In one embodiment, a method includes recording, by a camera of a client device, a video of a region of a first user's skin and estimating, from the recorded video, a current ratio of ratios (RoR) for the first user corresponding to the first user's current blood-oxygen saturation. The method further includes converting the first user's determined RoR to a transformed RoR for the first user based at least in part on a baseline user's RoR determined while generating a trained SpO2 prediction model. The trained SpO2 prediction model is trained to estimate the baseline user's SpO2 value based on an input RoR value from the baseline user. The method further includes determining, by the trained SpO2 prediction model and based on the transformed RoR for the first user, the first user's current estimated blood-oxygen saturation.