I use .011 set, but for example Nergal from Behemoth mixes .011 inch treble strings and .013 inch bass ones. What's the best strings diameter for C# tuning?

.011 should work fine. Have you taken your instruments scale length vs. what Nergal uses into consideration?

I keep my guitar tuned to C# and I use .011's. They're perfect for the job

If you normally use 10-46 strings, I recommend Ernie Ball's yellow-package slinkies that are 11-54.

The issue with saying "11s" is that the top string isn't the issue, it's the bottom. There are sets with a .011 on top with the a 48/50 on the bottom, and that's going to be too light, it's more for players who play a lot of chords and want a bit less tension on the first point of contact when they're strumming. You'll want at least a 52, if not a 54, for C# if you're used to a 46 for E.

I use 10-52 for Drop C and aside from some minor spring adjustment it's perfect. Tense enough to beat the shit out of when playing rhythm but light enough to really dig into those bends when soloing.

That aside, any gauge will work as long as you set up the guitar properly. Tony Iommi uses rather light strings for his low tuning, and Dave Mustaine uses 10-52 on E standard.

D'Addario makes a light top/heavy bottom set that I use (although I tune to C now, I used the same strings when I tuned to C#). They're 11-56, and I love them.

And somefella is right. Hell, stevie ray vaughn tuned only down to Eb, and he used 13s.

Oh, also wanted to say your scale length will play a big role in string size also. A set of 10s on a fender and on a gibson at the same tuning will feel completely different.

I'd say if you have a scale length of 25.5" then .52 - .11 should be fine.

I currently use Dunlop Heavy Core Heaviest tuned in C, but that thicker core really kills the natural distortion and it's way to heavy to vibrate properly, so I don't recommend those strings. Maybe their regular sets are better, but I can recommend Beefy Slinky if you like them thick.

I use the beefy slinky set for both C# and C. I believe the set is 11-54.

Tension feels just right around C# and a little loose for C but that's how I like it. Loose and sludgie.

Guess I gonna do the obnoxious "teach a man how to fish" thing and strongly advise you to learn to work with these.

Nice, thanks for the link. It would be interesting to play a bass with a .254 low string.