How this estimate works
Every birth control method has a published ‘typical use’ failure rate — the percentage of users who become pregnant within one year of typical (not perfect) use, based on large population studies. This calculator uses those commonly cited annual rates as a starting point.
Extending it over multiple years
To estimate the cumulative chance of pregnancy over several years of consistent use, the calculator applies the standard cumulative-probability formula: Chance = 1 − (1 − annual failure rate)^number of years, assuming the same method and behavior every year.
These are population-level averages, not a prediction for any individual, and actual effectiveness depends heavily on correct and consistent use. This tool is for general educational purposes only and is not a substitute for guidance from a qualified healthcare professional.