The latency of a computing system affects users' performance. One important component of end-to-end latency is display lag - the time required to turn framebuffer contents into photons emitted by a computer screen. However, there is no well-documented and widely available method for measuring display lag.Thus, the effect of display lag is rarely considered in scientific studies and system development. We developed DispLagBox, a simple open-source device for measuring display lag. It supports the International Display Measurements Standard but also offers additional metrics for characterizing display lag with a resolution of 0.1 ms. The device, based on a Raspberry Pi computer, measures the time between VSYNC and a change in brightness on the connected display. Repeated measurements can be conducted automatically, so that not only average latency but also latency distributions for each device can be reported. For most displays we tested, DispLagBox reports latencies that are close to those reported by a commercial black-box measurement device. Typically, the difference is 1 - 3 ms.