How do you calculate molar mass from a chemical formula?

Count how many atoms of each element the formula contains, multiply each count by that element's standard atomic mass, then add the products. For water, H₂O, that is 2 × 1.008 + 1 × 15.999 = 18.015 g/mol.

What units is molar mass in?

Molar mass is expressed in grams per mole (g/mol). Its numeric value matches the relative molecular (or formula) mass, which is dimensionless and reported in unified atomic mass units (u).

What is the difference between molar mass and molecular weight?

They are numerically equal but conceptually different. Molecular weight (relative molecular mass) is a dimensionless ratio of a molecule's mass to one twelfth of a carbon-12 atom. Molar mass is the mass per mole of substance and carries the unit g/mol.

How do I convert grams to moles?

Divide the mass in grams by the molar mass: moles = grams ÷ molar mass. For example, 90 g of glucose (180.156 g/mol) is 90 ÷ 180.156 ≈ 0.4996 mol. Use the converter in the tool above to do this instantly for any formula.

How do I handle parentheses and subscripts in a formula?

A subscript multiplies only the atom directly before it, so H₂O has two hydrogens and one oxygen. A number after a closing parenthesis multiplies the whole group, so Ca(OH)₂ means one calcium plus two oxygens and two hydrogens. The calculator parses both, plus nested brackets.

How do you find the molar mass of a hydrate like CuSO₄·5H₂O?

Add the molar mass of the anhydrous salt to the mass of the waters of crystallization. CuSO₄ is 159.602 g/mol and 5 H₂O adds 5 × 18.015 = 90.075 g/mol, giving 249.677 g/mol. Type the dot (·) and the calculator handles it.

What is percent composition by mass?

Percent composition is the share of a compound's total mass contributed by each element: (element mass ÷ molar mass) × 100. In water, oxygen is 15.999 ÷ 18.015 ≈ 88.8% and hydrogen the remaining 11.2%. The tool shows this as a bar and a table.

Chemistry

Molar Mass Calculator

Type any chemical formula — subscripts, parentheses, and hydrate dots included — to get its molar mass in g/mol, the per-element breakdown, percent composition, and an instant grams-to-moles converter.

Enter a chemical formula

g/mol

Subscripts as plain numbers · brackets ( ) [ ] · hydrate dot · supported

Convert with this molar mass

grams

= 0.5551 mol · 3.3427 × 10²³ molecules

Molar mass

180.156g/mol

3 elements · 24 atoms

Composition by mass

Per-element breakdown

Element	Count	Mass	%
O	6	95.994	53.28
C	6	72.066	40.00
H	12	12.096	6.71

Atomic weights are IUPAC standard values.

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The basics

What is molar mass?

Molar mass is the bridge between the atoms a chemist writes on paper and the grams they weigh on a balance. It is the mass of one mole — 6.022 × 10²³ particles — of a substance, and it equals the sum of the atomic masses of every atom in the formula. Two ideas sit underneath it.

M = Σ (count × atomic mass)

A sum of atomic masses

Every atom in the formula contributes its standard atomic weight. Add them all and you have the molar mass of the whole compound.

2×1.008 + 15.999 = 18.015 g/mol

1 mol = 6.022 × 10²³

Mass of one mole

A mole is a fixed count of particles. Molar mass tells you what that count weighs, so you can move between grams and moles.

18.015 g of water = 1 mole

Method

How to calculate molar mass by hand

Four steps turn any formula into a number — the same procedure the calculator runs for you.

1
Read the formula into atom counts
Note how many atoms of each element the formula has. A subscript multiplies the atom before it; a number after a bracket multiplies the whole group.
C6H12O6 → C:6, H:12, O:6
2
Look up each atomic mass
Find each element's standard atomic weight on the periodic table — the value printed under the symbol, in g/mol.
C 12.011 · H 1.008 · O 15.999
3
Multiply count × atomic mass
Multiply each element's atom count by its atomic mass to get that element's total contribution.
6×12.011, 12×1.008, 6×15.999
4
Add the contributions
Sum every element's contribution. The total is the molar mass, in grams per mole.
72.066 + 12.096 + 95.994 = 180.156

Worked examples

From salt to hydrate

The same add-it-up method scales from a two-atom salt to a bracketed compound to a hydrate — each figure below is computed live, not hardcoded.

Formula NaCl

One atom of each — just add the two atomic masses.

1 × Cl (35.45) = 35.45

1 × Na (22.99) = 22.99

Add: 35.45 + 22.99

M(NaCl) = 58.44 g/mol

Why it matters

What molar mass lets you do

Once you know a compound's molar mass, it becomes a conversion factor that powers most of quantitative chemistry.

n = m ÷ M

Grams to moles

Divide a measured mass by the molar mass to find how many moles you are holding — the starting point of nearly every calculation.

m = n × M

Moles to grams

Multiply a required number of moles by the molar mass to find what to weigh out on the balance.

N = n × Nₐ

Counting particles

Through moles, molar mass connects a weighable mass to the actual number of molecules or atoms present.

Quick chart

Molar mass of common compounds

The molar masses students look up most often, computed from the same atomic weights as the tool. Filter by name or formula.

18 shown

Compound	Formula	Molar mass
Water the reference solvent	H2O	18.015 g/mol
Sodium chloride table salt	NaCl	58.44 g/mol
Carbon dioxide exhaled gas, dry ice	CO2	44.009 g/mol
Oxygen gas the air we breathe	O2	31.998 g/mol
Glucose blood sugar	C6H12O6	180.156 g/mol
Sucrose table sugar	C12H22O11	342.297 g/mol
Sodium hydroxide lye, caustic soda	NaOH	39.997 g/mol
Hydrochloric acid stomach acid	HCl	36.458 g/mol
Sulfuric acid battery acid	H2SO4	98.072 g/mol
Ammonia cleaning agent	NH3	17.031 g/mol
Methane natural gas	CH4	16.043 g/mol
Ethanol drinking alcohol	C2H5OH	46.069 g/mol
Calcium carbonate chalk, limestone	CaCO3	100.086 g/mol
Calcium hydroxide slaked lime	Ca(OH)2	74.092 g/mol
Sodium bicarbonate baking soda	NaHCO3	84.006 g/mol
Potassium permanganate purple oxidizer	KMnO4	158.032 g/mol
Copper(II) sulfate pentahydrate blue crystals	CuSO4·5H2O	249.677 g/mol
Aspirin acetylsalicylic acid	C9H8O4	180.159 g/mol

Reference

Standard atomic masses

The building blocks — IUPAC standard atomic weights in g/mol. These are the numbers every molar mass is summed from.

HHydrogen

1.008

HeHelium

4.0026

LiLithium

6.94

BeBeryllium

9.0122

BBoron

10.81

CCarbon

12.011

NNitrogen

14.007

OOxygen

15.999

FFluorine

18.998

NeNeon

20.18

NaSodium

22.99

MgMagnesium

24.305

AlAluminium

26.982

SiSilicon

28.085

PPhosphorus

30.974

SSulfur

32.06

ClChlorine

35.45

ArArgon

39.948

KPotassium

39.098

CaCalcium

40.078

FeIron

55.845

CuCopper

63.546

ZnZinc

65.38

AgSilver

107.868

SnTin

118.71

IIodine

126.904

AuGold

196.967

HgMercury

200.592

PbLead

207.2

BrBromine

79.904

MnManganese

54.938

CrChromium

51.996

NiNickel

58.693

CoCobalt

58.933

Worth knowing

Rules and edge cases

A handful of conventions keep molar-mass work consistent and head off the most common mistakes.

g/mol = u

Two names, one number

The molar mass in g/mol matches the molecular weight in atomic mass units (u). They are numerically identical; only the unit and meaning differ.

( )ₙ multiplies the group

Brackets distribute

A subscript outside a bracket applies to every atom inside it. Miss it and you undercount the whole group — the classic exam slip.

·n H₂O

Hydrates count their water

Waters of crystallization are part of the formula mass. A hydrate weighs more than its anhydrous salt, so the dot matters.

ions ≈ atoms

Electron mass is negligible

The mass an electron adds or removes when an atom becomes an ion is so tiny that ionic and atomic molar masses are treated as equal.

averaged isotopes

Atomic weights are averages

Standard atomic weights already blend an element's natural isotopes by abundance, so you never add isotopes separately for ordinary work.

C₆H₁₂O₆ = (CH₂O)₆

Molecular, not empirical

Molar mass uses the actual molecular formula. Glucose and formaldehyde share an empirical formula but have very different molar masses.

Glossary

Mass terms, untangled

Several closely related terms describe almost the same number. Here is how they differ.

Molar mass

The mass of one mole of a substance. The only one of these terms that carries a unit of grams per mole.

mass of one mole · g/mol

Molecular weight

A dimensionless ratio of a molecule's mass to a twelfth of a carbon-12 atom. Numerically equal to molar mass.

relative · dimensionless

Formula mass

Used for ionic compounds like NaCl that have no discrete molecules — the mass of one formula unit instead of one molecule.

for ionic compounds

Atomic mass

The weighted-average mass of a single element's atoms. Molar mass is the sum of these across a formula.

one element · from the table

FAQ

Common questions

Molar mass is the mass of one mole of a substance — one mole being 6.022 × 10²³ particles (Avogadro's number). It equals the sum of the standard atomic masses of every atom in the chemical formula, and it is expressed in grams per mole (g/mol).

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