Molarity formula
Molarity (M) is the number of moles of solute divided by the volume of solution in litres: M = mol ÷ L. The unit mol/L is written M. To find moles first, use our moles calculator.
How to use this calculator
Enter the moles of dissolved solute and the total solution volume in litres, then read the molarity. If your volume is in millilitres, divide by 1000 first. To work out how to dilute a stock to a target molarity, switch to the dilution calculator.
- Millilitres are the usual trap. Entering 250 instead of 0.25 L makes the answer a thousand times too small.
- Grams are not moles. Convert mass to moles with the molar mass before entering it.
- Volume cannot be zero, since dividing by it is undefined.
Worked example
Dissolve 0.5 mol of sodium chloride in enough water to make 2 L of solution. Molarity = 0.5 ÷ 2 = 0.25 M. Doubling the volume to 4 L would halve the concentration to 0.125 M.
Working backwards is just as common: to prepare 2 L of a 0.25 M solution you need 0.25 × 2 = 0.5 mol of solute, which for sodium chloride at 58.44 g/mol is about 29.2 g. That inverse relationship between volume and concentration — add solvent, concentration falls proportionally — is the basis of every dilution you will do at the bench. For work at varying temperature, molality is the safer measure because mass does not expand.
Frequently asked questions
- What is molarity?
- Molarity is the concentration of a solution expressed as moles of solute per litre of solution (mol/L, written M).
- What units does molarity use?
- Molarity uses mol/L, abbreviated M. One molar (1 M) means one mole of solute per litre of solution.
- How do I find moles for this?
- Divide the solute mass in grams by its molar mass in g/mol. Our moles calculator does this for you.
- My volume is in mL — what do I do?
- Convert to litres by dividing by 1000 before entering it (e.g. 250 mL = 0.25 L).
- What is the difference between molarity and molality?
- Molarity divides moles by litres of total solution, while molality divides moles by kilograms of solvent alone. Molality is preferred when temperature changes, because volume expands with heat but mass does not.