has anyone done this before? someone fancy saving us many hours working out a detailed route?

I'm assuming this is a touring question

Maybe it isn't a touring question, and the OP wants to do a Diagonale, that is a ride across France from one corner to another, for example Dunquerqe-Menton, Brest-Menton, Strasbourge-Hendaye. The route from Calais to Nice almost corresponds to the Dunquerqe-Menton route. I think the link is https://diagonales.homelinux.net/adf/?page=415&menu=4. There is an old VO blog about the diagonales, and Bicycle Quarterly had a few articles about them in the Spring 2009 issue.

Calais to Paris... straight south to Montpellier... follow the med to Nice

well, the OP never showed up again to tell us, I figured the tourists would know routes better anyway.

Get on your bicycle in Calais, and start riding. France is quite easy to get around ... pick up a map at your local bookstore and just follow the roads as you come to them. No complicated mapping of routes involved.

You can even follow some main rivers: Calais to Paris, follow the Seine... then the Saone to the Rhone to the Mediterraenean and on to Nice.... OR: down to the Loire, and across to the Rhone... it's all good!

FranceMap03..gif

I would go Calais to Paris to Orleans. Orleans is on the Loire River, So follow the Loire River Bike Path until you get to Nevers, then Chalon sur Saone, then down the Saone and Rhone rivers to the med. Then follow the coastline to Nice.

OP: why Calais? If coming from London I prefer crossing Newhaven-Dieppe which puts you closer to Paris (there's even a "avenue-verte" (MUP) from Dieppe which gets you 40 km towards Paris (eventually it's meant to go all the way)... just a thought

I got off a boat once in Calais and just started hitchhiking from there. I wound up in Mont St. Michel on an exquisitely dreary Englsigh Channel Octobter day, with a flask of Calvados.

Best tour ever, and I wasn't even on a bicycle!

Get the good michelin maps and keep off the big roads. Go for scenic and slow.