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 :
Here,  is the amount of the solute in moles,  is the number of constituent particles present in volume  (in litres) of the solution, and  is the Avogadro constant, since 2019 defined as exactly . The ratio  is the number density .
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.
; Formality or analytical concentration
If a molecular entity dissociates in solution, the concentration refers to the original chemical formula in solution, the molar concentration is sometimes called formal concentration or formality (F) or analytical concentration (c). For example, if a sodium carbonate solution () has a formal concentration of c() = 1 mol/L, the molar concentrations are c() = 2 mol/L and c() = 1 mol/L because the salt dissociates into these ions.