How is Molecular Weight computed?

Molecular Weight is computed as the sum of the mass of each amino acid plus a water molecule.
The mass of the water molecule is 17.0073(OH) + 1.0079(H).
The default mass of each amino acid is using the average mass from table 2 of this website.

More advance users who would like to define the mass of each amino acid can do so via XML files.
For more details, please check out the examples given here.