One a.m.u. or one 'u' is equal to :

One a.m.u.(atomic mass unit) or one 'u' is $\frac{1}{12}$ of the mass of one carbon-$12$ atom. 
It is equal $1.6605389210\times 10^{-24}g$  or $1.6605389210\times 10^{-27}kg$.