By Sophie Johnson, Updated Mar 24, 2022

A mole is a fixed quantity of particles—atoms, molecules, ions, or electrons—equivalent to the number of atoms in 12 grams of the carbon‑12 isotope. This standard, established by the International Union of Pure and Applied Chemistry (IUPAC), allows chemists to count microscopic entities in bulk terms.

Step 1 – Understand the Relationship

There are 1,000 millimoles in one mole: 1 mol = 1,000 mmol. Equivalently, 1 mmol = 0.001 mol. In fractional form, this is expressed as 1 mol/1,000 mmol or its reciprocal.

Step 2 – Write the Conversion Formula

Choose the format that matches your calculation needs. For quick conversions, use the simple equation 1 mol = 1,000 mmol. For detailed work, write the fractional notation: 1,000 mmol/1 mol.

Step 3 – Apply the Formula

Multiply the number of moles you have by the conversion factor. For example, to convert 2 mol to millimoles:

2 mol × 1,000 mmol/1 mol = 2,000 mmol

If you start with millimoles, invert the fraction accordingly.

Things You’ll Need

• Pencil
• Paper
• Calculator (optional, for large numbers)

TL;DR

When converting, place the target unit on top of the fraction. For mole → millimole, use 1,000 mmol/1 mol; for millimole → mole, use 1 mol/1,000 mmol.

Avogadro’s number—currently 6.022 137 6 × 10²³—is the count of atoms in 12 g of carbon‑12. It is a measured constant that has evolved with precision; earlier values in the 1980s were 6.022 045 × 10²³.

Note on Definitions

Although Avogadro’s number is widely cited, the mole is defined by the exact quantity of atoms in 12 g of carbon‑12, not by the constant itself. This distinction underpins all stoichiometric calculations in chemistry.