Molarity Calculator

How to use the molarity calculator

Pick what you want to find. Molarity needs the mass you dissolved and the final volume of the solution. Mass of solute tells you how much to weigh for a target concentration and volume — the most common lab task. Volume tells you how far to fill up a given mass, and molar mass works backwards from a solution of known concentration. The last mode turns the mass percent and density printed on a bottle of concentrated acid or ammonia into molarity.

Type the formula of the solute and the calculator finds its molar mass. It understands brackets (Ca(OH)2, Al2(SO4)3, [Cu(NH3)4]SO4), hydrates written with a dot (CuSO4·5H2O, MgSO4·7H2O — a period or an asterisk works too) and subscript digits pasted from a document (H₂SO₄). Capital letters matter: Co is cobalt but CO is carbon monoxide. For a protein, a polymer or a reagent sold as a mixture, type its molar mass (from the label or certificate) into the optional field instead.

The formulas

Molarity is the amount of substance per liter of solution: c = n ÷ V, in mol/L (the symbol M). The amount comes from the mass: n = m ÷ M, where M is the molar mass in g/mol. Together they give m = c × V × M — the mass to weigh — and every other arrangement the calculator uses. The volume is the volume of the finished solution, not the volume of water you start with: dissolving a solid changes the volume, which is why solutions are made up to the mark in a volumetric flask.

For a concentrated reagent, one liter weighs 1000 × ρ grams, and the mass percent w of that is solute, so c = w × 10 × ρ ÷ M (w in %, ρ in g/mL). Hydrochloric acid at 37% with a density of 1.19 g/mL is 37 × 10 × 1.19 ÷ 36.458 ≈ 12.08 mol/L.

Worked examples

• Concentration: 29.22 g of sodium chloride (NaCl, 58.440 g/mol) is 0.5000 mol. In 500 mL of solution that is 0.5 ÷ 0.5 = 1.000 mol/L; in 2 L it is 0.25 mol/L.
• Mass to weigh: 500 mL of 0.1 M NaCl needs 0.1 × 0.5 × 58.44 = 2.922 g.
• Hydrate: copper(II) sulfate pentahydrate, CuSO4·5H2O, is 249.68 g/mol — the five waters of crystallization add 90.08 g/mol. Weighing the hydrate with the anhydrous molar mass (159.60 g/mol) gives a solution about 36% weaker than intended.
• Sulfuric acid: H2SO4 = 2 × 1.008 + 32.06 + 4 × 15.999 = 98.072 g/mol (older tables, with 1.00794 for hydrogen, give 98.079).

Molar mass, molecular weight and units

Molar mass in g/mol is numerically equal to the relative molecular mass (molecular weight, formula weight) printed on reagent labels. One mmol/L (mM) is a thousandth of a mol/L and one µmol/L (µM) a millionth; g/L equals mg/mL. The table under the result splits the molar mass by element, with each element's share by mass — handy for checking a formula or for elemental composition.

Atomic weights used

The calculator uses the standard atomic weights of the IUPAC Commission on Isotopic Abundances and Atomic Weights (CIAAW, Atomic Weights 2021, with the 2024 revisions of gadolinium, lutetium and zirconium). Elements whose atomic weight varies in nature, such as hydrogen, carbon, oxygen and chlorine, use IUPAC's conventional values. Elements in brackets have no standard atomic weight; the mass number of their longest-lived isotope is used, as on periodic tables.

Tips for accurate solutions

• Check the formula on the reagent bottle, especially the water of crystallization: Na2CO3, Na2CO3·H2O and Na2CO3·10H2O are three different molar masses.
• Correct for purity: if a reagent is 98% pure, divide the mass by 0.98.
• Molarity changes slightly with temperature because the volume does; molality (mol per kg of solvent) does not.
• To make a weaker solution from a stock, use the dilution calculator (C1V1 = C2V2).

Frequently asked questions

Last updated: October 19, 2026