How to use it
- Enter the first day of your last period.
- Enter your usual cycle length – the days from the start of one period to the start of the next. If it varies, use the average of the last few cycles.
- Read the estimated ovulation day, the fertile window and the dates for the next three cycles.
How it works
- Ovulation = first day of the period + (cycle length − 14) days. The 14 days are the average luteal phase; in real cycles it varies from person to person and from cycle to cycle.
- Fertile window = the 5 days before ovulation plus the day of ovulation, following the study of Wilcox, Weinberg and Baird (1995), which found conception happened almost only in this six-day interval.
- Next period = first day + cycle length; the table repeats the calculation for three cycles.
Worked example
Period started on 1 November 2026, 28-day cycle: ovulation 28 − 14 = 14 days later, on 15 November. The fertile window runs from 10 to 15 November, and the next period is expected on 29 November. With a 32-day cycle ovulation moves to 19 November.
Important
This is an estimate based on averages, not a medical test, and not a method of contraception. Stress, illness, travel, breastfeeding and hormonal changes can shift ovulation. For trying to conceive, ovulation tests give more precise timing; for birth control or fertility concerns, talk to a doctor.