Women's Health tool
Period Calculator
Predict your next six periods, your cycle day and your likely fertile days from the first day of your last period and your cycle length. Free, nothing stored.
Nothing you enter leaves your device.
Inputs
Results
Enter values to see your results.
Simple steps
How it works
- 01
Enter the first day of your last period.
- 02
Enter your average cycle length and how many days your period usually lasts.
- 03
Select Calculate to see your next periods, fertile window and ovulation estimate.
Good to know
Frequently asked questions
How is my next period predicted?
Your cycle length is added to the first day of your last period, again and again for the next six cycles. A cycle is counted from the first day of one period to the first day of the next.
What is a normal cycle length?
Most adult cycles last 21 to 35 days, and 28 days is the average. A cycle that changes by a few days from month to month is common.
How accurate are the dates?
They assume your cycles stay the same length. Stress, illness, travel, breastfeeding and hormonal contraception can all shift them. Use the dates as a guide.
Can I use it to avoid pregnancy?
No. The fertile days are estimates for regular cycles and are not a method of contraception.
What do the fertile days mean?
They are the five days before the estimated ovulation and the day of ovulation itself, when pregnancy is most likely.
When should I see a doctor?
If your cycles are often shorter than 21 or longer than 35 days, your periods stop for three months or more, or bleeding is very heavy or painful.
Is anything I enter stored or sent?
No. The calculation runs in your browser. Nothing you type is saved or sent to a server.
How it is calculated
Each period starts one cycle length after the previous one. Ovulation is estimated 14 days before the next period, and the fertile window runs from 5 days before ovulation to the day of ovulation (Wilcox and colleagues, 1995). Cycles of 21 to 45 days can be entered.
Sources
- American College of Obstetricians and Gynecologists. Committee Opinion 651: Menstruation in girls and adolescents
- Wilcox AJ et al. Timing of sexual intercourse in relation to ovulation. N Engl J Med 1995;333:1517–21
Last reviewed: October 14, 2026