Molar mass is the mass of one mole of a substance, measured in grams per mole. A mole is exactly 6.02214076 × 10²³ particles, so the molar mass tells you how much that many particles weigh. It is found by adding up the standard atomic weight of every atom in the formula. Water is two hydrogens at 1.008 plus one oxygen at 15.999, giving 18.015 g/mol — so one mole of water weighs 18.015 grams.

How do I calculate molar mass by hand?

Count how many atoms of each element the formula contains, look up each element's standard atomic weight on the periodic table, multiply each weight by its count, and add the results. For calcium carbonate, CaCO₃: calcium is 40.078, carbon is 12.011, and three oxygens give 3 × 15.999 = 47.997. Adding those gives 100.086 g/mol. Watch brackets carefully — in Ca(OH)₂ the subscript multiplies both the oxygen and the hydrogen.

What is the difference between molar mass and molecular mass?

They are numerically identical but carry different units. Molecular mass is measured in unified atomic mass units (u) and describes a single molecule; molar mass is measured in grams per mole and describes a mole of them. Water has a molecular mass of 18.015 u and a molar mass of 18.015 g/mol. For ionic compounds the term formula mass is more accurate than molecular mass, since a crystal of sodium chloride has no discrete NaCl molecules — only a repeating lattice in a 1:1 ratio.

How do I convert grams to moles?

Divide the mass by the molar mass: n = m ÷ M. Ten grams of water divided by 18.015 g/mol gives 0.5551 moles. To go the other way, multiply moles by molar mass. And to reach the actual particle count, multiply moles by Avogadro's constant — those 0.5551 moles contain about 3.34 × 10²³ water molecules. The converter on this page does all three steps at once.

How do I handle hydrates like CuSO₄·5H₂O?

The dot means water of crystallisation is bound into the crystal structure, and those water molecules count fully towards the mass. Copper sulfate pentahydrate is the anhydrous salt at 159.602 g/mol plus five waters at 18.015 each, giving 249.677 g/mol in total. Ignoring the water understates the mass by more than a third, which is a common source of error when weighing out reagents. This calculator accepts a middle dot, a full stop or an asterisk as the separator.

Why does percent composition matter?

It tells you what fraction of a compound's mass each element contributes, which is how empirical formulas are determined from laboratory analysis and how fertiliser and supplement labels are worked out. It also exposes a gap between atom count and mass share. Water is two-thirds hydrogen by atom count but only 11.19 percent hydrogen by mass, because oxygen is roughly sixteen times heavier per atom. The percentages always sum to exactly 100.

Why are atomic weights not whole numbers?

Because most elements are a mixture of isotopes, and the standard atomic weight is the average across their natural abundances. Chlorine sits at 35.45 because natural chlorine is roughly three-quarters chlorine-35 and one-quarter chlorine-37. Carbon is 12.011 rather than exactly 12 because about 1.1 percent of natural carbon is carbon-13. For elements with no stable isotope, such as technetium or the superheavy elements, a conventional mass number for the most stable isotope is used instead.

What is Avogadro's constant?

It is the number of particles in one mole: exactly 6.02214076 × 10²³ since the 2019 redefinition of SI units, which fixed it as a defined constant rather than a measured one. The number was chosen so that one mole of carbon-12 weighs almost exactly 12 grams, preserving continuity with the older definition. Its practical value is that it bridges scales — it converts a mass you can weigh on a balance into a count of atoms you could never tally directly.

How do I write formulas this calculator understands?

Use standard notation. Element symbols take a capital first letter and a lowercase second if they have one, so Co is cobalt while CO is carbon monoxide — the capitalisation matters. Subscripts are plain numbers after the symbol or bracket. Round, square and curly brackets all work and can be nested. Hydrates use a middle dot, a full stop or an asterisk. If something cannot be read the calculator says exactly what went wrong rather than returning a silently wrong answer.

Molar Mass Calculator

Type a chemical formula and get its molar mass in grams per mole, along with a full breakdown of what that mass is made of. The calculator reads ordinary chemical notation, including brackets and hydrates — Ca(OH)₂, K₄[Fe(CN)₆] and CuSO₄·5H₂O all parse correctly — and tells you plainly what is wrong...

CHEMICAL FORMULA

Brackets and hydrates are understood — try Ca(OH)2, K4[Fe(CN)6] or CuSO4·5H2O.

COMMON COMPOUNDS

MOLAR MASS

18.015 g/mol

One mole of H2O weighs 18.015 grams and contains 6.022 × 10²³ formula units.

ELEMENT BREAKDOWN

ELEMENT

ATOMS

ATOMIC

MASS

% BY MASS

Z 8

15.999

88.81%

Z 1

1.008

2.016

11.19%

TOTAL

18.015

100.00%

MASS TO MOLES

grams

Amount of substance

0.55509 mol

Formula units

3.3428e+23

STEPS

Count the atoms: O×1, H×2

Multiply each by its standard atomic weight: 15.999×1 + 1.008×2

Add them: molar mass = 18.0150 g/mol

WHAT IT IS MADE OF

Atom count and mass share are different things. H2O has 3 atoms in all, but O alone accounts for 88.8% of the weight. Heavy elements dominate the mass even when only a few atoms are present, which is why the largest slice is rarely the most numerous atom.

THE MOLE, IN CONTEXT

Molar mass

18.015 g/mol

Mass of one unit

2.991e-23 g

Units in 1 gram

3.343e+22

Avogadro constant

6.02214076 × 10²³

IUPAC 2021 standard atomic weights · updates as you type

Created with❤️byeaglecalculator.com

HOW TO USE

1
Type a chemical formula into the box using ordinary notation — element symbols with a capital first letter, subscripts as plain numbers. H2O, C6H12O6 and NaCl all work as written.
2
Brackets and hydrates are understood. Ca(OH)2 multiplies the bracketed group, K4[Fe(CN)6] handles nesting, and CuSO4·5H2O treats the water of crystallisation as five separate water molecules. A middle dot, a full stop or an asterisk all work as the hydrate separator.
3
Read the molar mass in the black box, then the element breakdown beneath it — atom count, standard atomic weight, mass contributed and percentage by mass for each element, with the totals checked against 100 percent.
4
Use the mass-to-moles box to convert a real quantity. Enter a mass in grams and the calculator returns the amount in moles and the number of formula units, which is what most laboratory calculations need.

THE FORMULAS

Molar massM = Σ (atomic weight × count)

Moles from massn = m ÷ M

Mass from molesm = n × M

Number of particlesN = n × N_A

Percent composition% = (mass of element ÷ M) × 100

Avogadro constantN_A = 6.02214076 × 10²³ /mol

Mass of one unitM ÷ N_A grams

Unitsg/mol, numerically equal to u

FREQUENTLY ASKED QUESTIONS

RELATED CALCULATORS

pH Calculator

Calculate instantly →

Moles to Grams Calculator

Calculate instantly →

Molarity Calculator

Calculate instantly →

Dilution Calculator

Calculate instantly →

MORE CHEMISTRY CALCULATORS

Dilution Calculator

Calculate instantly →

Ideal Gas Law Calculator

Calculate instantly →

pH Calculator

Calculate instantly →

Molarity Calculator

Calculate instantly →

Moles to Grams Calculator

Calculate instantly →

Grams to Moles Calculator

Calculate instantly →

Was this calculator helpful?

Last updated: July 28, 2026 · IUPAC 2021 atomic weights · Eagle-eyed accuracy for every calculation.