Read Your Body does not have an effectiveness rate because it’s simply a digital data recording and visualisation tool. It doesn’t include any predictions or auto-interpretations. It’s not a contraceptive device. Responsibility for all outcomes is in your hands.
You can use Read Your Body to record daily observations of scientifically validated biomarkers like cervical fluid, temperature and hormone tests. You can then interpret your patterns yourself according to the guidelines of your chosen fertility awareness-based method (FABM).
Modern fertility awareness-based methods (FABMs) can be highly effective for avoiding pregnancy with perfect use. Actual effectiveness depends on your chosen method, understanding, motivation and practices. You can read about the evidence base for FABMs here.