Molar concentration (also called amount-of-substance concentration or molarity) is the number of moles of solute per liter of solution. Specifically, It is a measure of the concentration of a chemical species, in particular, of a solute in a solution, in terms of amount of substance per unit volume of solution. In chemistry, the most commonly used unit for molarity is the number of moles per liter, having the unit symbol mol/L or mol/dm³ (1000 mol/m³) in SI units. Molar concentration is often depicted with square brackets around the substance of interest; for example with the hydronium ion [H₃O⁺] = 4.57 x 10^-9 mol/L.

Molar concentration or molarity is most commonly expressed in units of moles of solute per litre of solution. For use in broader applications, it is defined as amount of substance of solute per unit volume of solution, or per unit volume available to the species, represented by lowercase ${\displaystyle c}$:

Here, ${\displaystyle n}$ is the amount of the solute in moles, ${\displaystyle N}$ is the number of constituent particles present in volume ${\displaystyle V}$ (in litres) of the solution, and ${\displaystyle N_{\text{A}}}$ is the Avogadro constant, since 2019 defined as exactly 6.02214076×10²³ mol⁻¹. The ratio ${\displaystyle {\frac {N}{V}}}$ is the number density ${\displaystyle C}$.

In thermodynamics, the use of molar concentration is often not convenient because the volume of most solutions slightly depends on temperature due to thermal expansion. This problem is usually resolved by introducing temperature correction factors, or by using a temperature-independent measure of concentration such as molality.

The reciprocal quantity represents the dilution (volume) which can appear in Ostwald's law of dilution.

If a molecule or salt dissociates in solution, the concentration refers to the original chemical formula in solution, the molar concentration is sometimes called formal concentration or formality (F_A) or analytical concentration (c_A). For example, if a sodium carbonate solution (Na₂CO₃) has a formal concentration of c(Na₂CO₃) = 1 mol/L, the molar concentrations are c(Na⁺) = 2 mol/L and c(CO2−3) = 1 mol/L because the salt dissociates into these ions.

While there is clear consensus on the equivalence of units:

1 mol/m³ = 10⁻³ mol/dm³ = 10⁻³ mol/L = 10⁻³ M = 1 mM = 1 mmol/L