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Mole Calculator

A dozen for atoms — and since 2019 the number is fixed by definition, not measured.

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Used when no formula is given.
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Moles5.55093 mol100 g ÷ 18.015 g/mol of H2O
Mass100 g
Molar mass used18.015 g/mol
Formula units3.3428 × 10²⁴ unitsmoles × 6.02214076 × 10²³
Avogadro's constant6.02214076 × 10²³ per moleEXACT since the 2019 SI redefinition. The mole is now defined as that many things, and the kilogram no longer depends on a lump of metal in Paris
A sense of the scalea mole of grains of sand would bury the Earthsix hundred thousand billion billion. The number is unimaginable and the arithmetic with it is straightforward, which is the useful part
Mass of one formula unit2.9915 × 10⁻²³ gmolar mass divided by Avogadro

The formula

moles = mass ÷ molar mass; particles = moles × 6.02214076 × 10²³

A dozen for atoms

The mole solves one problem: atoms are far too small to weigh individually, and reactions happen in whole-number ratios of atoms.

Counting in moles lets you weigh out bulk material and know how many particles you have. It is a unit of amount, exactly as a dozen is, and the only unusual thing about it is the size.

Fixed by definition since 2019

Avogadro's constant used to be measured; now it is defined as exactly 6.02214076 × 10²³.

The redefinition also severed the kilogram from the platinum-iridium cylinder in Sèvres, tying the whole system to fundamental constants instead of to an object that could be scratched.

Standard atomic weights are averages

The masses used here are weighted means over the isotopes found naturally on Earth, which is why chlorine is 35.45 rather than a whole number.

For work with a specific isotope, or with material from an unusual source, the isotopic mass is the right figure and it is not this one.