What is Quantum Meruit?

Legal Definition
Quantum meruit is a Latin phrase meaning "what one has earned". In the context of contract law, it means something along the lines of "reasonable value of services".

In the United States, the elements of quantum meruit are determined by state common law. For example, to state a claim for unjust enrichment in New York, a plaintiff must allege that (1) defendant was enriched; (2) the enrichment was at plaintiff's expense; and (3) the circumstances were such that equity and good conscience require defendants to make restitution.
-- Wikipedia
Legal Definition
Latin for "as much as he has deserved." An equitable remedy that provides restitution for unjust enrichment. Damages awarded in an amount considered reasonable to compensate a person who has provided services in a quasi-contractual relationship. See quasi-contract.

A claim in quantum meruit is usually an action to recover the reasonable value of services rendered by one party to another.
Illustrative caselaw
See, e.g. Foman v. Davis, 371 U.S. 178 (1962).
See also
  • Contract
Legal Definition
Pleading. As much as he has deserved. When a person employs another to do work for him, without any agreement as to his compensation, the lawimplies a promise from, the employer to the workman that he will pay him for his services, as much as be may deserve or merit. In such case the plaintiff may suggest in his declaration that the defendant promised to pay him as much as he reasonably deserved, and then aver that his trouble was worth sucli a sum of money, which the defendant has omitted to pay. This is called an assumpsit on a quantum meruit. 2 Bl. Com. 162, 3 1 Vin. Ab. 346; 2 Phil. Ev. 82.

2. When there is an express contract for a stipulated amount and mode of compensation for services, the plaintiff cannot abandon the contract and resort to an action for a quantum meruit on an implied assumpsit. 18 John. R. 169; 14 John. R. 326; 10 Serg. & Rawle, 236. Sed vide 7 Cranch, 299; Stark. R. 277; S., C. Holt's N. P. 236; 10 John. Rep. 36; 12 John. R. 374; 13 John. R. 56, 94, 359; 14 John. R. 326; 5 M. & W. 114; 4 C. & P. 93; 4 Sc. N. S. 374; 4 Taunt. 475; 1 Ad. & E. 333; Addis. on Contr. 214.
-- Bouviers Law Dictionary
Legal Definition
As much as he deserved. In pleading. The common count in an action of assumpsit for work and labor, founded on an implied assumpsit or promise on the part of the defendant to pay the plaintiff os much as he reasonably deserved to have for his labor. 3 Bl. Comm. 161; 1 Tidd, Pr. 2.
-- Black's Law Dictionary
Legal Definition
One of the common counts for services rendered. See 72 Tenn. 494. See, also, Common counts.
-- Ballentine's Law Dictionary