Predict your most fertile days and ovulation date from your cycle length. Includes a 6-month fertility calendar.
Fertility phases throughout a typical 28-day menstrual cycle, with ovulation occurring on day 14.
| Cycle day | Phase | Fertility |
|---|---|---|
| 1–5 | Menstruation | Low |
| 6–11 | Pre-ovulatory (follicular) | Low–moderate |
| 12–14 | Fertile window peak | High (ovulation day 14) |
| 15–17 | Post-ovulation (luteal) | Moderate (egg survives 12–24h) |
| 18–28 | Luteal phase | Low |
Source: American College of Obstetricians and Gynecologists (ACOG); Wilcox et al. NEJM 1995.
Calculate your fertile window and ovulation date
Ovulation typically occurs ~14 days before your next period (the luteal phase). The fertile window includes the 5 days before ovulation (sperm can survive that long) plus the day of ovulation. Peak fertility is the 2 days before and including ovulation.
This calculator uses the calendar method and is most accurate for regular cycles. Irregular cycles may vary significantly.
Cross-checked against authoritative formulas (IRS, NIST, ACSM) and updated for 2025/2026 tax years and rates.
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FAQ
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An ovulation calculator is a free fertility tool that estimates your fertile window and ovulation day from your cycle length. Our ovulation calculator projects the next 3-6 months of fertile days. The ovulation calculator is used by women trying to conceive or avoid pregnancy and works best for women with regular 21-35 day cycles.
How it works
Our ovulation calculator lets you estimate when you ovulate and identify your most fertile days using your cycle length, in three steps.
Type the average length of your menstrual cycle in days, from the first day of one period to the first day of the next. Most women have 26-32 day cycles. The calculator uses the difference (cycle length - 14 days) to estimate ovulation day. If your cycles vary by more than 7 days, use your shortest recent cycle for the most conservative fertile window.
Type the start date of your most recent period. The calculator projects your next 3-6 months of fertile windows so you can plan ahead. If you have irregular cycles, you can also enter your shortest and longest recent cycles to see the full range of possible ovulation days.
The result shows your estimated ovulation date, the 6-day fertile window, and the next several months of projections. Peak fertility is the day before ovulation and ovulation day itself. To conceive, have sex every 1-2 days during the fertile window. To avoid pregnancy, use protection during the entire window.
Estimate your conception date and fertile window from your due date or last period.
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