Ovulation Calculator
Enter the first day of your last period and your average cycle length to estimate your ovulation days, fertile windows and next periods for the coming three cycles.
Enter the first day of your last period and tap Calculate ovulation.
| Cycle | Fertile window | Ovulation | Next period |
|---|
Ovulation is estimated 14 days before your next expected period; the fertile window runs from 5 days before to 1 day after ovulation.
How ovulation is estimated
After ovulation, the second half of the cycle (the luteal phase) lasts about 14 days in most people, while the first half varies. So the calculator counts back 14 days from your next expected period: with a 28-day cycle that is day 14, and with a 32-day cycle day 18.
The fertile window
Sperm can survive in the reproductive tract for up to about 5 days, and an egg lives for roughly 12–24 hours after ovulation. The fertile window is therefore about 6 days: the 5 days before ovulation and the day of ovulation, with one extra day shown after it to allow for error. In a classic study by Wilcox and colleagues, conception happened only after intercourse in the 6 days ending on ovulation day, with the highest chances on the day of ovulation and the 2 days before it.
Example
If your last period started on October 1, 2026 and your cycle is 28 days, your next period is due October 29. Estimated ovulation is October 15, and your fertile window is about October 10 to October 16.
Getting a better estimate
A calendar estimate is only as good as the regularity of your cycles. You can narrow down ovulation with:
- Ovulation predictor kits (LH tests): detect the hormone surge that comes 1–2 days before ovulation.
- Cervical mucus: becomes clear, slippery and stretchy, like raw egg white, around your most fertile days.
- Basal body temperature: rises slightly after ovulation. It confirms ovulation has happened rather than predicting it.
Once you are pregnant, the Pregnancy Due Date Calculator shows how far along you are and when common prenatal tests take place.
Frequently asked questions
Can I use this as birth control?
No. A calendar estimate is not a reliable form of contraception. Fertility awareness–based methods need training and daily tracking of several signs, and even then they fail more often than most other methods. Talk to a healthcare provider about birth control.
My cycles are irregular. Does it still work?
The estimate becomes much less accurate when cycle length changes from month to month. Track your cycles for a few months, and use LH tests or other signs. If your cycles are often shorter than 21 days or longer than 35 days, or you have been trying to conceive for a year (6 months if you are 35 or older), see a doctor.
Why does it skip ahead?
If the period date you entered was more than one cycle ago, the calculator assumes your periods have continued at the same length and starts with the current cycle. For the best result, enter the first day of your most recent period.
Sources
- ACOG FAQ, Fertility Awareness-Based Methods of Family Planning.
- Wilcox AJ, Weinberg CR, Baird DD, Timing of sexual intercourse in relation to ovulation, N Engl J Med 1995;333:1517–21.