Proven reserves (also called measured reserves, 1P, and reserves) is a measure of fossil fuel energy reserves, such as oil reserves, natural gas reserves, and coal reserves. It is defined as the "[q]uantity of energy sources estimated with reasonable certainty, from the analysis of geologic and engineering data, to be recoverable from well established or known reservoirs with the existing equipment and under the existing operating conditions." A reserve is considered proven if it is probable that at least 90% of the resource is recoverable by economically profitable means.

Operating conditions are taken into account when determining if a reserve is classified as proven. Operating conditions include operational break-even price, regulatory and contractual approvals, without which the reserve cannot be classified as proven. Price changes therefore can have a large impact on the classification of proven reserves. Regulatory and contractual conditions may change, and also affect the amount of proven reserves. If a reserve's resources can be recovered using current technology but is not economically profitable it is considered "technically recoverable" but cannot be considered a proven reserve. Reserves less than 90% recoverable but more than 50% are considered "probable reserves" and below 50% are "possible reserves".