Molar Mass Calculator
Enter a formula → inspect composition → get molar mass. Fast, precise, and browser-only.
Molar Mass
74.092g/mol
Ca(OH)₂ · 5 atoms
Formula mass: 74.092 u
| Element | Atoms | Atomic mass | Mass contribution | Mass % |
|---|---|---|---|---|
| H | 2 | 1.008 | 2.016 | 2.72 |
| Ca | 1 | 40.078 | 40.078 | 54.09 |
| O | 2 | 15.999 | 31.998 | 43.19 |
Atomic weights: CIAAW 2021 dataset. Values are versioned in the calculation engine. Full precision internally; display rounded (mass 3 decimals, % largest-remainder → 100.00).
Have a sample mass in mind?Open the Moles Calculator to convert it to moles.
How it works
From formula to molar mass
Parse the formula
Split the formula into elements, numbers and groups.
Expand groups
(OH)2 → O2 H2; hydrate parts are added separately.
Calculate atomic contributions
Multiply each element’s atom count by its atomic weight.
Add and calculate
Add the contributions to get molar mass, then calculate each element’s mass percentage.
What is molar mass?
Molar mass is the mass of one mole of a substance, expressed in grams per mole (g/mol). For a molecular substance, its numerical value corresponds to the molecular mass expressed in atomic mass units (u).
Water has a molecular mass of 74.092 u and a molar mass of 74.092 g/mol.
Concepts
Read formulas like a chemist
Molar mass vs molecular mass
Molar mass is the mass of one mole of a substance, expressed in g/mol. Molecular mass is the mass of one molecule, expressed in u or Da. For molecular substances, their numerical values are equal. Ionic compounds are described using formula mass rather than molecular mass.
Watch the capitals
CO is carbon monoxide; Co is cobalt; lowercase co is invalid. One letter changes the entire substance; it is the classic chemistry input error.
Subscripts multiply
A subscript multiplies the element or group immediately before it. Ca(OH)₂ means Ca = 1, O = 2, H = 2. A middle dot marks hydrates: CuSO₄·5H₂O.
Examples
Worked examples
| Formula | Molar mass | Atoms | Demonstrates |
|---|---|---|---|
| H2O | 18.015 g/mol | H 2, O 1 | Basic formula |
| Ca(OH)2 | 74.092 g/mol | H 2, Ca 1, O 2 | Parentheses |
| CuSO4·5H2O | 249.677 g/mol | H 10, Cu 1, O 9, S 1 | Hydrate |
FAQ
Frequently asked questions
What is molar mass?
The mass of one mole of a substance, expressed in grams per mole (g/mol). One mole contains approximately 6.022 × 10²³ specified entities.
How do you calculate molar mass from a formula?
Count the atoms of each element, multiply each count by that element’s atomic weight, and add everything up. Example: Ca(OH)₂ = 2×1.008 + 40.078 + 2×15.999 = 74.092 g/mol.
What is the difference between molar mass and molecular mass?
They are numerically equal but different quantities: molecular mass is the mass of one molecule in atomic mass units (u/Da), while molar mass is the mass of one mole in g/mol. “Molecular weight” strictly means the dimensionless relative molecular mass, though many calculators use it to mean molecular mass.
How do I enter a chemical formula?
Type it as plain text with numbers for subscripts: H2O, C6H12O6, Ca(OH)2. Unicode subscripts like H₂O work too.
Does the calculator support parentheses and brackets?
Yes. Parentheses ( ), square brackets [ ] and braces can be nested, with a multiplier after each group, e.g. Al2(SO4)3, K4[Fe(CN)6].
How do I calculate the molar mass of a hydrate?
Enter the hydrate with a middle dot, period, or asterisk, e.g. CuSO4·5H2O. The calculator includes the water molecules in the total molar mass (249.677 g/mol).
Why is the formula case-sensitive?
Element symbols are case-sensitive by definition: Co is cobalt, while lowercase co matches no element and is rejected with a precise error.
Which atomic weights are used?
Atomic weights: CIAAW 2021 dataset. Values are versioned in the calculation engine.
Why might my result differ from another calculator?
Small differences (usually ±0.01 g/mol) come from different atomic-weight datasets or vintages, and from rounding: atomic weights shown here are rounded for display while the total is computed at full precision.
Can I enter a charged formula?
Yes. Charged formulas such as [Fe(CN)₆]³⁻ are supported. The charge has a negligible effect on the reported mass at the precision shown because the mass of an electron is extremely small compared with atomic masses.