Ovulation Calculator
Enter the first day of your last period and your typical cycle length to estimate your ovulation day, your most fertile window, and when your next period is due.
Nothing you enter is sent anywhere — the calculation happens in your browser.
How the math works
fertile window = 5 days before ovulation through 1 day after
The second half of the cycle (after ovulation) is remarkably fixed at about 14 days for most women, while the first half is the part that stretches or shrinks with cycle length. That's why we count backward from the next expected period rather than forward from day 14: on a 26-day cycle you likely ovulate around day 12, on a 32-day cycle around day 18. The fertile window is wider than ovulation day itself because sperm can survive up to five days in reproductive tract fluid, while the egg is viable for only 12–24 hours after release — so conception is possible from intercourse starting about five days before ovulation until roughly a day after.
How accurate is a calendar prediction?
Reasonable as a planning tool, imperfect as a measurement. Even women with regular 28-day cycles don't always ovulate on day 14 — in a classic study of women with regular cycles, only about 30% of cycles put ovulation inside the textbook days-10-to-17 window, and the same woman's ovulation day could shift by several days from one cycle to the next. If precise timing matters, watch for egg-white cervical mucus, use LH ovulation predictor kits, or track a small rise in waking body temperature (which confirms ovulation after it has happened, since the temperature rise is driven by progesterone).
FAQ
- What if my cycles are irregular?
- Then any calendar estimate is a rough guess — the variation usually comes from the pre-ovulation phase this method assumes is predictable. Ovulation predictor kits or tracking cervical mucus will serve you much better than dates alone.
- What are the signs that ovulation happened?
- The most reliable everyday signs: stretchy, egg-white-like cervical mucus in the days before; a basal body temperature rise of roughly 0.5–1°F that stays elevated for about three days after; and sometimes mild one-sided lower-abdominal twinges (mittelschmerz).
- How many days after my period can I get pregnant?
- On a short cycle, potentially within days of bleeding ending — sperm survive up to five days, so intercourse during your period can lead to conception if you ovulate early. This is why the fertile window, not "safe days," is what matters.
This tool provides general information only and is not medical advice, and it is not a contraceptive method. For fertility or cycle concerns, talk to a qualified healthcare professional.