An injector testing system includes a testing rig having a receptacle and an injector mounting structure. The receptacle has an inlet for receiving fluid injected by an injector, a target outlet for receiving at least a portion of fluid injected through the inlet, and a runoff outlet for receiving fluid missing the target outlet. At least one fluid-quantity measuring device fluidly connected to at least one of the target and runoff outlets measures a quantity of fluid having flowed thereto. A controller receives signals representative of (i) a quantity of fluid having flowed to the at least one fluid-quantity measuring device via the target outlet, and (ii) a quantity of fluid having flowed to the at least one fluid-quantity measuring device via the runoff outlet or both of the runoff outlet and the target outlet. Based on these quantities, a proportion of fluid injected into the target outlet is determined.