Pearl Index Calculator (contraceptive failure rate per 100 woman-years)

Understanding the Pearl Index calculator — concept and principle

The Pearl Index is the classic way to report how often a contraceptive method fails in a study: the number of unintended pregnancies per 100 woman-years of use. This calculator computes it from the pregnancies and the exposure time, and projects the chance of pregnancy over several years of use at that rate.

Pregnancies per 100 years: with the default inputs the result is Pearl 3 (13.9% over 5 years).

How it is calculated

Pearl Index = pregnancies × 1,200 ÷ woman-months, or × 1,300 ÷ menstrual cycles, or × 100 ÷ woman-years

Woman-years = months ÷ 12, or cycles ÷ 13

Chance of pregnancy over n years = 1 − e^(−PI × n ÷ 100)

Pregnancies per 1,000 woman-years = PI × 10.

The principle

Raymond Pearl (1933) needed a fair way to compare methods tested on different numbers of people for different lengths of time. Dividing pregnancies by total exposure time gives a rate, like an incidence rate. The multi-year projection assumes a constant rate and uses the exponential (Poisson) model, in which the chance of no event over n years is e^(−rate × n).

Worked example

Default inputs: 3 pregnancies, 1,200 woman-months, 5 years.

Exposure = 1,200 ÷ 12 = 100 woman-years.
Pearl Index = 3 ÷ 100 × 100 = 3; per 1,000 woman-years = 30.
Over 5 years = 1 − e^(−0.03 × 5) = 1 − e^(−0.15) = 13.9%.

Each point is the calculator re-run with only “Total exposure” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Total exposure	Pearl Index (per 100 woman-years)
300	12
600	6
900	4
1,200	3
1,500	2.4
1,800	2
2,400	1.5

How to read the result

A lower index means a more effective method. Figures differ between perfect use and typical use, so read which one a study reports. The failure rate usually falls with time because people for whom a method fails drop out early and users gain experience, so long studies tend to show lower indices; life-table methods handle this better. The multi-year projection is therefore a cautious upper-end estimate.

Common mistakes

Comparing indices from studies of very different length.
Counting months when the method was not used.
Reading the index as a percentage chance for one person in one year without considering the study population.

Keep in mind: This is a statistical tool, not personal contraceptive advice. Discuss contraception with a doctor, nurse or pharmacist.

Frequently asked questions

What does a Pearl Index of 1 mean?

One unintended pregnancy per 100 women using the method for a year.

Why 13 cycles per year?

A year has about 13 menstrual cycles of 28 days, so 1,300 converts cycles to woman-years.

Is the Pearl Index still used?

Yes, especially in regulatory studies, alongside life-table rates.

Sources: Pearl R, "Factors in human fertility and their statistical evaluation", Lancet 1933;222:607–611 · Trussell J, "Contraceptive failure in the United States", Contraception 2011;83(5):397–404

Checked 9 Nov 2026. Standard definition: Pearl R, “Factors in human fertility and their statistical evaluation”, Lancet 1933;222:607–611 — pregnancies × 1,200 ÷ months of exposure (× 1,300 ÷ cycles); constant-rate projection is a standard exponential (Poisson) assumption.

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