The formula
How it works
Work out the molecular weight of a compound from its chemical formula. Type a formula such as H2O or Ca(OH)2 — brackets and subscripts included — and the calculator sums the atomic weights of every atom.
FAQ
How do I type a formula?
Use standard notation: element symbols with a capital first letter, subscripts as plain numbers, and brackets for groups. So water is H2O, glucose is C6H12O6 and calcium hydroxide is Ca(OH)2. Case matters — "Co" is cobalt but "CO" is carbon monoxide.
What are the units?
The result is in grams per mole (g/mol), the standard unit for molar mass. It equals the mass in grams of one mole — about 6.022 × 10²³ molecules — of the substance, which is what you weigh out when preparing a solution to a set concentration.
What is the difference between molecular weight and formula weight?
They are calculated the same way — summing atomic weights — but "molecular weight" is typically used for molecules with true covalent bonds, while "formula weight" is the more precise term for ionic compounds that do not form discrete molecules, like table salt.
Can I use this for hydrates, like CuSO4·5H2O?
Enter the water molecules as a separate group with their own count, such as writing it as CuSO4(H2O)5, since the calculator does not currently parse the dot notation used for hydrates.
Why do atomic weights have decimals instead of whole numbers?
Atomic weights are a weighted average of an element’s naturally occurring isotopes, each with a slightly different mass, so the result is rarely a round number — chlorine, for instance, averages to about 35.45.
How accurate is the result?
It uses standard atomic weights accurate to at least three or four significant figures, which is precise enough for virtually all classroom and lab calculations.
What if the calculator says my formula is invalid?
Check for a typo in an element symbol, an unmatched bracket, or a symbol the calculator does not recognize — every element must use its correct one- or two-letter symbol with the right capitalization.
About the molecular weight calculator
This calculator finds the molecular weight — also called the molar mass — of a chemical compound from its formula. It reads the formula atom by atom, including anything grouped inside brackets, multiplies each element by the standard atomic weight, and adds them all up. The result is the mass of one mole of the substance, a number chemists need constantly for weighing out reagents and mixing solutions.
How to use it
Type the chemical formula using normal notation: capitalised element symbols, subscript counts as ordinary numbers, and brackets for repeated groups. For example, water H2O gives about 18.02 g/mol, glucose C6H12O6 gives about 180.16 g/mol, and calcium hydroxide Ca(OH)2 gives about 74.09 g/mol. The answer updates as you type.
The formula
Molecular weight is the sum of the atomic weights of every atom in the molecule, , where is how many atoms of element the formula contains and is that element’s atomic weight. For water, that is g/mol. Brackets simply multiply the group inside them by their subscript before adding to the total.
Where it is used
Molecular weight is fundamental to chemistry. It converts between mass and moles, so it underpins preparing solutions to a target concentration, balancing the amounts of reactants, and calculating yields. Students meet it in every stoichiometry problem, while researchers, pharmacists and industrial chemists rely on it whenever a recipe is specified by moles rather than grams.