Comprehensive Calculator Guide
This calculator measures how well a contraceptive method works as the Pearl Index: pregnancies per 100 women using it for a year, from the number of pregnancies, women and months of use. 3 pregnancies among 100 women over 12 months is an index of 3. It also shows the chance of pregnancy over five years, useful for reading studies and comparing methods.
How it works
Raymond Pearl introduced the index in 1933: pregnancies × 1,200 ÷ total months of use (1,200 = 100 women × 12 months); studies counting cycles use 1,300. Typical use: IUDs and implants under 1, the pill about 7, male condoms about 13, withdrawal about 20.
It assumes a constant risk, though risk falls over time. Most scholars permit temporary family planning with both spouses’ agreement (Islamic Fiqh Academy, resolution 39, 1988), but not permanent sterilisation except in necessity.
How to Use
- Enter the unplanned pregnancies during use.
- Enter the number of women and months of use each.
- Press “Calculate” to see the index, effectiveness and the five-year chance.
Formula Used
Pearl Index = pregnancies × 1,200 ÷ total months of use | Chance over n years = 1 − (1 − index ÷ 100)^n
Practical Examples
- Example 1
- 3 pregnancies among 100 women over 12 months: 3 × 1,200 ÷ 1,200 = 3; about 14.1% over 5 years.
- Example 2: a larger study
- 26 among 400 women over 12 months: 26 × 1,200 ÷ 4,800 = 6.5; about 28.5% over 5 years.
Important Tips
- Compare typical-use figures, not just perfect use.
- Check the study’s size and length.
- Choose a method with a doctor.
Common Mistakes to Avoid
- Dividing pregnancies by women without the time of use.
- Comparing one-year and five-year studies.
- Reading an index of 1 as a 1% lifetime risk.
Frequently Asked Questions
- What does an index of 1 mean?
- About one pregnancy if 100 women used the method for a year.
- Which methods are most effective?
- IUDs and implants, under 1 in typical use.
- Why 1,200?
- The index is per 100 women per year: 100 × 12 months.