With wear levelling it usually takes a few GB off of user access to replace worn cells.
32GB = 30GB (2gb for cell replacement)
64GB = 60GB (4gb for cell replacement)
128GB = 120GB (8GB for cell replacement)
256GB = 250GB (6GB for cell replacement)
My X25-E 64GB read 59.6GB before I formatted it to install OS X.
After it showed 59.6GB (same amount, so 4GB approx is used for wear levelling).
After installing OS X I have this: 43.04GB available. OS X took around 10-15GB and a few extra's here and there (updates) took the rest.