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Ovulation Calculator

Enter the first day of your last period and your average cycle length. You get your estimated ovulation day, your fertile window and your next period. If your cycles vary, enter your shortest and longest cycle and the window widens to cover them.

Your numbers

Fill in with the longest cycle if your cycles are irregular.

The days from ovulation to your next period. 14 is typical.

Result

Estimate for information only, not professional advice. Last reviewed .

How we calculate it

Ovulation happens about 14 days before the next period starts, so it falls (cycle length − 14) days after the first day of your last period. In a 28 day cycle that is day 15, counting the first day of the period as day 1.

Sperm can live for up to about 5 days and the egg for about a day after it is released. The fertile window therefore runs from 5 days before ovulation to 1 day after, about 7 days.

With irregular cycles the window starts 19 days before the end of your shortest cycle’s ovulation count and ends one day after your longest cycle’s ovulation. It is wider because the day of ovulation moves from month to month.

Assumptions and limits

  • This is an estimate for general information. It is not medical advice and cannot replace a conversation with a doctor or midwife. It is not a form of birth control.
  • Stress, illness, travel, breastfeeding and some medicines move ovulation. Ovulation test strips or tracking body temperature give a better signal than a calendar.
  • See a doctor if you have been trying to conceive for 12 months (6 months if you are 35 or older), or if your cycles are shorter than 21 or longer than 45 days.
  • Sources: ACOG, "Trying to Get Pregnant? Here’s When to Have Sex" (ovulation about 14 days before the period; sperm live up to 5 days; the egg 12 to 24 hours); MedlinePlus, "Pregnancy - identifying fertile days"; Wilcox et al., "The timing of the fertile window in the menstrual cycle" (BMJ 2000, PMC27529).

Frequently asked questions

When do you ovulate?

About 14 days before your next period. For a 28 day cycle that is around day 14 to 15; for a 32 day cycle it is around day 18 to 19.

How long is the fertile window?

About six or seven days: the 5 days before ovulation, ovulation day and the day after. Sex in the days before ovulation is most likely to lead to pregnancy.

Can I use this with irregular cycles?

Yes, enter your shortest and longest recent cycles. The window gets wider, which is honest about the uncertainty. If cycles are very irregular, ovulation test strips work better than a calendar.

What is the luteal phase?

The time between ovulation and the start of your next period. It is usually 12 to 14 days and stays fairly constant for each person, which is why ovulation is counted back from the next period.

Is a calendar method a reliable way to avoid pregnancy?

No. Timing methods have a high failure rate compared with other birth control. Use this to plan, not to prevent, and talk to a doctor about contraception.

Related calculators

Worked example (a fixed example using the starting numbers, it does not change as you type)

Inputs: First day of your last period = January 1, 2026; Average cycle length (days, 21 to 45) = 28; Luteal phase (days, 10 to 16) = 14.

  • Fertile window Jan 10, 2026 to Jan 16, 2026
  • Estimated ovulation day Thursday, January 15, 2026
  • Next period expected Thursday, January 29, 2026
  • Days in the fertile window 7
  • Luteal phase used 14 days

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Last reviewed: . Formulas and assumptions are described on our methodology page. For information only; not professional advice.