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The standard atomic weight of a chemical element (symbol A_r°(E) for element "E") is the weighted arithmetic mean of the relative isotopic masses of all isotopes of that element weighted by each isotope's abundance on Earth. For example, isotope ⁶³Cu (A_r = 62.929) constitutes 69% of the copper on Earth, the rest being ⁶⁵Cu (A_r = 64.927), so Relative isotopic mass is dimensionless, and so is the weighted average. It can be converted into a measure of mass (with dimension M) by multiplying it with the atomic mass constant dalton.