Molecular Weight Calculator (Molar Mass)
Enter a chemical formula and get its molecular weight (molar mass), computed automatically from the atomic weight of every element it contains. Handles simple formulas like H2O, formulas with parentheses like Fe2(SO4)3, and hydrate notation like CuSO4·5H2O.
Atomic weights of common elements
Atomic weights for elements that commonly appear in chemical formulas (the calculator itself supports all 118 elements internally).
| Symbol | Element | Atomic weight |
|---|---|---|
| H | Hydrogen | 1.008 |
| He | Helium | 4.0026 |
| C | Carbon | 12.011 |
| N | Nitrogen | 14.007 |
| O | Oxygen | 15.999 |
| F | Fluorine | 18.998 |
| Ne | Neon | 20.18 |
| Na | Sodium | 22.99 |
| Mg | Magnesium | 24.305 |
| Al | Aluminum | 26.982 |
| Si | Silicon | 28.085 |
| P | Phosphorus | 30.974 |
| S | Sulfur | 32.06 |
| Cl | Chlorine | 35.45 |
| Ar | Argon | 39.948 |
| K | Potassium | 39.098 |
| Ca | Calcium | 40.078 |
| Fe | Iron | 55.845 |
| Cu | Copper | 63.546 |
| Zn | Zinc | 65.38 |
| Br | Bromine | 79.904 |
| Ag | Silver | 107.87 |
| I | Iodine | 126.9 |
| Ba | Barium | 137.33 |
| Au | Gold | 196.97 |
| Hg | Mercury | 200.59 |
| Pb | Lead | 207.2 |
What is a molar mass calculation?
Molar mass (molecular weight) is the sum of the atomic weights of every atom in a chemical formula, weighted by how many times each atom appears. It's expressed in g/mol, and that single number is what lets you convert freely between mass and the number of moles of a substance.
This tool computes the molar mass the moment you type in a formula, showing a breakdown of the atom count, atomic weight, and contribution for every element involved. It also handles formulas with nested parentheses, like Fe2(SO4)3, and hydrate notation, like CuSO4·5H2O.
How to use the molar mass calculator
- Enter a chemical formula Type in the formula of the substance you want to look up (e.g. H2O, NaCl, Fe2(SO4)3).
- Use "·" for hydrates For a hydrate, add "·" or "*" after the base compound, followed by the water coefficient (e.g. CuSO4·5H2O).
- Read the result The molar mass, along with each element's atom count, atomic weight, and contribution, updates instantly as you type.
- Check individual atomic weights Scroll down to the reference table to look up the atomic weight of any commonly used element.
Tips for getting more out of it
- Element symbols are one uppercase letter plus optional lowercase letters — note that capitalization changes meaning, e.g. "Co" (cobalt) versus "CO" (carbon monoxide).
- Parentheses can be nested as many times as needed (e.g. in Ca3(PO4)2, the number right after the parenthesis multiplies everything inside it).
- For a hydrate, write the base compound followed by "·" or "*", a coefficient, and the water formula (e.g. MgSO4·7H2O).
- If you omit a number after an element or group, it defaults to 1 (e.g. in NaCl, both Na and Cl have a count of 1).
When to use this calculator
Chemistry homework and exam prep
Check your answers on molar mass problems or practice working through formulas with parentheses and hydrates.
Planning reagent quantities
Use the molar mass as the basis for figuring out how much of a reagent you need to weigh out to reach a target number of moles.
Checking stoichiometric ratios
Quickly look up the molar mass of each species when converting the coefficients of a balanced equation into mass ratios.
Calculating hydrate masses
Get the precise molar mass of a hydrated reagent, such as copper(II) sulfate pentahydrate, including the water of crystallization.
Molar mass glossary
- Molar mass
- The mass of one mole of a substance, in g/mol. It's found by summing the atomic weight of each atom in the formula, multiplied by how many of that atom are present.
- Atomic weight
- The relative mass of a single atom of an element, expressed as the abundance-weighted average of the masses of its naturally occurring isotopes.
- Mole (mol)
- The SI base unit for amount of substance. Since the 2019 redefinition, it's been defined by fixing the Avogadro constant (about 6.022×10²³) as an exact value.
- Isotope
- Atoms of the same element (same number of protons) that differ in neutron count. Most elements have a non-integer atomic weight because several isotopes occur naturally in a fixed ratio.
- Hydrate
- A compound that incorporates water molecules (water of crystallization) into its crystal structure in a fixed ratio, written like CuSO4·5H2O with the water count set off by "·".
FAQ
Side Note — Why atomic weights aren't whole numbers
Carbon's atomic weight is 12.011, not a clean 12 — and the same is true for many elements. That's because most elements found in nature are mixtures of several isotopes (atoms with the same number of protons but a different number of neutrons), and the atomic weight is the abundance-weighted average of those isotopes' masses. Chlorine, for instance, is a mix of chlorine-35 (about 76% abundant) and chlorine-37 (about 24%), and that weighted average lands on the not-quite-round value of 35.45.
On the other hand, elements like fluorine (18.998) and sodium (22.990) sit very close to whole numbers — because in nature, those elements exist almost entirely as a single isotope, with essentially no mixing. The size of the fractional part in an atomic weight is actually a rough clue to how isotopically diverse an element is in nature.
The mole itself has an interesting history. It used to be defined as "the number of atoms in 12 grams of carbon-12," but the 2019 redefinition of the International System of Units (SI) fixed the Avogadro constant (about 6.022×10²³) itself as the defining value instead. That change made the mole's definition independent of any specific substance, including carbon-12.