Finger pressure offers a new dimension for touch interaction, where input is defined by its spatial position and orthogonal force. However, the limited availability and complexity of integrated force-sensing hardware in mobile devices is a barrier to exploring this design space. This paper presents a synthesis of two features in recent mobile devices - a barometric sensor (pressure altimeter) and ingress protection - to sense a user's touch force. When a user applies force to a device's display, it flexes inward and causes an increase in atmospheric pressure within the sealed chassis. This increase in pressure can be sensed by the device's internal barometer. However, this change is uncontrolled and requires a calibration model to map atmospheric pressure to touch force. This paper derives such a model and demonstrates its viability on four commercially-available devices (including two with dedicated force sensors). The results show this method is sensitive to forces of less than 1 N, and is comparable to dedicated force sensors.