How do you convert grams to moles?

Divide the mass by the molar mass: n = m ÷ M. Ten grams of sodium chloride, molar mass 58.44 g/mol, is 10 ÷ 58.44 = 0.1711 moles. The molar mass is the conversion factor and it is different for every substance, so the first step is always to establish it — from a periodic table, or by typing the formula into the box above.

Why does the same mass give such different amounts?

Because a mole counts particles, and particles differ enormously in mass. Ten grams of hydrogen gas is 4.96 moles while ten grams of copper sulfate pentahydrate is only 0.040 moles — the same balance reading representing about 124 times as many particles in one case as the other. A mass on its own tells you nothing about how much substance you have until you divide by the molar mass.

How many particles are in my sample?

Multiply the amount in moles by Avogadro's constant: N = n × 6.02214076 × 10²³. Ten grams of sodium chloride, being 0.1711 moles, contains about 1.03 × 10²³ formula units. Note the wording — for an ionic compound these are formula units rather than molecules, because a crystal of sodium chloride contains no discrete NaCl molecules, only sodium and chloride ions in a repeating one-to-one lattice.

How precise is my answer?

Exactly as precise as your weighing. Because n = m ÷ M and the molar mass is effectively exact, the relative uncertainty passes straight through: a mass known to one part in a thousand gives an amount known to one part in a thousand. In absolute terms the uncertainty in moles is your balance tolerance divided by the molar mass. Weighing 5.000 g on a balance good to ±1 mg gives about 0.02 percent; weighing 5 mg on the same balance gives 20 percent.

What if I do not know the molar mass?

Type the chemical formula and the calculator adds up the standard atomic weights for you. If you do not know the formula either but you do know both the mass and the amount — perhaps from a titration or a gas volume — enter those two instead and the calculator returns the molar mass, which is often how an unknown compound is identified in the first place.

Do I use the hydrated or anhydrous molar mass?

Whichever matches what is actually on the balance. If you weighed copper sulfate pentahydrate then the water of crystallisation is part of the sample and the molar mass is 249.677 g/mol, not the 159.602 of the anhydrous salt. Using the anhydrous figure would overstate your amount by more than half. Check the label on the bottle, because both forms are commonly sold and they look similar.

What is the difference between this and moles to grams?

Only the direction, but they answer different practical questions. Grams to moles is analytical: you have weighed something and want to know how much substance it is, which is where stoichiometry begins. Moles to grams is preparative: you know how much substance a procedure calls for and need to know what to weigh out. Both use the same relationship, m = n × M, rearranged.

How many significant figures should I report?

No more than your least precise measurement supports, which is almost always the mass. A balance reading of 4.37 g carries three significant figures, so the amount should be reported to three as well, however many digits the calculator displays. Molar masses from a periodic table typically carry four or five, so they rarely limit the result. Reporting more digits than the weighing supports implies a precision you do not have.

Does this work for elements as well as compounds?

Yes. For an element you use its atomic weight as the molar mass, so 10 g of iron at 55.845 g/mol is 0.179 moles of iron atoms. Take care with elements that exist as molecules in their normal state, though. Oxygen gas is O₂ at 31.998 g/mol, not O at 15.999, and hydrogen, nitrogen and the halogens behave the same way — writing the wrong one doubles or halves your answer.

Grams to Moles Calculator

Convert a weighed mass into an amount of substance using n = m ÷ M. Type what you weighed and the formula of the substance, and the calculator works out the molar mass for you, then returns the amount in moles and the number of particles it contains. This is the direction laboratory work almost alwa...

n = m ÷ M

WHAT DID YOU WEIGH? — optional, fills the molar mass

molar mass 58.4400 g/mol — brackets and hydrates understood

Solving for amount in moles.

MASS YOU WEIGHED

MOLAR MASS — from the formula

g/mol

AMOUNT IN MOLES

solved: 0.171116 mol

WEIGHED SAMPLES

AMOUNT IN MOLES

0.171116 mol

IN MILLIMOLES

171.1157

PARTICLES

1.030e+23

MOLAR MASS

58.440 g/mol

Amount

0.171116 mol

In millimoles

171.1157 mmol

Mass

10.0000 g

In milligrams

10,000.00 mg

Molar mass

58.4400 g/mol

Particles

1.0305e+23

STEPS

You weighed 10.0000 g.

NaCl has a molar mass of 58.4400 g/mol — the mass of one mole.

n = m ÷ M = 10.0000 ÷ 58.4400 = 0.171116 mol

WHAT A WEIGHED MASS ACTUALLY TELLS YOU

A balance reading is not an amount of substance. Ten grams is ten grams, but how much substance that represents depends entirely on what you weighed — and the range is enormous. Ten grams of hydrogen holds around 124 times as many molecules as ten grams of copper sulfate pentahydrate. The molar mass is the only thing that turns a reading on a scale into a count.

typical of a laboratory balance

Your answer is only as good as the weighing. Dividing by the molar mass changes the units but not the relative error, so a mass known to one part in a thousand gives an amount known to one part in a thousand. That is why small samples need a better balance: weighing 5 mg on a scale accurate to 1 mg leaves a twenty percent uncertainty before any chemistry has happened.

Created with❤️byeaglecalculator.com

HOW TO USE

1
Type the formula of what you weighed 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 to enter a molar mass yourself.
2
Enter the mass your balance showed. Units run from micrograms to kilograms, and the amount comes back in moles along with the number of formula units it contains.
3
With no formula given, enter any two of mass, amount and molar mass and the calculator returns the third — including working backwards to identify a molar mass from a weighed sample of known amount.
4
Pick your balance class in the second panel to see how much of your answer is real. The uncertainty in moles is your weighing uncertainty divided by the molar mass, and the relative error is unchanged by the conversion.

THE FORMULAS

Grams to molesn = m ÷ M

Moles to gramsm = n × M

Molar massM = m ÷ n

Number of particlesN = n × Nᴀ

Avogadro constant6.02214076 × 10²³ /mol

Relative uncertaintyΔn/n = Δm/m

Uncertainty in molesΔn = Δm ÷ M

Unitsg ÷ (g/mol) = mol

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