Quantum meruit, meaning "as much as one has earned," is a remedy allowing a person to recover reasonable payment for work genuinely done or services genuinely rendered, in circumstances where there is no fully enforceable, express contract fixing the exact amount to be paid, or where an existing contract has broken down in a way that makes suing on the contract itself unavailable or inappropriate. It is a restitutionary remedy, aimed at preventing one party from being unjustly enriched by accepting the benefit of another's work without paying a reasonable amount for it.

The remedy typically arises in several recurring situations. First, where work is performed under what both parties believed to be a valid contract, but that contract is later found to be void, unenforceable, or otherwise incapable of being sued upon directly, a party who has genuinely conferred a benefit through their work may recover a reasonable sum for it. Second, where a contract for a fixed lump sum exists but the other party wrongfully prevents its complete performance partway through, the party who was ready and willing to perform may recover a reasonable sum for the work actually completed before being prevented from finishing. Third, where a party voluntarily accepts partial or substituted performance of a contract, knowing it falls short of what was originally agreed, a reasonable sum may be recoverable for what was actually accepted and used. Fourth, where services are requested and genuinely rendered with no express agreement on price at all, a reasonable sum may be recovered for the value of those services.

Quantum meruit does not apply where a party has simply, and without justification, abandoned a contract partway through of their own accord; a party who voluntarily walks away from a lump-sum contract without lawful excuse generally cannot recover anything for the incomplete work left behind, since the remedy exists to prevent unjust enrichment, not to reward a party's own unjustified failure to complete what was promised. Nor does it apply where a party has fully performed under a valid, enforceable contract that itself fixes the exact price payable; in that situation, the party must sue on the contract itself for the agreed price, rather than seeking a separate, open-ended reasonable sum.

Where quantum meruit does apply, the amount recoverable is not the contract price originally agreed, but a reasonable sum assessed according to the value of the work actually done or the benefit actually and genuinely conferred on the other party.