Health
Ovulation Calculator
Find your ovulation date and fertile window from your last menstrual period and cycle length.
Typical range: 21–35 days. Default is 28.
Ovulation Date
Fertile Window
Window Opens
Day
Ovulation
Day
Window Closes
Day
Next Period
Days to Ovulation
Cycle Calendar
How Ovulation Works
Follicular Phase
Day 1 – OvulationPeriod begins and estrogen rises, stimulating follicle growth. Ends at ovulation. Length varies cycle to cycle.
Ovulation
One dayA surge of LH triggers egg release from the dominant follicle. The egg survives 12–24 hours. Sperm can survive 3–5 days beforehand.
Luteal Phase
~14 daysThe corpus luteum produces progesterone to prepare the uterus. This phase is consistently ~14 days regardless of cycle length.
Ovulation by Cycle Length
| Cycle | Ovulation Day | Fertile Window |
|---|---|---|
| 21 days | Day 7 | Days 2–7 |
| 24 days | Day 10 | Days 5–10 |
| 26 days | Day 12 | Days 7–12 |
| 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 |
Ovulation Formula
Calendar-based ovulation estimate:
The luteal phase (ovulation → next period) is nearly always 14 days. Only the follicular phase length changes between women and cycles.
Frequently Asked Questions
Ovulation typically occurs 14 days before your next expected period — not 14 days after your period starts. The formula is: Ovulation day = First day of LMP + (cycle length − 14). For a 28-day cycle that starts January 1, ovulation falls around January 15. For a 32-day cycle starting January 1, ovulation falls around January 19 (32 − 14 = day 18 of the cycle).
The fertile window is the span of days when pregnancy is possible. Sperm can survive in the reproductive tract for up to 5 days, and an egg is viable for about 12–24 hours after ovulation. This means the fertile window spans the 5 days before ovulation through the day of ovulation itself — 6 days total. The two days before ovulation and ovulation day itself carry the highest conception probability.
It depends on your cycle length. Ovulation occurs at cycle day (cycle length − 14). For a 28-day cycle, that is day 14. For a 26-day cycle, day 12. For a 32-day cycle, day 18. The luteal phase (ovulation to next period) is consistently ~14 days in most women; what varies is the follicular phase (period to ovulation). Shorter cycles mean earlier ovulation, not a shorter luteal phase.
Yes. Stress, illness, significant weight change, travel, and hormonal shifts can push ovulation earlier or later than usual. Irregular cycles make calendar-based prediction less reliable. Tracking basal body temperature (BBT) or using LH ovulation test strips provides a more precise confirmation of actual ovulation timing each cycle.
Common physical signs include: (1) a rise in basal body temperature (BBT) of 0.2–0.5°C / 0.4–1.0°F that persists for 3+ days; (2) cervical mucus becoming clear, stretchy, and egg-white-like; (3) a mild one-sided pelvic ache called mittelschmerz; (4) a positive LH surge on an ovulation predictor kit (OPK), which typically peaks 24–36 hours before ovulation. BBT confirms ovulation has occurred; LH strips predict it 24–36 hours in advance.
A calendar-based ovulation calculator is most accurate for women with regular cycles of consistent length. Studies suggest calendar methods predict ovulation within ±2–3 days for regular cycles. For irregular cycles, accuracy drops significantly. OPK test strips are more accurate (detecting the LH surge 24–36 hours before ovulation), and ultrasound follicle tracking is the most precise method available.
Last updated: July 18, 2026