Of course 25 GB is a nominal value.

How many real bytes has a BD of 25 GB and one of 50 GB?
A 25 GB DB-R has about 22.5 usefulll GB, But I need to be sure of it. So I have asked!

I don't know how much bytes has a 25 GB BD-R, but I am sure that 25 GB is a comercial and nominal value, not the real amount of bytes contained in the media.