How to use the Molarity Calculator
- Weigh your solute and enter the mass in grams.
- Enter the molar mass of the compound — use the molar mass calculator if you only have the formula.
- Enter the final volume of the solution in litres, not the volume of solvent added.
- Read the molarity in mol/L, plus millimolar for dilute biological buffers.
- Check the g/L figure against your protocol if it specifies concentration by mass rather than by mole.
How the calculation works
Molarity is moles of solute per litre of solution. The distinction between solution volume and solvent volume is the one that trips people up: dissolving 58.44 g of sodium chloride in one litre of water gives slightly more than one litre of solution and therefore slightly less than 1 M. Correct practice is to dissolve in part of the solvent and then make up to the mark in a volumetric flask.
Because molarity is defined per litre of solution, it is temperature-dependent — the solution expands when warmed, so the same moles occupy more volume and the molarity falls. For work where that matters, molality (moles per kilogram of solvent) is the better unit because mass does not change with temperature. Colligative-property calculations such as freezing-point depression use molality for exactly this reason.
Dilution follows C₁V₁ = C₂V₂, which lets you prepare working solutions from a concentrated stock without reweighing. The relationship holds because the number of moles is conserved when you add solvent. Prepare serial dilutions where the factor is large: a single 1,000-fold dilution has far worse precision than three consecutive tenfold steps.
M = n ÷ V, where n = mass ÷ molar mass; dilution: C₁V₁ = C₂V₂Source: IUPAC Compendium of Chemical Terminology (Gold Book), 'amount concentration'; BIPM SI Brochure, 9th edition, mole definition (2019).
Worked example
Preparing 250 mL of 0.1 M sodium hydroxide (molar mass 40.00 g/mol).
- Moles required: 0.1 mol/L × 0.25 L = 0.025 mol.
- Mass required: 0.025 × 40.00 = 1.00 g.
- Enter mass 1.0, molar mass 40, volume 0.25 to confirm.
- The calculator returns 0.1 mol/L, or 100 mM, or 4 g/L.
Weigh 1.00 g of NaOH, dissolve in about 200 mL of water, then make up to exactly 250 mL for a 0.1 M solution.
Frequently asked questions
What is the difference between molarity and molality?+
Molarity is moles per litre of solution and varies with temperature. Molality is moles per kilogram of solvent and does not. Use molality for colligative-property work.
Should I use the volume of water or the final volume?+
The final solution volume, always. Dissolve first, then make up to the mark.
How do I dilute a stock solution?+
Use C₁V₁ = C₂V₂. To make 100 mL of 0.1 M from 1 M stock, take 10 mL of stock and add solvent to 100 mL.
What does 1 mM mean?+
One millimolar, or 0.001 mol/L. Biological buffers are usually quoted in millimolar because the concentrations involved are small.
Last reviewed September 1, 2026. We review this page whenever the underlying formula, tax year, published rate or standard changes.