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Ovulation calculator

Your cycle, minus the mental arithmetic. Your fertile window, your likely ovulation day and when a test can tell you, for this cycle and the next two.

How the calculator works

Ovulation usually happens about 14 days before your next period starts. The time from ovulation to the next period is the most consistent part of the cycle. The time before ovulation varies more. So a longer cycle usually moves ovulation later, and a shorter one moves it earlier. The calculator counts back 14 days from your next expected period. Your own cycle may differ by a few days, so treat the dates as a guide.

Your fertile window

In a study of 221 women trying to conceive, nearly all pregnancies came from sex in the six days that ended on ovulation day. Sperm can live for up to five days, and an egg lives about a day after ovulation. The chance was highest in the last few days of that window, including ovulation day itself. ACOG gives a slightly wider window, from 5 days before ovulation to 1 day after, so the day after ovulation may count too.

Making the estimate better

  • Ovulation tests detect the hormone surge (LH) that comes before ovulation. A positive test usually means ovulation in the next 24 to 36 hours.
  • Cervical mucus often becomes clear, slippery and stretchy, like raw egg white, around ovulation.
  • Basal body temperature rises slightly after ovulation, so it confirms ovulation has happened rather than predicting it.

When to talk to your doctor

  • Your cycles are often shorter than 21 days or longer than 35 days, or very irregular.
  • You’ve been trying for 12 months, or for 6 months if you’re 35 or older. If you’re over 40, talk to your doctor now.
  • You have no periods, or a condition known to affect fertility, such as PCOS or endometriosis. Don’t wait the full 6 or 12 months.

This calculator isn’t a way to prevent pregnancy.

Sources

  1. American College of Obstetricians and Gynecologists, “Fertility Awareness-Based Methods of Family Planning.” acog.org
  2. Wilcox AJ, Weinberg CR, Baird DD. “Timing of sexual intercourse in relation to ovulation.” New England Journal of Medicine, 1995. nejm.org
  3. American College of Obstetricians and Gynecologists, “Evaluating Infertility.” acog.org
  4. MedlinePlus, “Ovulation home test.” medlineplus.gov
  5. American College of Obstetricians and Gynecologists, “Abnormal Uterine Bleeding.” acog.org
  6. American Society for Reproductive Medicine, “Fertility evaluation of infertile women: a committee opinion,” 2021. asrm.org