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The molar mass constant converts relative atomic mass or relative molecular mass into molar mass. It is approximately $1\mathrm{g}\mathrm{mo}{\mathrm{l}}^{-1}$. Its SI numerical value is determined experimentally: the exact Avogadro constant multiplies an experimentally determined unified atomic mass unit in kilograms. Treating $M_{\mathrm{u}}$ as exactly $1\mathrm{g}\mathrm{mo}{\mathrm{l}}^{-1}$ is therefore an excellent elementary-calculation approximation, rather than a defining equality of the modern SI.^[ref]

The relative molecular mass is the dimensionless ratio of a specified mean molecule mass to the unified atomic mass unit, written $m_{\mathrm{u}}$ when treated as a physical quantity:Thus a relative molecular mass of approximately $18.015$ and a molar mass of approximately $18.015\mathrm{g}\mathrm{mo}{\mathrm{l}}^{-1}$ describe related quantities with different units. The conversion uses the molar mass constant, rather than identifying a dimensionless number with a mass per mole.