How it works
Ovulation is when an ovary releases an egg. In most cycles it happens about 14 days before the next period, not 14 days after the last one — the phase after ovulation (the luteal phase) is fairly constant, while the phase before it varies. So the calculator first predicts the next period, then counts back 14 days.
next period = last period + cycle length · ovulation ≈ next period − 14 daysSperm can survive up to about five days, and the egg about one day, so the fertile window is the five days before ovulation, the day of ovulation and the day after.
fertile window = ovulation − 5 days … ovulation + 1 dayWorked example. Last period started on 1 November 2026 with a 28-day cycle. Next period ≈ 29 November; ovulation ≈ 29 November − 14 = 15 November 2026; fertile window ≈ 10 to 16 November. With a 32-day cycle, ovulation moves to about 19 November.
How reliable is this?
Calendar methods are estimates. Cycles vary from month to month, and stress, illness, breastfeeding and conditions such as polycystic ovary syndrome can shift ovulation. The predictions are most useful when your cycles are regular. Ovulation test kits (which detect the LH surge), basal body temperature and changes in cervical mucus give more direct signs.
- Trying to conceive: intercourse every one to two days during the fertile window gives the best chance.
- Avoiding pregnancy: a calendar calculator is not a reliable method of contraception. Talk to a doctor about effective options.
- When to seek advice: if you have tried for 12 months (or 6 months if you are over 35), or if your cycles are very irregular.
Already pregnant? Estimate your due date with the pregnancy calculator. To count days to a date, use days from today.