Convert between the mass, molar mass, number of moles, and number of molecules of a substance. Enter any two values — typically a mass and a molar mass — and the calculator finds the rest using moles = mass ÷ molar mass and Avogadro's number.
The mole calculator is an essential chemistry tool that converts between the four quantities that describe an amount of a substance: its mass, its molar mass (molecular weight), the number of moles, and the number of molecules or atoms. Enter any two of these and the calculator instantly works out the others, using the fundamental relationship moles = mass ÷ molar mass together with Avogadro's number. Whether you are balancing a reaction, preparing a solution, or working through a stoichiometry problem, this tool takes the arithmetic off your hands.
The mole is the chemist's counting unit — the bridge between the mass of a substance that you can weigh on a balance and the number of particles it actually contains. Because atoms and molecules are far too small and too numerous to count directly, chemistry relies on the mole to translate between the laboratory scale (grams) and the atomic scale (individual particles). This calculator makes that translation effortless.
A mole (symbol: mol) is the SI unit for the amount of a substance. One mole contains exactly 6.02214076 × 10²³ elementary entities — atoms, molecules, ions, or whatever particle you are counting. This enormous number is called Avogadro's number (or Avogadro's constant), named after the Italian scientist Amedeo Avogadro. Just as "a dozen" always means 12, "a mole" always means about 602 sextillion particles.
The mole is defined so that one mole of a substance has a mass in grams equal to its molecular weight in atomic mass units. For example, one mole of carbon-12 weighs exactly 12 grams and contains 6.022 × 10²³ atoms. This clever definition is what links the weighable mass of a sample to the countable number of particles inside it — the whole point of the mole.
The core relationship this calculator uses is:
number of moles = mass ÷ molar mass
where mass is in grams and molar mass is in grams per mole (g/mol). Rearranged, the same equation gives:
mass = moles × molar mass
molar mass = mass ÷ moles
To convert between moles and the number of individual particles, the calculator multiplies by Avogadro's number:
number of molecules = moles × 6.02214076 × 10²³
Because these equations link all four quantities, knowing any two of them (as long as one lets you find the moles) is enough to determine the rest.
Suppose you have 10 g of hydrochloric acid (HCl), which has a molar mass of about 36.5 g/mol. To find the number of moles:
Now imagine you want to neutralize that acid with sodium hydroxide (NaOH), which has a molar mass of 40 g/mol. Because the reaction needs one mole of NaOH per mole of HCl, you need 0.274 mol of NaOH, which is 0.274 × 40 = 10.96 g of NaOH. The mole calculator lets you run each of these conversions in seconds.
From any two inputs, the calculator reports the complete picture of your sample:
To use the calculator you usually need the molar mass of your compound. The molar mass is the sum of the atomic masses of all the atoms in the formula, read from the periodic table. For water (H₂O), for instance, you add two hydrogen atoms (about 1.008 g/mol each) and one oxygen atom (about 16.00 g/mol): 2 × 1.008 + 16.00 = 18.016 g/mol. For table salt (NaCl), sodium (22.99) plus chlorine (35.45) gives 58.44 g/mol. Once you have the molar mass, enter it along with your mass or moles, and the calculator does the rest. Molar mass and molecular weight are numerically identical — "molecular weight" is the older term, while "molar mass" carries the explicit g/mol unit.
The tool is fully flexible. The most common use is entering a mass and a molar mass to find the moles and molecule count — for example, "how many moles are in 5 g of glucose?" But you can just as easily go the other way: enter the moles and molar mass to find the mass you need to weigh out, which is exactly what you do when preparing a solution of a given concentration. You can also enter a mass and moles to back out an unknown molar mass, a handy trick for identifying a compound. As long as the two values you provide let the number of moles be determined, the calculator fills in everything else.
Avogadro's number, 6.02214076 × 10²³, is one of the defining constants of modern science. Since the 2019 redefinition of the SI units, the mole is defined as exactly this many elementary entities, making Avogadro's number an exact value rather than a measured one. It is almost unimaginably large: a mole of water molecules, for example, is only about 18 grams — a small mouthful — yet contains more molecules than there are stars in the observable universe by a wide margin. This is why chemists count in moles rather than in individual particles: the numbers involved in even a tiny sample are astronomically large. The mole calculator handles these huge figures for you and presents the molecule count in clean scientific notation.
One of the most practical uses of the mole is preparing solutions of a known concentration. Molarity (M) is defined as the number of moles of solute per liter of solution, so molarity = moles ÷ volume in liters. To make a solution, you work backwards: decide the molarity and volume you want, multiply them to get the moles of solute needed, and then use this mole calculator to convert those moles into a mass you can actually weigh out. For example, to prepare 0.5 liters of a 2 M sodium chloride solution, you need 2 × 0.5 = 1 mole of NaCl; entering 1 mole and NaCl's molar mass of 58.44 g/mol tells you to weigh out 58.44 grams. This two-step routine — target concentration to moles, then moles to grams — is one of the most common tasks in any chemistry lab, and the mole is the essential link that makes it possible. The same logic underpins diluting stock solutions, mixing reagents in the right proportions, and scaling a recipe up or down. Whenever a procedure specifies a concentration but your balance measures mass, the mole is what bridges the gap, and this calculator does the conversion instantly.
Having a few molar masses on hand makes the calculator even faster to use. Here are some that come up constantly in chemistry class and the lab:
To find the molar mass of any compound yourself, simply add the atomic masses of every atom in its formula, using the values from the periodic table. Remember to multiply by the subscript for each element — glucose, for instance, has six carbons, twelve hydrogens, and six oxygens.
Let's trace a full conversion to see how the mole ties everything together. Suppose you dissolve 9 grams of water and want to know how many molecules that is. First, find the moles: 9 g ÷ 18.015 g/mol = 0.4996 mol — almost exactly half a mole. Then convert to molecules: 0.4996 mol × 6.02214076 × 10²³ ≈ 3.01 × 10²³ molecules. So a mere 9 grams of water — less than two teaspoons — contains over 300 sextillion molecules. Going further, because each water molecule contains two hydrogen atoms and one oxygen atom, that same sample holds about 6.02 × 10²³ hydrogen atoms (one full mole of them) and 3.01 × 10²³ oxygen atoms. This chain — grams to moles to molecules to individual atoms — is the backbone of quantitative chemistry, and every link in it runs through the mole. The calculator handles the grams-to-moles-to-molecules steps directly; for atom counts, just multiply the molecule count by the number of that atom in the formula.
Divide the mass of the substance (in grams) by its molar mass (in g/mol): moles = mass ÷ molar mass. For 10 g of HCl with a molar mass of 36.5 g/mol, that is 10 ÷ 36.5 ≈ 0.274 mol.
Avogadro's number is 6.02214076 × 10²³, the number of elementary entities (atoms, molecules, ions) in one mole. It is the conversion factor between moles and the actual number of particles.
Multiply the number of moles by Avogadro's number: molecules = moles × 6.02214076 × 10²³. So 0.274 mol contains about 1.65 × 10²³ molecules.
Molar mass is the mass of one mole of a substance, in g/mol. It is numerically the same as the molecular weight; "molecular weight" is the older name and is technically unitless, while "molar mass" explicitly carries the g/mol unit.
Multiply the moles by the molar mass: mass = moles × molar mass. Enter the moles and the molar mass in this calculator and it returns the mass to weigh out.
Yes. Divide the mass by the moles: molar mass = mass ÷ moles. Enter those two values and the calculator returns the molar mass, which can help identify an unknown compound.
This Mole Calculator is provided for educational and general informational purposes. It uses the standard mole relationships and the 2019 SI value of Avogadro's number; verify results independently for critical laboratory or academic work.