Ovulation — the release of an egg from the ovary — happens once per cycle, and the days leading up to it are the only time intercourse can lead to pregnancy. This calculator estimates when you ovulate from the first day of your last period and your usual cycle length, marks your fertile window on a calendar and projects the next six cycles.
How to use the ovulation calculator
- Enter the first day of your last period — the first day of real bleeding, not spotting.
- Enter your average cycle length, counted from the first day of one period to the first day of the next.
- Enter your usual period length so the calendar can shade those days.
- Leave the luteal phase at 14 days unless you have measured yours with ovulation tests or temperature charting.
- Read your estimated ovulation date and fertile window, check the calendar, and scroll down for the next six cycles.
How ovulation is estimated
The calculator counts back from the next expected period, because the time after ovulation is steadier than the time before it:
Later cycles are projected by adding the cycle length again and again.
Worked example
Last period started March 2, 2026; cycles average 28 days with a 14-day luteal phase.
Next period: March 2 + 28 = March 30. Ovulation: March 30 − 14 = March 16 (cycle day 15).
Fertile window: March 11 – 16, with March 14, 15 and 16 the most fertile days.
With a 32-day cycle, everything after the period moves four days later: ovulation on March 20 and a fertile window of March 15 – 20.
The often-quoted “day 14” counts from zero; counting the first day of bleeding as day 1, a textbook 28-day cycle ovulates on day 15.
How the fertile window works
Fertility rises steadily through the six-day window rather than switching on all at once. Intercourse five days before ovulation has a small chance of leading to pregnancy because few sperm survive that long; the odds climb over the following days and peak in the two days before ovulation and on the day itself. The day after ovulation adds little, because the egg survives only about a day once released. For couples trying to conceive, having intercourse every one to two days across the window covers it without needing to pinpoint the exact day.
Signs that ovulation is near
- Ovulation predictor kits detect the surge of luteinizing hormone (LH) in urine, which usually comes 24 to 36 hours before ovulation.
- Cervical mucus becomes clear, slippery and stretchy, similar to raw egg white, in the most fertile days.
- Basal body temperature rises by about 0.5 to 1 °F (0.3 to 0.5 °C) after ovulation and stays up until the next period. It confirms ovulation after the fact rather than predicting it.
- Mittelschmerz, a one-sided twinge of pelvic pain, is noticed by some people around ovulation.
When calendar predictions fall short
Average-based dates work best for regular cycles. Predictions are less dependable with cycles that vary by more than about a week, in the months after childbirth or stopping hormonal birth control, while breastfeeding, during perimenopause, and with conditions such as polycystic ovary syndrome (PCOS) or thyroid disorders. If cycles are frequently shorter than 21 days or longer than 35 days, that is worth raising with a clinician. For tracking upcoming periods and your current cycle day, use the period calculator.
This calculator is informational only and is not medical advice, and it is not a method of contraception. If you have been trying to conceive for 12 months (or 6 months if you are 35 or older), consult a healthcare professional.
Frequently asked questions
When am I most fertile?
The chance of pregnancy is highest from intercourse in the two days before ovulation and on ovulation day itself. In a regular 28-day cycle with a 14-day luteal phase, that is about cycle days 13 to 15, counting the first day of your period as day 1.
How long does the fertile window last?
About six days: the five days before ovulation plus ovulation day. Sperm can survive up to roughly five days, but an egg lives only about 12 to 24 hours after release, so the window closes soon after ovulation.
Do I ovulate on the same day every cycle?
Not necessarily. Even people with regular periods often ovulate a few days earlier or later from one cycle to the next, and stress, illness, travel and breastfeeding can shift it further. Calendar predictions are a starting point; ovulation tests or temperature charting show what actually happened.
Can I use an ovulation calculator for birth control?
No. A calendar estimate cannot account for cycle-to-cycle variation, so it is not a reliable way to avoid pregnancy. Talk with a healthcare professional about effective contraception or formal fertility-awareness methods taught by trained instructors.
What is the luteal phase?
It is the part of the cycle after ovulation and before the next period. It usually lasts 12 to 16 days and is more consistent than the first half of the cycle, which is why ovulation is estimated by counting back from the expected next period.