What Size Collet Does My Router Take?
The fastest way to find out is to look up your model in our brand-by-model compatibility tables. But if your router isn’t listed or you want to check for yourself, here are four reliable methods.
Method 1: Check the Specifications
Every router lists its collet size in the technical specifications — usually on the box, in the manual, and on the manufacturer’s website. Search for your model number followed by “specifications” or “data sheet”. Retailers like Screwfix, Toolstation, and Amazon also list the collet size in the product details.
The spec will say something like “Collet capacity: 1/4″ and 1/2″” or “Collet size: 6mm, 8mm, 1/4″”. This tells you what’s included in the box — and often what additional collets are available as accessories.
Method 2: Measure the Collet Bore
Remove the collet from your router (unscrew the collet nut and the collet will come out with it, or drop out separately depending on the design). Measure the internal diameter of the collet when it’s in its relaxed, open state:
| Measurement | Collet Size |
|---|---|
| ~6.0mm | 6mm (metric) |
| ~6.3-6.4mm | 1/4″ (6.35mm) |
| ~8.0mm | 8mm (metric) |
| ~9.5mm | 3/8″ (9.53mm) |
| ~12.0mm | 12mm (metric) |
| ~12.7mm | 1/2″ (12.7mm) |
Digital callipers are ideal for this — you can pick up a decent pair for under £15. A ruler will get you close enough to tell the difference between 1/4″ and 1/2″, but won’t reliably distinguish between 6mm and 1/4″ (only 0.35mm apart) or between 8mm and 1/4″ (1.65mm apart).
Method 3: Try a Known Bit
The simplest practical test: grab a router bit you already know the shank size of and try it in the collet. If you’ve got any standard UK-bought router bits, they’re almost certainly 1/4″ shank (that’s what most starter sets are). Slide it into the collet without tightening:
Fits snugly? You’ve got a 1/4″ collet.
Rattles around with lots of space? You’ve probably got a 1/2″ collet (or you’ve left the 1/2″ collet in and need to swap to the 1/4″ one that came in the box).
Slightly too tight or won’t go in? You might have an 8mm collet — common on Festool and some older European routers.
Method 4: Check the Collet Markings
Many collets have the size stamped or engraved on them — though it can be hard to read if the collet is old and gunked up with resin. Clean the collet with white spirit and look for markings. You might see “1/4”, “1/2”, “8”, “6.35”, or similar. Trend collets are particularly well-marked. Some budget router collets have no markings at all, in which case you’re back to measuring.
Quick Guide by Router Type
If you’re not sure of your exact model, the type of router gives you a strong clue:
| Router Type | Most Likely Collet Size | Notes |
|---|---|---|
| Palm router / trim router | 1/4″ only | Almost universal — very few exceptions |
| Cordless compact router | 1/4″ (+ sometimes 8mm) | DeWalt DCW604 and Milwaukee include 8mm |
| Small plunge router (under 1,000W) | 1/4″ only | Some include 8mm as well |
| Medium plunge router (1,000-1,500W) | 1/4″ + 1/2″ | Both usually included in box |
| Large plunge router (1,500W+) | 1/2″ + 1/4″ | Both usually included — 1/2″ is primary |
| Festool (any model) | Multiple options | Buy additional collet inserts as needed |
Need a Replacement Collet?
Once you know your size, here are the most popular replacement collets: