On their site, they say to size down one---I'm a 32 in regular, preshrunk jeans, but the fact that they're raw but sanforized is confusing the shit out of me.
To deal with the shrinkage, ...
They're sanforized, which means pre-shrunk, so even with a hot wash, they'll shrink like 1/2" in the waist, and legs. I had a pair for 2 years (UB101, size 33), and they measure exactly 33" around the waist. They hardly stretched either.
They only come in 1 length, so prepare to cuff, stack, or get them hemmed.
What you want to do for sizing, is to measure your current pair of jeans. Then match up that measurement with the actual waist measurement of the Unbrandeds to find your size.
You have a pair of tagged 32 jeans, right? Hypothetically, You measure them, and they actually measure 34". You would then go to http://www.rueandstate.com/products/unbranded-321-heavyweight-21-oz-straight-japanese-raw-denim-jeans
And look at their sizing chart. As you can see, size 33 has a measured waist of 34".
Therefore, you should order a size 33". (Or go down a size, and plan on brute forcing the waist)
(Measure and compare the thigh, knee, rise, leg opening as well)