Last updated: 2026-06-14

Understanding molarity

Molarity is the most widely used measure of concentration in chemistry. It tells you how many moles of a dissolved substance (the solute) are present in each litre of solution. Because chemical reactions depend on the number of particles involved, expressing concentration in moles makes it easy to predict how much of one substance will react with another.

The molarity formula

Rearranging the same equation lets you solve for any of the three quantities:

• Moles of solute: $n = M \times V$
• Volume of solution: $V = n \div M$

This calculator handles all three cases — choose what you want to find and enter the other two values.

Worked example

How many grams of sodium chloride (NaCl) are needed to make 250 mL of a 0.40 M solution?

First find the moles of solute:

Then convert to grams using the molar mass of NaCl (58.44 g/mol):

So you would weigh out about 5.84 g of NaCl and add water to a final volume of 250 mL.

Volume of solution, not solvent

A common mistake is to add the stated volume of water to the solute. Molarity is defined by the final volume of the solution, which is slightly larger than the solvent alone because the solute takes up space. In practice, chemists dissolve the solute in a smaller amount of solvent, then top up to the calibration mark of a volumetric flask to reach the exact final volume.

Molarity, millimolar, and micromolar

Concentrations span an enormous range, so chemists use prefixes:

This calculator accepts and reports concentrations in any of these units and converts between them automatically.

Molarity vs. molality

Molarity (M) is moles per litre of solution and changes slightly with temperature because liquids expand when heated. Molality (m) is moles per kilogram of solvent and is temperature-independent. For routine bench work, molarity is standard; molality is used when temperature changes matter, such as in freezing-point-depression and boiling-point-elevation calculations.