In chemistry, you often need to know the molar mass of the substance for calculations.
There is nothing hard calculating it - you just take the standard atomic mass for all elements in the compound, multiply it by the number of atoms of the corresponding element in compound and multiply it by 1 gram/mol, the molar mass constant, and that is to obtain the corresponding dimension (gram/mole), as the molar mass is the mass of 1 mole of the substance. Standard atomic units for elements can be found in the Periodic table handbook.
To use the calculator you have to input the formula of the compound and get your result by clicking "Calculate" button. Please comply with the standard notation, for example,
C2H5OH, H2O, Na2SO4. Besides Na2SO4 has to be written properly, not like that - na2so4, to prevent ambiguity in parsing elements of the substance.