Ovulation Calculator
Estimate your ovulation day and fertile window from your last period and typical cycle length.
The cycle math
Fertile window = ovulation day − 5 through ovulation day
28-day cycle: ovulation around day 14, fertile window days 9–14, most fertile days 12–13.
32-day cycle: ovulation shifts to about day 18 — the luteal phase (ovulation to period) stays ~14 days, so longer cycles ovulate later.
24-day cycle: ovulation = cycle length − 14 = 24 − 14 = day 10. Fertile window = day 10 − 5 through day 10, so days 5–10; peak fertility days 8–9.
Compare that to the 28-day example above: the 4-day-shorter cycle pulls ovulation forward by exactly 4 days (day 14 → day 10), because the 14-day luteal phase is fixed and it's the earlier, more variable follicular phase that changes length. In a cycle this short, the fertile window can begin only a few days after the period ends.
Reading the fertile window
Because sperm can survive up to five days and the egg only about a day, the fertile window opens several days before ovulation and closes shortly after — the two days before ovulation are peak. Calendar prediction works best with regular cycles; if yours vary, ovulation predictor kits, basal body temperature charts, and cervical mucus changes give real-time confirmation this estimate can't. Pair with the due date calculator once pregnancy is confirmed.
Ovulation day by cycle length
| Cycle length | Estimated ovulation day | Fertile window |
|---|---|---|
| 21 days | Day 7 | Days 2–7 |
| 24 days | Day 10 | Days 5–10 |
| 28 days | Day 14 | Days 9–14 |
| 30 days | Day 16 | Days 11–16 |
| 32 days | Day 18 | Days 13–18 |
| 35 days | Day 21 | Days 16–21 |
Day count is measured from day 1 of the period (the first day of bleeding), not the last day. Each row assumes the standard 14-day luteal phase used throughout this page.
Common ovulation-tracking mistakes
- Assuming ovulation always falls on day 14. That's only true for a 28-day cycle — ovulation is counted back from the next period, so a 32-day cycle ovulates around day 18, not day 14 (see the table above).
- Counting the fertile window forward from the period instead of back from ovulation. The window is defined relative to ovulation day, not cycle start, which is why shorter cycles shift the entire window earlier.
- Trusting a calendar estimate on a cycle that varies by more than a few days month to month. Averages break down quickly with irregular cycles — an ovulation predictor kit or basal body temperature chart gives a real-time signal a calendar can't.
Frequently asked questions
When do I ovulate?
About 14 days before your next period — day 14 in a 28-day cycle, day 18 in a 32-day cycle.
What is the fertile window?
The ~6 days ending on ovulation; the two days before are most fertile.
How accurate is calendar prediction?
A cycle-average estimate — less reliable if irregular. Kits and BBT tracking are more precise.
Can I get pregnant right after my period ends?
Yes, especially with a short cycle — in a 24-day cycle, the fertile window can start around day 5, just a day or two after the period ends, since sperm can survive up to five days.
Does this work for irregular cycles?
Less reliably. The estimate assumes a consistent average, so a cycle that varies by more than a few days is better tracked with an ovulation kit or basal body temperature rather than a calendar alone.
Related calculators
Medical disclaimer: A calendar-based estimate, not medical advice or a contraceptive method. For fertility concerns or reliable family planning, consult a healthcare provider. Last reviewed: September 2026.