The quantities enrichment and abundance are ofter confused. Whereas the abundance refers to a fraction of the number of atoms or atom percent, the enrichment refers to mass. The abundancies are given, for example, in the Karlsruhe Nuclide Chart and in Nucleonica.
Denoting the masses of uranium isotopes by m4 (mass U-234), m5 (mass U-235), and m8 (mass U-238), the enrichment of U-235 in a uranium sample is defined as:
x5 = m5 / mU = m5 / (m4 + m5 + m8)
For an interesting discussion on this see the Nucleonica forum post: