Ovulation Calculator
Enter the first day of your last period and your cycle length to find your ovulation date and 6-day fertile window.
Ovulation Day by Cycle Length
| Cycle Length | Ovulation Day | Fertile Window Opens |
|---|---|---|
| 21 days | Day 7 | Day 2 |
| 24 days | Day 10 | Day 5 |
| 28 days | Day 14 | Day 9 |
| 30 days | Day 16 | Day 11 |
| 32 days | Day 18 | Day 13 |
| 35 days | Day 21 | Day 16 |
Day 1 = first day of menstrual period. Ovulation = cycle length − 14 days. Fertile window = 5 days before ovulation through ovulation day.
Frequently Asked Questions
When do I ovulate?
Ovulation typically occurs about 14 days before your next period, not 14 days after your last. For a 28-day cycle, ovulation is around day 14. For a 32-day cycle, it is around day 18. The formula is: Ovulation Day = Cycle Length − 14. The 6-day fertile window (5 days before ovulation + ovulation day) is when pregnancy is possible. Sperm can survive up to 5 days in the reproductive tract, so intercourse in the days leading up to ovulation is effective. The egg is only viable for 12–24 hours after ovulation.
What are signs of ovulation?
Fertile cervical mucus: becomes clear, stretchy, and slippery (like raw egg whites) as ovulation approaches. Basal body temperature (BBT): rises 0.2–0.5°F after ovulation and stays elevated. Ovulation pain (mittelschmerz): a one-sided pelvic ache or twinge lasting minutes to hours. LH surge: at-home ovulation predictor kits (OPKs) detect the LH surge 12–36 hours before ovulation — the most reliable at-home indicator. Breast tenderness and slight increase in libido may also occur.
How accurate is this ovulation calculator?
This calculator is based on the standard assumption that ovulation occurs 14 days before the next period. It is accurate for women with regular, predictable cycles. However, many factors can shift ovulation: stress, illness, travel, significant weight changes, and medications (including hormonal birth control). Cycles vary month to month. For the highest accuracy, combine this calculator with LH predictor tests (OPKs) and basal body temperature (BBT) charting. If your cycles are irregular (varying by more than 7 days), consult a gynecologist or reproductive endocrinologist.
How long is the fertile window?
The fertile window is approximately 6 days: the 5 days leading up to ovulation plus ovulation day itself. The highest pregnancy rates occur from intercourse 1–2 days before ovulation. Sperm can survive up to 5 days in fertile cervical mucus, which is why the window opens 5 days before ovulation. After the egg is released, it is only viable for 12–24 hours. Timing intercourse every 1–2 days during the fertile window gives the best chance of conception.