Ovulation Calculator
Ovulation date calculator using your cycle length and luteal phase instead of day 14: the fertile window, the two peak days, next periods and when to test.
Results
Assumptions and formula
The luteal phase — ovulation to the next period — is the part of the cycle that varies least between people, so ovulation is counted back from the end: cycle length less luteal length, plus one, gives the cycle day. The fertile window is the five days before ovulation plus the day of it, because sperm can survive up to five days in fertile cervical mucus while the egg is viable for about one. The two highest-probability days are the day before ovulation and the day of it.
This is calendar arithmetic on the cycle length you entered, projected forward on the assumption that every cycle is that length. It cannot detect that ovulation has happened; stress, illness, travel and many conditions move it. Ovulation predictor kits, basal body temperature charting and cervical mucus observation measure the cycle you are in rather than predicting from past ones. A pregnancy test becomes reliable about 14 days after ovulation.
Worked example
A period starting 1 March 2026 on a 28-day cycle with a 14-day luteal phase puts ovulation on day 15, 15 March, with the fertile window 10–16 March and the peak on 14–15 March. On a 32-day cycle ovulation moves to day 19, 19 March — four days later than the day-14 assumption would give. On a 24-day cycle it moves to day 11.
Frequently asked questions
When do I ovulate if my cycle is not 28 days?
About 14 days before your next period, not 14 days after your last one. The luteal phase — ovulation to period — is the stable part of the cycle, usually 12 to 14 days, while the phase before ovulation is what stretches or shortens. So a 32-day cycle ovulates around day 18 and a 24-day cycle around day 10. Calculators that fix ovulation on day 14 are wrong for everyone whose cycle is not 28 days, which is most people.
How long is the fertile window?
About six days: the five days before ovulation plus the day of ovulation itself. Sperm can survive up to five days in fertile cervical mucus, while the egg is viable for roughly 12 to 24 hours after release. Intercourse after ovulation has passed is very unlikely to result in conception, which is why the window closes a day after rather than opening then.
Which days are most likely to result in conception?
The two days ending with ovulation — the day before and the day of. Probability rises through the window and falls sharply after ovulation. That said, the studies behind these figures are averages across cycles; your own ovulation may not land on the estimated day, which is why the window matters more than the single date.
Can I use this as contraception?
No. Calendar-based prediction is the least reliable form of family planning — typical-use failure rates are high because ovulation moves with stress, illness, travel and cycle irregularity, and the calendar cannot detect the change. If you are avoiding pregnancy, use a method designed for it and discuss options with a clinician.
When can I take a pregnancy test?
About 14 days after ovulation, which is around the day your period is due. Implantation typically happens 6 to 12 days after ovulation and hCG then takes a few days to reach detectable levels, so testing earlier often gives a false negative even when pregnant. If a test is negative and the period does not arrive, retest a few days later.
Can I use this as an ovulation tracker or fertility calculator month to month?
As a starting point, yes. Enter the first day of your last period and your usual cycle length and it lists the fertile window for the next several cycles like an ovulation calendar. A fertility calculator based on dates can only estimate, though: ovulation moves with stress, illness and travel, and cycles that vary by more than a few days make the prediction wide. If you are trying to conceive, pairing the calendar with ovulation tests or basal temperature narrows the window far better than dates alone.
Is a fertile window calculator the same as an ovulation calculator?
Nearly. An ovulation calculator estimates the single day the egg is released; a fertile window calculator widens that to the six days when conception is possible, the five days before ovulation plus the day itself, because sperm survive for up to five days and the egg for about one. This page reports both, and marks the two peak days inside the window, because the chance of conception is highest on the day before and the day of ovulation rather than spread evenly across the six.
Sources
Sources reviewed 13 September 2026: checked against their current editions on that date.
- Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. New England Journal of Medicine, 1995; 333(23): 1517–1521.
- Wilcox AJ, Dunson D, Baird DD. The timing of the "fertile window" in the menstrual cycle: day specific estimates from a prospective study. BMJ, 2000; 321(7271): 1259–1262.
- American College of Obstetricians and Gynecologists. FAQ: Fertility Awareness-Based Methods of Family Planning.
This page is an educational estimate and is not medical advice. Follow individual guidance from your clinician.