Each cycle, an egg is released roughly in the middle — and it survives only 12–24 hours. Sperm, however, can wait in the reproductive tract for up to 5 days. That mismatch creates a fertile window of about 6 days ending on ovulation day, and timing intercourse within it is what matters for conception.
This calculator predicts ovulation as 14 days before your next expected period (the luteal phase is the most consistent part of the cycle), then marks the 5 days before plus ovulation day as your fertile window. It also projects your next few periods so you can plan ahead.
Part of the Health Calculators collection on CountCrunch.
How ovulation day is predicted
Example: with a 30-day cycle and a last period starting 1 October, ovulation lands around 16 October and the fertile window runs 11–16 October. With a 28-day cycle, ovulation is around day 14.
Why the window is 6 days
Pregnancy is most likely from intercourse in the 2–3 days before ovulation. The day before ovulation typically shows the highest conception probability; by the day after ovulation, the chance drops close to zero.
Body signs that confirm ovulation
- Cervical mucus turns clear, stretchy and egg-white-like on fertile days.
- LH surge: ovulation predictor kits detect the hormone spike 12–36 hours before release.
- Basal body temperature rises slightly after ovulation — it confirms ovulation happened rather than predicting it.
- Mittelschmerz: some people feel a one-sided twinge at ovulation.
Frequently asked questions
When am I most fertile in my cycle?
The 2–3 days before ovulation through ovulation day itself. With a 28-day cycle, that is roughly days 11–14 counting from the first day of your period.
Can I get pregnant outside the fertile window?
Very unlikely. The egg lives 12–24 hours after ovulation, so intercourse more than a day after ovulation — or more than 5 days before — rarely leads to pregnancy.
How accurate are ovulation calculators?
They predict based on averages; real cycles vary. They work best with regular cycles. Ovulation test strips or temperature tracking confirm what actually happened in a given cycle.
My cycles are irregular. What should I do?
Calendar predictions get shaky with irregular cycles. LH test strips used daily around mid-cycle, or tracking cervical mucus, give more reliable real-time signals. See a clinician if cycles are consistently very short, very long or unpredictable.
Can this be used to avoid pregnancy?
Calendar-only methods have a high failure rate (around 15–25% with typical use) because ovulation timing shifts. Do not rely on a calculator alone for contraception.