Many mobile AR apps that use the front-facing camera can benefit significantly from knowing the metric scale of the user’s face. However, the true scale of the face is hard to measure because monocular vision suffers from a fundamental ambiguity in scale. The methods based on prior knowledge about the scene either have a large error or are not easily accessible. In this paper, we propose a new method to measure the face scale by a simple user interaction: the user only needs to swing the phone to capture two selfies while using the recently popular Dual Camera mode. This mode allows simultaneous streaming of the front camera and the rear cameras and has become a key feature in many social apps. A computer vision method is applied to first estimate the absolute motion of the phone from the images captured by two rear cameras, and then calculate the point cloud of the face by triangulation. We develop a prototype mobile app to validate the proposed method. Our user study shows that the proposed method is favored compared to existing methods because of its high accuracy and ease of use. Our method can be built into Dual Camera mode and can enable a wide range of applications (e.g., virtual try-on for online shopping, true-scale 3D face modeling, gaze tracking, and face anti-spoofing) by introducing true scale to smartphone-based XR.