Commercial smartwatches and fitness trackers are integrating increasingly advanced physiological sensors. For optimal performance, such devices need to be firmly coupled to the body, yet also remain comfortable when worn for extended periods of time. Existing solutions for measuring the contact force in order to ensure it is in an optimal tightness range typically depend on direct force measurement, but this adds hardware, and therefore cost, to the devices. This paper presents a novel method for estimating contact force by using only an optical heart rate sensor, as already found in many wearable devices. Initial tests indicate that the proposed method can estimate contact force with a mean absolute error of 0.36N, on par with FSRs. This new approach has the potential to expand the utility of existing sensors for both researchers and end-users, with anticipated applications not only in optimizing physiological sensing, but also in haptic information delivery.