Enter below a structural formula (like H2O, CaO2) and click the button.
Note: the formula must not contain other characters than the elements and numbers.
E.g. spaces, parentheses, hyphens or other types of interpunction will not be recognized.
However, elements may occur at more than one location (e.g. HOH works fine; same as H2O).
And of course an appropriate use of caps is required...
A list of elements (atom numbers 1-100) and their atom mass, you can find here.