Unified atomic mass unit (amu) or (u) = $$\frac{1} {12}$$ the mass of one carbon $$(C^{12})$$ atom.
By Avogadro's number $$6.023 \times 10^{23}$$ we know that mass of $$6.023 \times 10^{23}$$ atoms of carbon $${_6}C^{12}$$ is equal to 12 gm.
So mass of one atom of $${_6}C^{12} = \frac {12} {6.023 \times 10^{23}} \space gm$$
$$1 \space amu = \frac{1} {12}$$ mass of one $${_6}C^{12} = \frac{1} {12} \times \frac {12} {6.023 \times 10^{23}} \space gm$$
$$1.66 \times 10^{-24} \space gm$$
$$1 \space amu = 1.66 \times 10^{-27} \space kg$$