What size router bit to use?

[jonnybee123]

Forgive me if this sounds like a daft question but I've just bought a new router and I'm thinking of routing grooves for shelves to go into for a wooden book case I'm making. The shelf is 18mm thick. Should I use an 18mm router bit or a 19mm bit? Just wondering if the 18 would be too snug a fit.

 

[BB3Lions]

Try 18, then if too small, try 19

 

[jonnybee123]

I havent bought the bit yet. Didn't really want to buy 2 as it'll be only a one off job.

 

[evangelion]

Is your router a 1/2" collet? Not sure I'd trust an 18mm cutter in a 1/4" collet
Does the fence have micro adjustment? You could tweak the groove with the micro adjust if a tad too tight.
I'd probably go with making a jig, batch out the cuts, use a 12mm cutter and strip of wood/packer to make up the difference to 18mm, or whatever the measurement needs to be.