Stoichiometry Calculator (Moles, Mass & Particles)
Convert between the mass, the amount in moles and the number of particles for a single substance. Enter a mass and its molar mass and the calculator returns the number of moles and the exact particle count using Avogadro's number.
Enter Values
Minimum 0
How to use this calculator
- Enter the mass of your sample in grams.
- Enter the molar mass of the substance in grams per mole (from the periodic table or a molar-mass lookup).
- Read off the amount in moles and the number of particles (atoms, molecules or formula units).
How it works
The amount of substance in moles equals the mass divided by the molar mass: n = m / M. Each mole contains Avogadro's number of particles, 6.022 x 10^23, so the total particle count is n x 6.022e23. For a chemical reaction you then multiply the moles by the mole ratio taken from the balanced equation to find the amount of any other reactant or product.
Worked example
Worked example. For 18 g of water (molar mass 18 g/mol), moles = 18 / 18 = 1 mol, which contains 1 x 6.022e23 = 6.022e+23 molecules. Halving the sample to 9 g gives 0.5 mol and 3.011e+23 molecules.
Common mistakes
- Using the wrong molar mass — always sum the atomic masses of every atom in the formula (e.g. water H2O is 2x1.008 + 16.00 = 18.02 g/mol).
- Forgetting units; mass must be in grams and molar mass in grams per mole for the division to give moles.
- Treating the particle count as atoms when the substance is molecular — for H2O one 'particle' is a molecule containing three atoms.
Frequently asked questions
What is Avogadro's number?
Avogadro's number, 6.022 x 10^23, is the number of particles in one mole of any substance. It links the microscopic scale of atoms and molecules to macroscopic quantities you can weigh out in the lab.
How do I use this for a chemical reaction?
First find the moles of your known substance here, then multiply by the mole ratio from the balanced equation (the ratio of coefficients) to get the moles of the substance you want. Multiply that by its molar mass to convert back to grams.
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Cite this tool
Zerdly. (2026). Stoichiometry Calculator (Moles, Mass & Particles). Retrieved from https://www.zerdly.com/tools/stoichiometry-calculator/
Tip: Enter any known values to calculate the remaining results.
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