How to calculate molarity

Molarity is moles of solute per litre of solution. The formula is simple; the parts people miss are getting moles from grams, converting millilitres to litres, and diluting to a target concentration. Here is each one, worked.

By the Calculator Den team · Updated 2026-10-22

Molarity is the most common way chemists describe how concentrated a solution is, and calculating it is straightforward once you keep two things in order: the amount of dissolved substance, measured in moles, and the volume of the whole solution, measured in litres. The formula itself is one line. What actually trips people up in class and in the lab is getting from grams to moles, handling millilitres, and working out how to dilute a stock solution to the concentration you need. A free molarity calculator handles the arithmetic, but the method below is what makes the answer make sense.

The molarity formula

Molarity, written with a capital M, is:

Molarity (mol/L) = moles of solute ÷ litres of solution

So a solution with 2 moles of solute in 1 litre has a molarity of 2 ÷ 1 = 2 mol/L, usually written 2 M and read "two molar". Two details hide in that sentence. The solute is the thing being dissolved; the solution is the solute and the solvent together, so the volume is the final volume of the mixture, not the volume of water you started with. And the volume is in litres, which matters because glassware is marked in millilitres.

How to calculate molarity from grams

You are rarely handed moles directly. Usually you have a mass in grams and have to convert first. Moles = mass in grams ÷ molar mass in grams per mole, where the molar mass comes from the chemical formula (our molar mass calculator works it out from a formula like NaCl or H2SO4).

Worked example: dissolve 58.5 g of table salt, NaCl, in enough water to make 1 litre of solution. The molar mass of NaCl is 58.44 g/mol, usually rounded to 58.5, so 58.5 ÷ 58.5 = 1 mole. The molarity is 1 mole ÷ 1 litre = 1 M. If instead you dissolved 117 g of NaCl in a litre, that is 117 ÷ 58.5 = 2 moles, giving 2 M.

Going the other way is just as common: to prepare a target molarity you need the grams, so rearrange to mass = molarity × litres × molar mass. For 2 litres of 0.5 M NaCl that is 0.5 × 2 × 58.44 = about 58.4 g, which you dissolve and make up to 2 litres.

Handling millilitres

Lab volumes come in millilitres, and molarity needs litres, so convert first by dividing by 1,000. Say you dissolve 0.5 moles of a substance in 250 mL of solution. That volume is 250 ÷ 1,000 = 0.25 litres, so the molarity is 0.5 ÷ 0.25 = 2 M. Forgetting to convert the millilitres is the single most common molarity error, and it throws the answer out by a factor of 1,000, so it is worth a deliberate check every time.

Diluting to a target molarity

Often you already have a concentrated stock solution and need a weaker one. The relationship is the dilution equation, C1V1 = C2V2, where C is concentration and V is volume. The moles of solute do not change when you add solvent, so concentration times volume stays constant.

Worked example: you have a 5 M stock and want 500 mL of 0.5 M solution. Rearrange for V1, the volume of stock you need: V1 = C2V2 ÷ C1 = (0.5 × 500) ÷ 5 = 50 mL. So you measure 50 mL of the 5 M stock and add solvent up to a final volume of 500 mL. Always add the concentrated solution to the solvent and make up to the final volume, rather than guessing the water, because it is the final solution volume that defines the molarity.

Molarity, molality and normality

A quick word on the near neighbours, because the names blur. Molarity is moles per litre of solution and is by far the most common. Molality is moles per kilogram of solvent, used where temperature changes the volume, since mass does not change with temperature. Normality is about reactive units per litre and shows up in acid-base and redox work. For everyday concentration, molarity is the one you want, but it helps to know the others exist so you do not mix up the formulas.

Common molarity mistakes

• Using millilitres as litres. Divide mL by 1,000 first. This is the error that most often gives an answer 1,000 times too big.
• Measuring the solvent, not the solution. The volume in the formula is the final volume of the mixture, so make up to the mark rather than adding a set amount of water.
• Skipping the molar mass step. Grams are not moles. Convert mass to moles with the molar mass before dividing by volume.
• Reaching for the wrong equation when diluting. Use C1V1 = C2V2 for dilutions; use moles over litres only when you know the moles and the final volume directly.

Keep moles and litres straight, convert grams and millilitres before you divide, and molarity is a one-line calculation. When you want it checked instantly, the free molarity calculator takes moles and volume and returns the concentration, and the molar mass calculator covers the grams-to-moles step from any chemical formula.