Molecular Weight Calculator

Molecular weight — more precisely called molar mass — is the mass of one mole (6.022 × 10²³ particles) of a substance, measured in grams per mole (g/mol). It's calculated by adding up the atomic weights of every atom in the molecule's formula: water (H2O) has two hydrogen atoms (about 1.008 g/mol each) and one oxygen atom (about 15.999 g/mol), giving a molar mass of about 18.015 g/mol. This calculator parses any valid chemical formula and performs exactly this sum automatically.

A subscript number after an element symbol tells you how many atoms of that element are present — H2O means 2 hydrogen atoms and 1 oxygen atom (a missing subscript always means 1). Parentheses group atoms that repeat together, with the subscript after the closing parenthesis multiplying everything inside: Ca(OH)2 means one calcium atom plus two of the (OH) group, for a total of 1 calcium, 2 oxygen, and 2 hydrogen atoms. This calculator accepts nested parentheses and brackets, so formulas like Mg3(PO4)2 are parsed correctly.

These three terms are often used interchangeably, but have slightly different traditional meanings. 'Molecular weight' technically applies to molecular compounds made of discrete molecules (like water or glucose). 'Formula weight' is the more general term, used for ionic compounds like sodium chloride (NaCl) that don't form true molecules but exist as repeating crystal lattices. 'Molar mass' is the modern, unit-explicit term (g/mol) that applies to both cases equally. In practice, all three describe the same underlying calculation: the sum of atomic weights in a formula.

Atomic weight isn't simply the number of protons and neutrons in an atom's nucleus, because most elements naturally occur as a mixture of isotopes — atoms of the same element with different numbers of neutrons. The standard atomic weight is a weighted average across all naturally occurring isotopes, based on how common each one is. Chlorine, for example, is a mix of about 76% chlorine-35 and 24% chlorine-37, giving it an average atomic weight of about 35.45 — a number that would never occur in any single atom, but accurately predicts the mass of a large real-world sample.

Every element beyond bismuth (element 83), along with technetium and promethium, has no stable isotopes at all — every version of the atom is radioactive and eventually decays. For these elements, chemists can't calculate a natural-abundance weighted average, so tables instead list the mass number of the longest-lived known isotope in square brackets, like technetium's [98] or plutonium's [244]. This calculator uses those bracketed values for its heavier, synthetic elements, though such elements essentially never appear in the kind of everyday molecular formulas this tool is designed for.