Calculate the Pearl Index for contraceptive failure rate per 100 women-years and compare typical versus perfect use.
The Pearl Index is a historical statistical measure of contraceptive effectiveness that expresses failure rate as the number of unintended pregnancies per 100 women-years of exposure. It remains a familiar summary metric for comparing contraceptive methods and for calculating trial-style failure rates from woman-time data.
The distinction between perfect use and typical use still matters for counseling because user-dependent methods usually have a much wider gap between the two. This page supports both a custom Pearl Index calculation and a comparison view for common contraceptive methods.
Multi-year projections on the page are illustrative only and should not be treated as the same thing as life-table efficacy data.
The Pearl Index is still a simple way to summarize annual contraceptive failure rate in a format many patients and clinicians recognize. It is also useful when you have raw study data and need to convert pregnancies and exposure time into a standard rate.
It is best used as a failure-rate summary rather than as an exact long-term forecasting tool.
Pearl Index = (Number of pregnancies / Women-months of exposure) × 1,200 Annual failure rate = Pearl Index Annual effectiveness = 100% - Pearl Index Illustrative constant-rate projection = 1 - (1 - annual rate/100)^k × 100
Result: Pearl Index 7.0 (typical use)
A typical-use Pearl Index of 7 means about 7 pregnancies per 100 women-years of use. That is useful as a counseling summary, but it should not be confused with the more rigorous life-table methods used in many contraceptive studies.
It gives a compact annual failure-rate summary that is easy to understand and compare across methods.
The Pearl Index can be misleading when follow-up is long or when failure rates change over time. That is why trial reports often pair or replace it with life-table methods.
Use this page to understand annual contraceptive failure rate and the difference between typical and perfect use, not to produce exact long-term pregnancy forecasts.
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This page calculates the Pearl Index from pregnancies divided by observed woman-time at risk, reported per 100 woman-years or equivalently per 1,200 woman-months. That is the historical and regulatory trial-style definition of the Pearl Index, and it is the defensible core output on the page.
The page should be read as a contraceptive-failure-rate summary rather than an exact long-term forecasting tool. Multi-year pregnancy projections depend on assumptions about constant annual hazard and are only illustrative; life-table or Kaplan-Meier methods remain the better way to describe cumulative contraceptive efficacy over time.
Perfect use assumes the method is used correctly and consistently every time. Typical use reflects real-world behavior such as missed pills, late injections, or incorrect barrier-method use.
Long-acting reversible contraception depends much less on day-to-day user behavior, so the typical-use and perfect-use rates are both very low.
It is based on pregnancies divided by woman-time actually at risk, usually reported per 100 women-years or per 1,200 woman-months.
The Pearl Index assumes a constant pregnancy rate over time, which is not always true. It can mislead in longer studies, and it does not describe cumulative efficacy as well as life-table or Kaplan-Meier methods. That is why the multi-year projections on this page are only rough illustrations, not official effectiveness estimates.
Dual method use is more complicated than plugging two annual rates into a simple formula, so real-world combined effectiveness should be discussed carefully rather than assumed from a rough multiplication shortcut.
Emergency contraception is usually described differently, because it is not a continuous ongoing method like the ones the Pearl Index was designed to compare.