PeptideCalculator.Net is an online peptide and PNA molecular weight calculator.  It can calculate the average or monoisotopic molecular weights of peptides including phosphorylated peptides.  One can select from a variety of N-terminal modifications like Biotin, 5-FAM etc. and optional linkers like aminobutyric acid, aminohexanoic acid etc.  The calculator displays the length of the peptide in addition to the overall charge on the peptide.  The peptide charge is color coded displaying green for neutral peptides, red for acidic peptides and blue for basic peptides.  This is extremely useful in predicting the solubility of peptides.  It is also capable of calculating the molecular weights of cyclic, lactam bridged and hydrocarbon stapled peptides. The PNA calculator calculates the molecular weights of peptide nucleic acids.