety of router bits to bulk up your toolbox and prepare you for all manner of projects. Browse our collection here


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