Molar Mass Calculator
Type a chemical formula. You get the molar mass, how much each element contributes, and a grams ↔ moles converter.
Where the mass comes from
| Element | Atoms | Atomic mass | Subtotal (g/mol) | Mass % |
|---|---|---|---|---|
| Carbon (C) | 6 | 12.011 | 72.066 | 40.00% |
| Hydrogen (H) | 12 | 1.008 | 12.096 | 6.71% |
| Oxygen (O) | 6 | 15.999 | 95.994 | 53.28% |
| Total | 180.156 | 100% |
Grams ↔ moles
Stuck on the concept? Ask why
AI answers can be wrong. The calculators on this page are deterministic; trust them for numbers.
How to find molar mass by hand
- Count the atoms. A subscript applies to the symbol just before it. A subscript after a bracket multiplies everything inside. A number in front of a hydrate’s water multiplies the whole H₂O.
- Look up each atomic mass on the periodic table with atomic masses.
- Multiply and add. Atoms × atomic mass for each element, then sum. The unit is g/mol.
Example: calcium phosphate, Ca₃(PO₄)₂. Ca: 3 × 40.078 = 120.234. P: 2 × 30.974 = 61.948. O: 8 × 15.999 = 127.992. Total 310.174 g/mol.
Percent composition
The mass % column is each element’s share of the molar mass: subtotal ÷ total × 100. It is what you need for mass-percent questions and as the starting point for the reverse problem, finding a formula from percentages, which the empirical formula calculator solves.
Questions students ask
How do you calculate molar mass?
Multiply each element’s atomic mass by the number of its atoms in the formula, then add. For water, H₂O: 2 × 1.008 + 1 × 15.999 = 18.015 g/mol. Brackets multiply everything inside: Ca(OH)₂ has 1 Ca, 2 O and 2 H.
What is the difference between molar mass and molecular weight?
They are the same number with different units. Molecular weight (relative molecular mass) is unitless or in daltons (u) and describes one molecule; molar mass is in g/mol and describes one mole. H₂O: 18.015 u and 18.015 g/mol.
How do I convert grams to moles?
Divide the mass by the molar mass: moles = grams ÷ (g/mol). 36 g of water ÷ 18.015 g/mol = 2.00 mol. To go from moles to grams, multiply instead.
How do I enter hydrates and complex ions?
Use a dot, asterisk or · for water of crystallisation (CuSO4·5H2O or CuSO4*5H2O), round or square brackets for groups (K4[Fe(CN)6]), and a caret for charge (SO4^2-). Symbols are case-sensitive: Co is cobalt, CO is carbon monoxide.
Which atomic masses does the calculator use?
IUPAC standard atomic weights (conventional values such as H 1.008, C 12.011, O 15.999). Your textbook may round differently, so the last decimal can differ.