Raymond Pearl introduced the index in 1933. It is still used on contraceptive labels, although life-table analysis is often preferred because effectiveness changes over time. The lower the index, the more effective the method.
How to use the Pearl Index Calculator
- Pregnancies
- Exposure measured in
- Total exposure (all users)
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the number of pregnancies observed and the total exposure time from a study (or your own use).
Typical-use failure rates (first year)
| Method | Pregnancies per 100 women |
|---|---|
| Implant | 0.1 |
| Hormonal IUD | 0.4 |
| Copper IUD | 0.8 |
| Injection (every 3 months) | 4 |
| Pill | 7 |
| Patch | 7 |
| Vaginal ring | 7 |
| Diaphragm | 17 |
| Male condom | 13 |
| Female condom | 21 |
| Withdrawal | 20 |
| Fertility awareness methods | 15 |
| Spermicide | 21 |
| No method | 85 |
Inputs explained
| Field | What to enter |
|---|---|
| Pregnancies | A number |
| Exposure measured in | Choose one: Months, Menstrual cycles |
| Total exposure (all users) | A number |
What each result means
- Chance of pregnancy in one year of use = [Pearl Index]
- Chance of pregnancy over 5 years = (1 – (1 – [Pearl Index]÷100)^5)×100
Health note: this calculator is for education and screening only and is not a diagnosis or treatment advice. Discuss your results with a qualified clinician.
Formula
Pearl Index = pregnancies × 1200 ÷ months of exposure
or = pregnancies × 1300 ÷ cycles of exposure (13 cycles a year)
Worked example
Inputs — Pregnancies: 3; Exposure measured in: Months; Total exposure (all users): 1,200.
Results — Pearl Index = 3; Chance of pregnancy in one year of use = 3 %; Chance of pregnancy over 5 years = 14.1 %.
Key terms
- Woman-years
- Total time of exposure, e.g. 100 women for 1 year.
Frequently asked questions
What is a good Pearl Index?
Below 1 for highly effective methods such as implants, IUDs and sterilisation.
Why is it per 100 woman-years?
It standardises results so studies of different sizes can be compared.
What is the difference between perfect and typical use?
Typical use includes mistakes such as missed pills, so the index is higher.