How do you convert moles to grams?

Multiply the number of moles by the molar mass: m = n × M. One mole of water, with a molar mass of 18.015 g/mol, weighs 18.015 grams, and half a mole weighs 9.008 grams. The molar mass is the conversion factor, and it is different for every substance, which is why the first step is always to find it — either from a periodic table or, on this page, by typing the formula.

How do you convert grams to moles?

Divide the mass by the molar mass: n = m ÷ M. One hundred grams of glucose, molar mass 180.156 g/mol, is 100 ÷ 180.156 = 0.5551 moles. This is the direction used most often in the laboratory, because you weigh a sample and then need to know how much substance you actually have for the stoichiometry.

A mole is a count, in the same way that a dozen is a count. It is exactly 6.02214076 × 10²³ particles, a number fixed by definition since the 2019 revision of SI units. Chemists use it because reactions happen between individual particles in whole-number ratios, and a mole is a quantity large enough to be weighable while still counting particles exactly.

Why do equal moles have such different masses?

Because a mole counts particles and particles differ enormously in mass. Half a mole of hydrogen gas weighs about 1.01 g while half a mole of copper sulfate pentahydrate weighs 124.84 g — more than a hundred times as much — yet both contain the same 3.01 × 10²³ formula units. This is exactly why chemical equations are balanced in moles rather than grams: a reaction consumes particles, not weight.

How do I find the molar mass?

Add up the standard atomic weight of every atom in the formula. Water has two hydrogens at 1.008 and one oxygen at 15.999, giving 18.015 g/mol. Brackets multiply everything inside them, so Ca(OH)₂ counts two oxygens and two hydrogens. Typing the formula into the box on this page does the whole sum for you using the IUPAC 2021 atomic weights.

Do I include the water in a hydrate?

Yes. Water of crystallisation is part of the crystal and contributes fully to the mass you weigh out. Copper sulfate pentahydrate is 249.677 g/mol, made up of the anhydrous salt at 159.602 plus five waters at 18.015 each. Using the anhydrous figure by mistake understates the required mass by more than a third, which is one of the commonest reagent errors.

How many particles are in my sample?

Multiply the number of moles by Avogadro's constant: N = n × 6.02214076 × 10²³. A quarter of a mole contains about 1.51 × 10²³ particles. Going the other way, divide a particle count by Avogadro's constant to get moles. Note that for ionic compounds these are formula units rather than molecules, since a crystal of sodium chloride contains no discrete NaCl molecules.

What is the difference between molar mass and atomic mass?

Atomic mass describes a single atom and is measured in unified atomic mass units; molar mass describes a mole of them and is measured in grams per mole. The numbers are identical, which is not a coincidence — the unit system was built that way so a single carbon-12 atom is 12 u and a mole of them weighs 12 grams. That deliberate correspondence is what makes the conversion so straightforward.

Why does my answer differ slightly from a textbook?

Almost always because of the atomic weights used. This calculator uses the abridged IUPAC 2021 values, so oxygen is 15.999 rather than 15.9994 and water comes to 18.015 rather than 18.0153. The difference is under 0.02 percent and never matters for laboratory work, but it can shift the last decimal place. Some elements also have a published range rather than a single value, because their isotopic composition varies with source.

Moles to Grams Calculator

Convert between moles and grams using m = n × M, either by typing a chemical formula and letting the calculator work out the molar mass, or by entering the molar mass yourself. Enter any two of mass, amount and molar mass and it returns the third, along with the number of formula units that amount c...

m = n × M

CHEMICAL FORMULA — optional, fills the molar mass

molar mass 18.0150 g/mol — brackets and hydrates understood

Solving for mass.

AMOUNT IN MOLES

MASS

solved: 18.0150 g

MOLAR MASS — from the formula

g/mol

COMMON COMPOUNDS

MASS

18.0150 g

MASS

18.015 g

AMOUNT

1.000 mol

PARTICLES

6.022e+23

Mass

18.0150 g

In milligrams

18,015.00 mg

Amount

1.000000 mol

In millimoles

1,000.0000 mmol

Molar mass

18.0150 g/mol

Mass of one unit

2.991e-23 g

STEPS

H2O has a molar mass of 18.0150 g/mol — the mass of one mole.

Multiply by the number of moles: m = n × M.

m = 1.000000 × 18.0150 = 18.0150 g

WHAT A MOLE ACTUALLY DOES

The mole is a counting unit, not a mass. A balance measures grams, but reactions happen between individual particles, and those two things are only connected through the molar mass. Dividing a weighed mass by the molar mass converts something you can measure into a count of particles you never could.

Equal moles are equal counts, never equal masses. Every bar above holds exactly the same number of formula units, yet the heaviest weighs more than a hundred times the lightest. That is precisely why chemical equations are balanced in moles rather than grams — a reaction consumes particles, and it has no way of knowing what they weigh.

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HOW TO USE

1
Type a chemical formula into the top box and the molar mass fills in automatically. Brackets and hydrates are understood — try Ca(OH)2, K4[Fe(CN)6] or CuSO4·5H2O. Leave it blank if you would rather enter the molar mass yourself.
2
Enter the amount in moles to get the mass, or the mass to get the amount. Units run from micrograms to kilograms and from nanomoles to moles, and everything converts before the arithmetic runs.
3
With no formula given, enter any two of mass, amount and molar mass and the calculator returns the third — including working backwards to find a molar mass from a weighed sample.
4
Look at the comparison on the right. It shows what your number of moles weighs for eight different substances, all containing exactly the same number of particles.

THE FORMULAS

Moles to gramsm = n × M

Grams to molesn = m ÷ M

Molar massM = m ÷ n

Number of particlesN = n × Nᴀ

Particles to molesn = N ÷ Nᴀ

Avogadro constant6.02214076 × 10²³ /mol

Mass of one unitM ÷ Nᴀ grams

Unitsg, mol and g/mol

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Last updated: July 29, 2026 · IUPAC 2021 atomic weights · Eagle-eyed accuracy for every calculation.