Quick answer: Molarity = moles of solute ÷ liters of solution (M = n ÷ V). Enter any two of the three values and this molarity calculator solves for the third, with unit conversion built in. Pick what to solve for below.
Last reviewed: September 2026
Key facts
- M = n ÷ V — moles of solute per liter of solution, the standard concentration unit in chemistry.
- Rearranged: n = M × V and V = n ÷ M — enter any two, solve the third.
- V is the total final solution volume, not the volume of solvent added.
- 1 M = 1,000 mM = 1,000,000 µM; 1 L = 1,000 mL.
Molarity is the number of moles of solute dissolved per liter of solution, written M = n ÷ V. This free molarity calculator solves for molarity, moles or volume from the other two values, converting between M, mM, mol, mmol, L and mL automatically.
Pick the value to solve for, enter the two known values, and get the answer with the working shown. No sign-up, works on mobile and desktop.
How to use this molarity calculator
- Choose what to solve for: molarity, moles or volume.
- Enter the two known values and pick their units.
- The unknown field is ignored — read the answer and the steps below.
Background: the mole and molar concentration
The mole is the SI unit of amount of substance; since the 2019 SI redefinition it is defined by fixing the Avogadro constant at exactly 6.02214076 × 1023 elementary entities per mole. Molarity expresses that amount per liter of solution, which makes it the natural concentration unit for stoichiometry: equal volumes of equal-molarity solutions contain equal numbers of moles, so balanced-equation ratios translate directly into measurable volumes.
Because reactions happen mole-to-mole, a molarity calculator turns bench quantities — grams weighed on a balance, milliliters measured in a cylinder — directly into the mole ratios that chemical equations use.
From grams to molarity: the standard workflow
- Weigh the solute: mass m in grams.
- Convert to moles: n = m ÷ molar mass (g/mol), using atomic weights — e.g. NaCl is 58.44 g/mol.
- Dissolve and bring to the final total volume V in liters: in a volumetric flask the solute is dissolved and solvent is added up to the mark, not added as a measured V of solvent.
- Divide: M = n ÷ V.
The molarity formula
| Term | Meaning |
|---|---|
| M (molarity) | Moles of solute per liter of solution (mol/L). |
| n (moles) | Amount of solute = mass ÷ molar mass (g ÷ g/mol). |
| V (volume) | Total volume of the finished solution in liters. |
To get moles from a weighed mass: n = mass (g) ÷ molar mass (g/mol). Then divide by liters.
Molarity vs molality vs normality vs mass percent
| Measure | Definition | Units | Temperature-dependent? | Used for |
|---|---|---|---|---|
| Molarity | Mol solute / L solution | M (mol/L) | Yes (liquid expands) | General lab work, titrations |
| Molality | Mol solute / kg solvent | m (mol/kg) | No | Colligative properties, precise physical chemistry |
| Normality | Equivalents / L solution | N (eq/L) | Yes | Acid–base and redox titrations |
| Mass percent | (Mass solute / mass solution) × 100 | % w/w | No | Pharmacy, food, commercial formulations |
Normality counts "equivalents" — reactive units. Sulfuric acid provides two H+ per mole, so 1 M H2SO4 is 2 N for acid–base work. Molarity is the default in most labs; reach for molality when temperature varies, since kilograms of solvent do not expand.
Worked examples
Molarity of 5.85 g NaCl in 0.5 L
NaCl molar mass = 58.44 g/mol, so n = 5.85 ÷ 58.44 = 0.1001 mol.
M = 0.1001 ÷ 0.5 = 0.2 M.
Volume holding 0.25 mol at 0.5 M
Solve for V: V = n ÷ M = 0.25 ÷ 0.5.
V = 0.5 L (500 mL).
Dissolve 25.0 g NaOH and bring to 250 mL
NaOH molar mass = 40.00 g/mol, so n = 25.0 ÷ 40.00 = 0.625 mol. The molarity calculator then gives M = n ÷ V = 0.625 ÷ 0.25.
M = 2.5 M.
Common concentration units
| Unit | Equals |
|---|---|
| 1 M (molar) | 1 mol/L |
| 1 mM (millimolar) | 0.001 M |
| 1 µM (micromolar) | 0.000001 M |
| 1 mmol | 0.001 mol |
| 1 mL | 0.001 L |
Limitations
- Temperature shifts the volume. Because V is a volume, molarity changes slightly as the liquid expands or contracts; prepare and use solutions near the calibration temperature of your glassware.
- Only the total solution volume counts — never the solvent volume alone (see the workflow above).
- The solute must dissolve fully. Beyond the solubility limit the excess stays solid, so the true concentration is lower than the calculation implies.
- Weigh dry solute. Absorbed water in a hygroscopic solid inflates the measured mass and with it the calculated molarity.
Frequently asked questions
What is molarity?
Molarity is the number of moles of solute per liter of solution, written M or mol/L. A 1 M solution contains 1 mole of solute in every liter of solution. It is the most common way to express concentration in chemistry.
How do you calculate molarity from grams?
First convert grams to moles: n = mass (g) ÷ molar mass (g/mol). Then M = n ÷ V. Example: 5.85 g NaCl (molar mass 58.44 g/mol) is 0.1001 mol; in 0.5 L that gives 0.2 M.
Is the volume the solvent or the total solution?
The total final volume of the solution. If you dissolve a solid and then dilute the mixture to 500 mL in a volumetric flask, V = 0.5 L — not just the water you started with.
What is the difference between molarity and molality?
Molarity is moles per liter of solution; molality is moles per kilogram of solvent. Molarity changes slightly with temperature (liquids expand), while molality does not, so molality is preferred for precise physical-chemistry work.
Can molarity be greater than the solubility?
No — you cannot dissolve more solute than the solubility limit allows. If your calculation implies an impossibly high molarity, the excess solute would simply remain undissolved.
Sources
- LibreTexts: Molarity — molar concentration, preparation of solutions, and the dilution equation (OpenStax-based)
- OpenStax: Chemistry 2e — free college chemistry textbook covering moles, molarity and solution stoichiometry
Key takeaways
- Molarity = moles of solute ÷ liters of solution.
- Enter any two of M, n, V to solve for the third.
- Convert mass to moles with molar mass first.
- V is always the total solution volume, measured after dissolving.
- Use molality instead when temperature changes matter.
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