This app uses bpm from the pulse in your finger to calculate a change in body temperature. Theoretically, an increase in your resting heart rate can be linked to an increase in temperature (among many other things.) The nice thing about the app is that it allows you to set your normal temperature and your average resting heart rate. This way, if you are not doing anything to raise your normal resting heart rate, you can calculate if your body temperature has increased by assuming that it goes up one degree for every 20ish beats your heart rate increases. The only problem with that assumption is that anything that increases your heart rate while scanning your finger (from cold medicine, physical exertion, coughing, etc.) will increase the apps "calculation" of your temperature. This also assumes that your body increases its temperature at one degree per 20 beat increase. This app can be fairly accurate if you can monitor and regulate your resting heart rate and know your body's normal temperature. By testing the app against a regular thermometer, you can get a good idea of how accurate it is for your own body, make adjustments to your settings in the app, and then you will have an idea of your body temperature with the app without using a conventional thermometer every time.