We present CarpetVR - a new system for marker-based positional tracking suitable for mobile VR (Figure 1). The system utilizes all sensors present on a modern smartphone (a camera, a gyroscope, and an accelerometer) and does not require any additional sensors. CarpetVR uses a single floor marker that we call the magic carpet. CarpetVR augments a standard mobile VR setup with a slanted mirror that can be attached either to the smartphone or to the head mount in front of the smartphone camera. As the person walks over the marker, the smartphone camera is able to see the marker thanks to the reflection in the mirror. Our tracking engine then uses a computer vision module to detect the marker and to estimate the smartphone position with respect to the marker at 40 frames per second. This estimate is integrated with high framerate signals from the gyroscope and the accelerometer. The resulting estimates of the position and the orientation are then used to render the virtual world. Our sensor fusion algorithm ensures minimal-latency tracking with very little jitter.