Ovulation Calculator
Enter the first day of your last period and your usual cycle length to estimate your ovulation day, fertile window and next period for the coming three cycles.
Last updated November 2, 2026
From the first day of one period to the first day of the next
Result
Estimated ovulation: November 16, 2026
| Measure | Value |
|---|---|
| Estimated ovulation day | Monday, November 16, 2026 |
| Fertile window | November 11, 2026 to November 16, 2026 |
| Next period expected | Monday, November 30, 2026 |
| Cycle | Fertile window | Ovulation | Next period |
|---|---|---|---|
| Cycle 1 | November 11, 2026 to November 16, 2026 | November 16, 2026 | November 30, 2026 |
| Cycle 2 | December 9, 2026 to December 14, 2026 | December 14, 2026 | December 28, 2026 |
| Cycle 3 | January 6, 2027 to January 11, 2027 | January 11, 2027 | January 25, 2027 |
Ovulation is estimated at 14 days before the next period, and the fertile window is the 5 days before it plus ovulation day. Real ovulation varies, especially with irregular cycles, so this is not a form of birth control. These dates are estimates. Cycles and pregnancies vary from person to person, and this tool cannot confirm a pregnancy or replace care from a health professional.
Show the work
Formula: Next period = last period + cycle length. Ovulation = next period - 14 days. Fertile window = 5 days before ovulation through ovulation day.
- Last period starts 2026-11-02; average cycle 28 days.
- Next period: 2026-11-02 + 28 days = 2026-11-30.
- Ovulation: 2026-11-30 - 14 days = 2026-11-16.
- Fertile window: 2026-11-16 - 5 days = 2026-11-11, through 2026-11-16.
Result: Estimated ovulation: November 16, 2026
How ovulation is estimated
ovulation = next period − 14 days
fertile window = 5 days before ovulation through ovulation day
Ovulation usually happens about 14 days before the next period starts, so with a 28-day cycle it falls around day 15 counting the first day of your period as day 1. Sperm can survive for several days, so the fertile window is estimated as the five days before ovulation plus ovulation day.
The fertile window shifts with cycle length: it covers days 8 to 13 of a 26-day cycle and days 12 to 17 of a 30-day cycle. Ovulation timing changes from cycle to cycle with stress, illness and other factors, so a calendar estimate can be off by several days. It is not a method of birth control.
See how we calculate for the formulas, sources and review schedule behind every calculator.
This is an estimate, not medical advice
The dates on this page are estimates. Cycle length and pregnancy length vary from person to person and from month to month, so real dates can differ by days or weeks. The tool cannot confirm a pregnancy, ovulation or fertility, is not a method of birth control, and does not replace care from a doctor or midwife. This page has not been reviewed by a medical professional.
Sources
Official and primary sources for the statements on this page. Their authors have not reviewed or endorsed this page.
- Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. N Engl J Med 1995;333(23):1517-1521. In 625 cycles, pregnancies came only from intercourse in the six days ending on the day of ovulation, the basis for a fertile window of the five days before ovulation plus ovulation day
- Wilcox AJ, Dunson D, Baird DD. The timing of the "fertile window" in the menstrual cycle: day specific estimates from a prospective study. BMJ 2000;321(7271):1259-1262. Finds the fertile window hard to predict even with regular cycles: only about 30% of women have it entirely within days 10 to 17
- Fehring RJ, Schneider M, Raviele K. Variability in the phases of the menstrual cycle. J Obstet Gynecol Neonatal Nurs 2006;35(3):376-384. In 141 regularly cycling women, cycle length averaged 28.9 days (SD 3.4) with 95% of cycles between 22 and 36 days, so real dates differ from a fixed-length estimate
The default of 14 days between ovulation and the next period is a common convention. We did not find a checked source for it, and the Fehring paper above reports that cycle phases vary widely between women.
How to use the Ovulation Calculator
- Enter the first day of your last period.
- Enter your average cycle length, from the first day of one period to the first day of the next.
- Read the estimated ovulation day, fertile window and next period, then use Copy, Print or Share.
Worked example
Last period began January 1, 2026 with a 28-day cycle. The next period is expected January 29, so ovulation is estimated on January 15 and the fertile window runs January 10 to 15.
| Cycle length | Ovulation (cycle day) | Fertile window (cycle days) |
|---|---|---|
| 26 days | 13 | 8 to 13 |
| 28 days | 15 | 10 to 15 |
| 30 days | 17 | 12 to 17 |
| 32 days | 19 | 14 to 19 |
Frequently asked questions
When do I ovulate?
Usually about 14 days before your next period. With a 28-day cycle that is around day 15 of the cycle.
How long is the fertile window?
About six days: the five days before ovulation and the day of ovulation.
What if my cycle is irregular?
Calendar estimates are unreliable for irregular cycles. A doctor can suggest other ways to track ovulation.
Can I use this to avoid pregnancy?
No. It is an estimate, not contraception. Talk to a health professional about birth control.
Are my numbers stored or sent anywhere?
No. The calculation runs in your browser and nothing you enter is saved or uploaded.