Mole (abbreviated as “mol”) is a unit of the amount of substance defined in the International System of Units (SI). One mole of any compound contains a number of molecules equal to Avogadro's number, 6.022E23. “Mmol” stands for millimoles and the prefix “milli” indicates “one thousandth,” i.e. the magnitude of 0.001 (see resources). Hence, 1 millimole equals 1 mole divided by 1,000. To illustrate how to calculate millimoles, consider these two examples: (1) calculate the number of mmol in 0.15 g of calcium carbonate (CaCO3); and (2) calculate the number of mmol in 25 ml of 0.2 mol/L of sodium hydroxide (NaOH).

Calculate the molar mass of a compound. Molar mass is calculated as the sum of mass of all atoms in the molecule. For mass of the atoms, use atomic weights of the corresponding elements in the periodic table of the chemical elements (see resources). In our example it would be Molar mass (CaCO3)=M(Ca)+M(C)+3 x M(O)=40+12+3x16=100 g/mol.

Calculate the amount of a compound in moles using the formula Amount (in moles) = mass (compound)/molar mass (compound) In our example, Amount (CaCO3)=0.15 g /100 g/mol= 0.0015 mol.

Convert moles to millimoles using the formula Amount (in millimoles)= Amount (moles) x 1,000. In our example, Amount (CaCO3)= 1,000 x 0.0015 mol=1.5 mmol.

Calculate the amount of a compound in moles using molarity. Note that molarity is the number of moles of the dissolved substance in one liter of the solution. Hence Amount (in moles)=molarity x solution volume (in L). In our second example, molarity is 0.2 mol/L and the volume of the solution is 25 ml or 0.025 L. Amount (NaOH)=0.2 mol/L x 0.025 L=0.005 mol. Use the formula from Step 3 to obtain mmol: 0.005 mol equals to 5 mmol.