High latency in an interactive system limits its usability. In order to reduce end-to-end latency of such systems, it is necessary to analyze and optimize the latency of individual contributors, such as input devices, applica-tions, or displays. We present a simple tool for measur-ing the latency of USB-connected input devices with sub-millisecond accuracy. The tool, based on a Rasp-berry Pi 2 microcomputer, repeatedly toggles a button of a game controller, mouse, or keyboard via an opto-coupler soldered to the button and measures the time until the input event arrives. This helps researchers, developers and users to identify and characterize sources of input lag. An initial comparison of multiple input devices shows differences not only in average latency but also in its variance.