Molecular Weight Calculator
Parse a chemical formula and calculate its molecular weight.
Parse any chemical formula and calculate its molecular weight by summing the standard atomic weights of every atom it contains, including formulas with parentheses.
How Molecular Weight Calculator Works
The calculator reads your formula character by character, recognizing element symbols (an uppercase letter, optionally followed by a lowercase letter) and any digit immediately after as that element's count in the formula — for example, "H2" means two hydrogen atoms.
Parenthesized groups, like (OH)2, are parsed as a nested unit first and then multiplied by the number following the closing parenthesis, so every atom inside is counted the right number of times.
Each element's count is multiplied by its standard atomic weight (using values from the periodic table) and all the results are summed to give the total molecular weight in grams per mole.
See It In Action
Who Uses Molecular Weight Calculator and Why
- Working out the molar mass of a compound needed to prepare a molarity solution in a chemistry lab.
- Checking a chemical formula's molecular weight for a stoichiometry homework problem.
- Verifying the molecular weight of a compound with a complex formula involving parentheses, like a hydrate or coordination compound.
- Quickly cross-checking a molecular weight value found in a textbook or safety data sheet.
Mistakes to Avoid
- Entering an element symbol with the wrong capitalization, like "CO" (carbon + oxygen) when "Co" (cobalt) was intended — chemical notation is case-sensitive and changes the entire meaning of the formula.
- Forgetting a subscript number for an element that has more than one atom, since a missing digit after a symbol is read as a count of exactly one.
- Mistyping a parenthesized group's closing multiplier, like writing Ca(OH)2 as Ca(OH) 2 with a space — formatting matters for the parser to correctly multiply everything inside the parentheses.
Tips for Best Results
- Double-check capitalization carefully for two-letter element symbols (like Na, Mg, Cl) versus separate one-letter symbols written together (like N + a, which isn't a valid combination) to avoid an unrecognized-symbol error.
- For compounds with repeated groups, like Ca(OH)2, use parentheses rather than typing the group out twice — the calculator correctly multiplies everything inside by the number following the closing parenthesis.
Fixing Common Problems
I got an error naming an unrecognized element symbol. — Check the capitalization and spelling of that symbol against the periodic table — a common cause is typing a two-letter symbol in the wrong case, like "na" instead of "Na," or "CL" instead of "Cl."
My molecular weight doesn't match a reference source. — Recount the atoms in your formula, especially inside any parentheses, and verify each element's count against the formula rather than the calculated total, since a single missed subscript changes the sum.
Terms Explained
Molecular weight (molar mass): The sum of the atomic weights of every atom in a chemical formula, expressed in grams per mole (g/mol).
Atomic weight: The standard average mass of one atom of an element, as listed on the periodic table, used as the building block for molecular weight calculations.