Molarity Calculator

A molarity calculator finds moles of solute per liter of solution: M = (mass ÷ molar mass) ÷ volume in liters. Dissolving 5.844 g of NaCl (58.44 g/mol) gives 0.1000 mol, and making it up to 250 mL (0.2500 L) gives 0.1000 ÷ 0.2500 = 0.4000 M. Run it backward to find the grams to weigh for any target molarity.

How to use the molarity calculator

1. Choose the solute from the compound list, or type its formula (for example CuSO4·5H2O or Ca(OH)2). You can also type a molar mass in g/mol, such as the formula weight printed on a reagent bottle.
2. Pick what you want to find: Molarity from a mass and a volume, the Mass to weigh for a target molarity, the Volume a given mass will make, or the molarity of a stock from its % w/w and density.
3. Enter the numbers with their units. Grams, milligrams, liters, milliliters, mM and µM are all converted for you.

The answer appears with the moles, mass, volume, mass concentration in g/L, a preparation recipe, the molar mass breakdown by element, and each step with units shown so you can follow the dimensional analysis.

Molarity formula

Molarity, also called molar concentration or amount concentration, is the amount of solute divided by the volume of the whole solution:

M = n / V, where n is moles of solute (mol) and V is the volume of solution in liters. Since n = m / M_r, where m is the mass in grams and M_r is the molar mass in g/mol:

• Molarity: M = m / (M_r × V)
• Mass to weigh: m = M × V × M_r
• Volume of solution: V = m / (M_r × M)
• From a mass percent label: M = 10 × (% w/w) × density (g/mL) / M_r

The unit M means mol/L (mol dm⁻³). One millimolar (mM) is 0.001 M and one micromolar (µM) is 0.000001 M. IUPAC calls this quantity amount concentration and writes it [B] for a substance B.

Molar mass from a chemical formula

The molar mass is the sum of the atomic weights of every atom in the formula. For glucose, C₆H₁₂O₆: 6 × 12.011 + 12 × 1.0080 + 6 × 15.999 = 180.156 g/mol. The calculator uses the IUPAC abridged standard atomic weights published by the Commission on Isotopic Abundances and Atomic Weights (CIAAW), the same values printed in most textbooks. It reads brackets, so (NH4)2SO4 counts 2 nitrogen and 8 hydrogen atoms, and a dot or · adds water of crystallization: CuSO4·5H2O is 249.677 g/mol.

Worked examples

Find the molarity

5.844 g of NaCl is dissolved and made up to 250 mL. Moles: 5.844 g ÷ 58.44 g/mol = 0.1000 mol. Volume: 250 mL = 0.2500 L. Molarity: 0.1000 mol ÷ 0.2500 L = 0.4000 M.

Find the mass to weigh

You need 500 mL of 0.5 M NaCl. Moles: 0.5 mol/L × 0.500 L = 0.250 mol. Mass: 0.250 mol × 58.44 g/mol = 14.61 g. Dissolve it in less than 500 mL of water, then fill to the 500 mL mark.

Watch the hydrate

To make 1 L of 0.1 M copper(II) sulfate from blue crystals (CuSO4·5H2O, 249.677 g/mol) you weigh 0.1 × 249.677 = 24.97 g. Using the anhydrous molar mass (CuSO4, 159.602 g/mol) would give 15.96 g, and the solution would be only about 0.064 M.

Molarity of a concentrated stock

Suppose a bottle label reads 37% (w/w) hydrochloric acid with density 1.18 g/mL. One liter weighs 1,180 g, of which 37% is HCl: 436.6 g. Divided by 36.458 g/mol, that is 11.98 M. Use the label of your own bottle, since density changes with concentration and temperature.

Molar masses of common compounds

Common mistakes with molarity

• Dividing by the volume of solvent. Molarity uses the final volume of the solution. Adding 0.5 mol of salt to 1.000 L of water gives slightly more than 1 L of solution, so the result is a little under 0.5 M. Dissolve first, then fill to the mark.
• Forgetting to convert mL to L. 0.1 mol in 250 mL is 0.4 M, not 0.0004 M.
• Ignoring water of crystallization. Check whether your bottle is the hydrate or the anhydrous salt; the molar masses can differ by more than 50%.
• Mixing up molarity and molality. Molality (m) is moles per kilogram of solvent and does not change with temperature; molarity does, because volume changes.
• Rounding too early. Keep extra digits until the last step, then round to the significant figures of your least precise measurement.

Need a weaker solution from a stock you already have? Use C₁V₁ = C₂V₂ in the dilution calculator. To convert a mass and volume to grams per milliliter, see the density calculator.

Sources

• CIAAW (IUPAC), Abridged standard atomic weights, 2024 table including the 2024 revisions for gadolinium, lutetium and zirconium (checked November 2026).
• IUPAC Gold Book, amount concentration: definition, symbol [B] and unit mol L⁻¹ (checked November 2026).
• NIST Special Publication 811, Guide for the Use of the International System of Units, for the exact pound, ounce and US gallon conversions (checked November 2026).

Molarity practice quiz: 10 questions

Test yourself with 10 multiple-choice questions. Choose an answer and press Submit to see if you are right and why. Your score appears at the end.

Frequently asked questions

How do you calculate molarity?

Convert the mass of solute to moles by dividing by its molar mass, convert the volume of solution to liters, then divide moles by liters. For 9.0 g of glucose in 0.500 L: 9.0 ÷ 180.156 = 0.0500 mol, and 0.0500 ÷ 0.500 = 0.0999 M.

How many grams do I need for a 1 M solution?

For 1 liter of a 1 M solution you need exactly the molar mass in grams: 58.44 g of NaCl or 180.16 g of glucose. For other volumes and concentrations, multiply molarity × liters × molar mass.

What is the difference between molarity and molality?

Molarity is moles per liter of solution; molality is moles per kilogram of solvent. In dilute water solutions near room temperature they are close, but molality is preferred when temperature changes, because it does not depend on volume.

Is molarity the same as molar concentration?

Yes. Molarity, molar concentration and amount concentration all mean moles of solute per liter of solution, with the unit mol/L, written M.

How do I convert mass percent to molarity?

Multiply the percent by the density in g/mL and by 10 to get grams per liter, then divide by the molar mass. 37% HCl with density 1.18 g/mL is 10 × 37 × 1.18 / 36.458 = 11.98 M.

Related calculators

• Dilution calculator: make a working solution from a stock with C₁V₁ = C₂V₂
• Density calculator: mass, volume and density with unit conversions
• Significant figures calculator: round lab answers correctly
• Scientific notation converter: handle very small concentrations
• Unit converter: mass and volume units