How do you calculate moles from mass?

Use the formula n = m / M, where n is moles, m is mass in grams, and M is molar mass in grams per mole (g/mol). For example, 36.03 g of water (molar mass 18.015 g/mol) gives 36.03 / 18.015 = 2.000 mol. The molar mass of an element or compound is found by summing the atomic masses from the periodic table for each atom in the formula.

How do you find the molar mass of a compound?

Add the atomic masses (from the periodic table) of every atom in the chemical formula. For water (H2O): 2 x 1.008 (hydrogen) + 1 x 15.999 (oxygen) = 18.015 g/mol. For sodium chloride (NaCl): 22.990 (Na) + 35.45 (Cl) = 58.44 g/mol. For glucose (C6H12O6): 6 x 12.011 + 12 x 1.008 + 6 x 15.999 = 180.156 g/mol.

How many molecules are in one mole?

One mole contains exactly 6.02214076 x 10^23 particles (atoms, molecules, or formula units). This is Avogadro's number (N_A). So if you have 2 moles of water, you have 2 x 6.022 x 10^23 = 1.204 x 10^24 water molecules. Multiply moles by Avogadro's number to find the number of particles.

Moles Calculator (Mass to Moles)

Convert mass to moles using the formula n = m / M, where n is moles, m is mass in grams, and M is molar mass in g/mol. You can also reverse-calculate mass from moles. Select a preset substance or enter any molar mass directly.

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Standard chemistry formula Molar masses sourced from IUPAC atomic weights (2021 table).

n = m / M | m = n x M | M = m / n

1. Substance

Preset substance (sets molar mass)

Molar mass (g/mol)

g/mol

2. Calculate

Mass (grams)

Results

Moles
1.0000
mol

Mass

18.015

grams

Molecules / Atoms

6.022

x 10²³ particles

Molar Mass Used

18.015

g/mol

Calculation Breakdown

ModeMass to Moles

Formulan = m / M

Mass (m)18.015 g

Molar mass (M)18.015 g/mol

Moles (n)1.0000 mol

Particle Count

Avogadro's number (N_A)6.02214 x 10²³

Moles calculated1.0000 mol

Particles (n x N_A)6.022 x 10²³

Mass per particle-

SubstanceWater (H2O)

Result: 18.015 g of water (H2O) contains 1.0000 mol.

How to Calculate Moles from Mass

The mole is the SI unit for amount of substance. The relationship between mass, moles, and molar mass is given by:

n = m / M

Where n is the number of moles (mol), m is the mass in grams (g), and M is the molar mass in grams per mole (g/mol). Rearranging gives you mass (m = n x M) or molar mass (M = m / n).

Worked Example

How many moles are in 18.015 g of water (H2O)?

Find the molar mass of water: H2O = (2 x 1.008) + (1 x 15.999) = 2.016 + 15.999 = 18.015 g/mol
Apply the formula: n = m / M = 18.015 / 18.015 = 1.0000 mol
Number of molecules: 1.0000 x 6.02214 x 10²³ = 6.022 x 10²³ molecules

This matches the calculator's default output exactly.

Finding Molar Mass from the Periodic Table

The molar mass of an element equals its relative atomic mass in g/mol. For a compound, add the molar masses of all atoms in the formula.

Substance	Formula	Molar Mass (g/mol)	Calculation
Water	H2O	18.015	2(1.008) + 15.999
Sodium chloride	NaCl	58.44	22.990 + 35.45
Carbon dioxide	CO2	44.01	12.011 + 2(15.999)
Glucose	C6H12O6	180.156	6(12.011) + 12(1.008) + 6(15.999)
Sulfuric acid	H2SO4	98.079	2(1.008) + 32.06 + 4(15.999)
Calcium carbonate	CaCO3	100.087	40.078 + 12.011 + 3(15.999)

Avogadro's Number and Particle Count

One mole of any substance contains exactly 6.02214076 x 10²³ particles (atoms, molecules, or formula units). This is Avogadro's number (N_A). To find the number of particles, multiply moles by N_A: particles = n x 6.02214076 x 10²³.

Common Conversions

Substance	Mass	Moles	Molecules
Water (H2O)	18.015 g	1.000 mol	6.022 x 10²³
Water (H2O)	9.008 g	0.500 mol	3.011 x 10²³
NaCl	58.44 g	1.000 mol	6.022 x 10²³
CO2	44.01 g	1.000 mol	6.022 x 10²³
Glucose (C6H12O6)	180.156 g	1.000 mol	6.022 x 10²³

Related Calculators

Science and Engineering Calculators: full hub for science tools.
Dilution Calculator (C1V1 = C2V2): find volumes and concentrations for solution dilutions.
Avogadro Number Calculator: calculate particles from moles using Avogadro's number.
Equilibrium Constant (Keq) Calculator: find Keq for chemical reactions.
Charles's Law Calculator: temperature and volume relationships for gases.

Sources and method: International Union of Pure and Applied Chemistry (IUPAC), 2021 atomic weights table (doi.org/10.1515/pac-2019-0603). IUPAC SI unit definitions, 2019 revision (mole defined as exactly 6.02214076 x 10²³ elementary entities). Formula n = m / M is standard stoichiometry as taught in NCEA Level 2 and 3 Chemistry.

This calculator is for educational use. Molar masses are rounded to three decimal places for most compounds. For precise laboratory work, use IUPAC standard atomic weights for your specific isotopic composition.

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