The calculation of the molecular weight of a signal in the mass spectrum is easy when you can see isotopes. But once you cannot see isotopes anymore it becomes a bit more tricky. Often you use deconvolution programs for this purpose, but when it is just for a quick check on the molecular weight of a protein, the calculator below might come in handy.

You just need a mass spectrum and two neighbouring m/z values which belong to the same protein or protein complex. Entering their m/z values gives you the molecular weight as well as the charge of the lower m/z value. From there you can assign the entire charge envelope in the ESI spectrum.

Remark: The calculator in its current form is for positively charged proteins. You can also calculate oligonucleotide spectra, yet then the calculated molecular weight will be slightly off by 2*1.00727*z as we subtract protons in negative ESI instead of adding them!