Ovulation Calculator

How to use the ovulation calculator

1. Enter the First day of your last period, the first day of bleeding.
2. Enter your Average cycle length (days), counted from the first day of one period to the day before the next (20 to 45 days).
3. Leave the Luteal phase (days) at 14 unless you know yours, for example from temperature charts or ovulation tests (10 to 16 days accepted).
4. Choose how many Cycles to show (1, 3 or 6).
5. Read the Estimated fertile window, the Most fertile days (estimate), the Estimated ovulation with its cycle day, the Next period expected and the date to take a Pregnancy test from. The table lists the following cycles. Clear resets the fields.

Everything is calculated in your browser; your dates are not sent anywhere.

Important: this tool must not be used as contraception. The results are estimates that can be wrong by several days, even with regular cycles.

How the estimate works

A menstrual cycle has two phases. The first (follicular) phase, from the period to ovulation, varies a lot between people and between cycles. The second (luteal) phase, from ovulation to the next period, is more stable and lasts about 14 days on average, although 10 to 16 days is common. The calculator therefore works backwards from the next expected period:

• next period = last period + cycle length;
• ovulation ≈ next period − luteal phase (14 days by default);
• fertile window = the 5 days before ovulation plus the day of ovulation.

Worked example: last period on 1 January 2026 with a 28-day cycle. The next period is expected on 29 January, ovulation around 15 January (cycle day 15), and the fertile window runs from 10 to 15 January. With a 32-day cycle, ovulation moves to around 19 January.

The six-day window comes from the classic study by Wilcox and colleagues (New England Journal of Medicine, 1995): sperm can survive up to about five days in the female tract, while the egg lives for roughly 12 to 24 hours. The chance of conception was highest for intercourse in the two days before ovulation and on the day itself, which the tool shows as the Most fertile days (estimate).

Why cycles make estimates unreliable

Large studies of recorded cycles show that ovulation often occurs on a different day than calendar methods predict, and that even people with regular periods have cycles that vary by several days. Stress, illness, travel, breastfeeding, stopping hormonal contraception, thyroid problems or polycystic ovary syndrome (PCOS) can all shift or suppress ovulation. Ovulation predictor kits (which detect the LH surge), basal body temperature and cervical mucus observation give information about the current cycle rather than an average.

If you are trying to conceive, regular intercourse every two to three days across the cycle covers the fertile window without relying on exact dates. See a doctor if you have been trying for 12 months (6 months if you are 35 or older), or sooner if your cycles are very irregular or absent.

For contraception, speak with a doctor, midwife or family planning service. Fertility-awareness methods require training and daily observations, not a calendar estimate.