Ovulation Calculator

What the calculator estimates

Ovulation is the release of an egg from an ovary, once in each menstrual cycle. Pregnancy is possible only for a few days around it, because the egg survives for roughly a day and sperm can survive in the body for up to about five days. Those days together are called the fertile window. This calculator uses the dates of your period to estimate when ovulation is likely, which days make up the fertile window, and when your next period should start, for the current cycle and the two after it.

How to use it

1. Enter the first day of your most recent period, meaning the first day of real bleeding, not spotting.
2. Enter your average cycle length: the number of days from the first day of one period to the first day of the next. If you have tracked a few months, use the average.
3. Leave the luteal phase at 14 days unless you know yours from tracking or from a doctor.

The method

A cycle has two halves. The first half, before ovulation, changes length from person to person and month to month. The second half, the luteal phase, from ovulation to the next period, is more constant and is commonly taken as 14 days. So the calculator works backwards from the expected next period:

• Next period = first day of last period + cycle length.
• Ovulation = next period - luteal phase length.
• Fertile window = 5 days before ovulation to 1 day after it.

The following cycles repeat the same steps from each expected period. If the period you enter was more than one cycle ago, the calculator moves forward to the cycle you are in now and tells you that the dates are projected.

A worked example

Your last period started on 6 October 2026 and your cycles are usually 30 days long. Your next period is expected on 5 November. Taking away a 14-day luteal phase puts ovulation on 22 October, so the fertile window runs from 17 to 23 October. The next cycle then starts on 5 November with ovulation around 21 November, and the third starts on 5 December with ovulation around 21 December. If tracking showed your luteal phase is 12 days, ovulation in the first cycle would move two days later, to 24 October.

Why it can be wrong for you

• Calendar methods assume your cycle repeats. Stress, illness, travel, breastfeeding, coming off hormonal contraception and conditions such as PCOS can all shift ovulation or stop it for a cycle.
• If your cycles vary by more than a few days, treat every date here as a rough guide. Ovulation test kits, basal body temperature charts and cervical mucus changes give signs from your own body.
• The calculator cannot confirm that ovulation happened.

Not medical advice

These dates are estimates for general information and are not medical advice. Do not use them to avoid pregnancy; calendar estimates are not reliable contraception. If you have been trying to conceive without success, have very irregular or painful periods, or have any concern about your health, please consult a doctor.