Ovulation Calculator
Ovulation is the one day each cycle when a released egg can be fertilised — yet the fertile window spans about six days, because sperm survive roughly five days while the egg lasts about one. Intercourse from five days before through the day after can result in pregnancy. This page estimates ovulation from your last period and cycle length, and returns the window and next period — an estimate, not contraception.
Enter values
Results
- Estimated ovulation day
- 15 September 2026
- Fertile window
- 11 Sept – 16 Sept 2026
- Expected next period
- 29 September 2026
How is the Ovulation calculated?
Enter the day, month and year of the first day of your last period and your typical cycle length in days, from 21 to 45. The calculator counts forward cycle length − 14 days to the estimated ovulation day — the standard assumption that ovulation happens 14 days before the next period. The fertile window opens four days before that day and closes the day after it, and the expected next period lands exactly one cycle length after your LMP. All three dates come back together.
Formula
Ovulation = LMP + cycle length − 14 days Fertile window = ovulation − 4 days to ovulation + 1 day Next period = LMP + cycle length where: LMP = first day of the last menstrual period The −14 encodes the luteal phase: ovulation arrives about 14 days before the next period, whether the cycle runs 24 days or 35.
Example
Last period: 1 November 2026 · Cycle: 28 days Estimated ovulation: 15 November 2026 · Fertile window: 11–16 November · Expected next period: 29 November 2026 With a 35-day cycle the same LMP puts ovulation on 22 November and the window at 18–23 November. Cycles vary month to month, so treat these as the month's best guess, not a measurement.
Things to remember
Apps narrow the window but rarely pin the day. To conceive, pair this estimate with an ovulation predictor kit, which detects the LH surge a day or so before release. Calendar arithmetic is not reliable contraception — late ovulation and sperm survival break it. After a year of trying, see a specialist.
Frequently Asked Questions
When in my cycle do I actually ovulate?
About 14 days before your next period begins — not always on day 14. In a 28-day cycle that is day 14; in a 35-day cycle it is closer to day 21. The stretch before ovulation varies from person to person and month to month; the stretch after it is remarkably fixed.
How long do sperm survive before ovulation?
Up to about five days in fertile cervical mucus, which is why the window opens four days before the estimated ovulation day. The egg itself survives roughly 24 hours after release, so the window closes the day after — a six-day span containing just one or two genuinely fertile moments.
Can I ovulate twice in one cycle?
Rarely. Two eggs released within about a day of each other can happen — that is how fraternal twins conceive — but a second fertile window a week later is not a normal pattern. Basal temperature tracking or ovulation predictor kits add real evidence to the calendar estimate.
Is this calculator reliable as contraception?
No. Calendar estimates cannot confirm the day ovulation actually happened, and cycles shift with stress, illness, travel and postpartum changes. Treat the dates as an estimate for planning conception; for prevention, speak to a clinician about a method that does not depend on predicting ovulation.
Related Calculators
- Due Date CalculatorPregnancy & Family PlanningEstimate your baby's due date from the first day of your last period, with cycle length adjusted and your gestational age worked out for today.
- Add Days CalculatorDate & AgeAdd or subtract any number of days from a date to find the resulting date and its weekday.
- Day of Week CalculatorDate & AgeFind the weekday, day of the year and ISO week number for any date, and see which ISO week early-January and late-December dates sit in.
- Date Difference CalculatorDate & AgeFind the exact number of days, weeks, months and years between any two dates, with inclusive and exclusive counts.