Period Calculator

Use this Period Calculator to estimate your next period, approximate ovulation date, and likely fertile window from your last period date and average cycle length.

Calculation Method

Next period start = first day of last period + average cycle length

Next period end = estimated next period start + period length − 1 day

Approximate ovulation = estimated next period start − 14 days

Estimated fertile window = about 5 days before ovulation through the estimated ovulation day

Example

If your last period started on January 1, your average cycle is 28 days, and your usual period lasts 5 days:

Next period start: January 29

Next period end: February 2

Approximate ovulation: January 15

Estimated fertile window: approximately January 10–15

Important Note

These are calendar estimates only. Actual ovulation and period timing can change from cycle to cycle, particularly when cycles are irregular.

Do not use this calculator as a birth-control method or to confirm pregnancy. If a period is late and pregnancy is possible, use an appropriate pregnancy test.

Sources

• American College of Obstetricians and Gynecologists — Ovulation and Fertile Timing
• NHS — Periods and Fertility in the Menstrual Cycle