How to Calculate Moles

1.What a mole is

One mole contains Avogadro’s number of entities — atoms, molecules, or formula units. Molar mass M is the mass of one mole of that substance, in g/mol when mass is in grams. Water’s molar mass is about 18.015 g/mol, so a mass near 18 g is about one mole. Particle count is a separate conversion (N = n × N_A); this tutorial focuses on the mass–mole bridge.

2.The formula

Moles n equal mass m divided by molar mass M: n = m / M. Rearranged, mass is m = n M. M must be positive. Keep units consistent — grams with g/mol, or kilograms with kg/mol — or n will be wrong by a factor of a thousand. The Mole Calculator on this site uses the same identity.

\[ n=\frac{m}{M} \]

3.Worked example

Take m = 18 g and M = 18.015 g/mol — the same mass-to-moles defaults as the Mole Calculator. Then n = 18 / 18.015 ≈ 0.999 mol, essentially one mole of water. The reverse check is m = n M: 0.999 × 18.015 ≈ 18 g. If you start from 1 mol instead, m = 1 × 18.015 = 18.015 g.

\[ n=\frac{18}{18.015}\approx 0.999 \]

4.Mass–mole bridge

Switch mass → moles or moles → mass, then move the sliders. The bridge keeps M in the middle: divide mass by M to get moles, or multiply moles by M to get mass. This is a teaching widget — no favorites, no full calculator chrome. Open the Mole Calculator when you want particle count as well.

5.Open the full calculator

The Mole Calculator uses the same n = m / M identity, the same 18 g / 18.015 g·mol⁻¹ example, and also converts particles with Avogadro’s number. Browse other chemistry calculators if you need dilution or molar-mass style checks next.