Last updated October 5, 2026 · Report an error
The Pearl Index is the standard published public-health statistic for contraceptive effectiveness: Pearl Index = (number of unintended pregnancies x 1300) / total months of contraceptive exposure. The result is expressed as unintended pregnancies per 100 woman-years of use. This is a population-level statistical formula used in contraceptive research, not a prediction of individual risk. For general reference and educational purposes only.
How it works
Enter the number of unintended pregnancies observed and the total months of contraceptive exposure across the study population. The tool applies the standard Pearl Index formula.
- Enter number of unintended pregnancies.
- Enter total months of contraceptive exposure.
- Click Calculate to see your results.
Examples
3 pregnancies over 1200 months of exposure
Pearl Index = (3 x 1300) / 1200 = 3900 / 1200 = 3.25.
Who should use it
- Reproductive health research and literature review.
- Public health and epidemiology education.
Industry applications
- Contraceptive research
- Public health statistics education
Advantages
- Simple, standardized, long-used research statistic.
- Allows comparison across different studies and methods.
Limitations
- Does not account for changing risk over time (e.g., first-year use vs. later years).
- Small studies produce imprecise estimates.
Common mistakes to avoid
- Using calendar months of the study instead of actual total months of contraceptive exposure (which accounts for dropouts).
- Treating a small study's Pearl Index as precise — small sample sizes produce wide confidence intervals.
Best practices
- Use the Pearl Index alongside its confidence interval and study size when comparing contraceptive methods.
Tips
- Newer contraceptive research increasingly reports life-table (Kaplan-Meier) failure rates alongside or instead of the Pearl Index, since it better captures changing risk over time.