In some cases, the highest mass peak in a mass spectrum is not the molecular ion. For example, alcohols sometimes give a parent peak that has a molar mass of M-18. "Molar Mass Finder" provides a list of possible molecular ion molar masses from given fragment masses. You can then use the Fragment Finder or Formula Finder applications to find possible formulas for your compound.
The basic idea for this application is that:

There is a high probability that any mass lost from the molecular ion will appear as a fragment ion or will be observed as a loss from a fragment ion(1).

Therefore, to guess the molar mass of a compound we make a list of all the low mass fragment masses and the losses from all the fragments. We then add these masses, one at a time to the parent peak. We then apply bad and poor loss rules to all the possibilities to reject the masses that don't make chemical sense. This process gives a few possible molar masses.

Your parent peak mass is also tested for bad and poor losses to see if it is a good possibility for the molecular ion. The more fragments you enter, the better this guessing game will work!