Grams to Moles Calculator
Convert grams to moles by dividing mass by molar mass. Formula: moles = mass (g) ÷ molar mass (g/mol). For education only.
How to use this tool
- Enter mass and molar mass in the fields above.
- Results update instantly as you type — or click Calculate.
- Read your amount and the full breakdown beneath it.
To convert grams to moles divide mass by molar mass: moles = g ÷ molar mass. For example, 36.03 g of water (18.015 g/mol) = 2.00 mol.
For education only.
Formula
moles = mass (g) ÷ molar mass (g/mol)
Millimoles: mmol = moles × 1000
How it works
This calculator divides the mass of a substance (in grams) by its molar mass (in g/mol) to determine how many moles are present — the inverse of the moles-to-grams conversion and an essential step in stoichiometric calculations.
Results are rounded to five decimal places for moles and four for millimoles. The formula assumes a pure, single-component substance; mixtures or hydrated salts require the combined formula weight as the molar mass input.
Worked example
Worked example: moles in 36.03 g of water
- Identify inputs: mass = 36.03 g, molar mass of H₂O = 18.015 g/mol.
- Apply the formula: moles = 36.03 ÷ 18.015 = 2.0 mol.
- Convert to millimoles: mmol = 2.0 × 1000 = 2000 mmol.
Moles = 2.0 mol; Millimoles = 2000.0 mmol
Common mistakes to avoid
- Dividing by atomic mass instead of molar mass for a compound -- water's molar mass is 18.015 g/mol, not 16 (oxygen's atomic mass alone).
- Forgetting to sum molar masses for all atoms in the formula -- for CaCO3 it is 40.08 + 12.01 + 3 x 16.00 = 100.09 g/mol, not just one element.
- Entering mass in milligrams instead of grams without converting -- 500 mg = 0.500 g, so using 500 inflates the mole count by 1000x.
Key terms
- Mole (mol)
- The SI unit for amount of substance; one mole contains exactly 6.022 × 10²³ entities (atoms, molecules, ions, etc.).
- Molar mass
- The mass of one mole of a substance in g/mol; for a compound, it is the sum of all constituent atomic masses.
- Millimole (mmol)
- One thousandth of a mole (0.001 mol); used when working with small sample quantities.
- Formula weight
- The sum of atomic masses of all atoms in a chemical formula, expressed in atomic mass units (amu) or g/mol.
Frequently asked questions
- What molar mass should I use?
- Look up the molecular formula and sum the atomic masses. Common examples: H₂O = 18.015, CO₂ = 44.009, NaCl = 58.44, glucose C₆H₁₂O₆ = 180.16 g/mol.
- What is a millimole?
- 1 mmol = 0.001 mol. Millimoles are common in biochemistry and medicine where concentrations are small.