Definition - What does Sum Certain mean?
A sum certain is a predetermined, fixed price described in the terms of a contract or other negotiable instrument.
A sum certain may be favored by parties who wish to avoid any ambiguity about how much money should be paid for the performance of a contractual obligation. A sum certain helps to prevent any conflicts about the calculation of payment.
Justipedia explains Sum Certain
A sum certain helps parties in a contract understand precisely what they will pay or be paid for fulfilling a contract.
Sometimes, when a contract describes a payment rate like "$20 an hour plus expenses and incidentals," litigation will ensue over how many hours a party worked or what counts as expenses and incidentals. In contrast, the contract could contain a sum certain that provides something like: "I will pay you $1,000 to write an article for me." In this example, it is unlikely that the payment is open to interpretation.