Estimate your next ovulation, fertile window, and period dates from your last period — plus a three-month cycle calendar.
Ovulation dates are estimates based on cycle length and cannot confirm ovulation. Cycle timing varies with stress, illness, sleep, travel, and hormonal conditions. This tool is not a contraceptive method and is not medical advice — speak with a clinician about fertility or irregular cycles.
Predictions assume a regular cycle of 28 days with a 14-day luteal phase. Dates are estimates, not a guarantee.
A regular 28-day cycle is shown as an example. Ovulation is estimated backwards from the next period using the luteal phase, which is more stable than the follicular phase for most people.
The luteal phase — the time from ovulation to the next period — averages about 14 days and is relatively stable from cycle to cycle. The fertile window is estimated as the five days before ovulation plus the day after, because sperm can survive several days while the egg survives roughly 12–24 hours.
Jordan's last period started on October 6, her cycle averages 28 days, and her luteal phase is 14 days.
| Phase | Typical timing (28-day cycle) | What happens |
|---|---|---|
| Menstrual | Days 1–5 | The uterine lining sheds. Cycle day 1 is the first day of full flow. |
| Follicular | Days 6–13 | Follicles mature and estrogen rises, rebuilding the lining. |
| Ovulation | Around day 14 | An egg is released. The fertile window spans the days just before. |
| Luteal | Days 15–28 | Progesterone supports the lining; if no pregnancy, the next period begins. |
Timing shifts with cycle length. The calculator places ovulation relative to your next period rather than assuming day 14.
Estimate your next ovulation date, six-day fertile window, and upcoming periods from your last period. See a three-month cycle calendar and a private fertility chart.
This calculator estimates when you are likely to ovulate and which days make up your fertile window. You enter the first day of your last period, your average cycle length, and — if you know it — your luteal phase, the number of days between ovulation and the next period. From those inputs it works backwards from your next predicted period to place ovulation, marks the six-day window around it, forecasts upcoming periods, and draws a three-month calendar. It also shows the estimated due date that would follow conception on the predicted ovulation day. Everything is calculated in your browser, so cycle dates never leave your device.
(1) Enter the first day of your last period and your average cycle length. (2) Adjust the luteal phase if you know it; 14 days is the default because it is the population average and tends to be more stable than the first half of the cycle. (3) The tool finds the start of your current cycle, adds the cycle length to get the next period, and subtracts the luteal phase to estimate ovulation. (4) It highlights the five days before ovulation plus the following day, marks your peak days, and projects the same pattern forward for future cycles. If today falls after this cycle’s ovulation, the next ovulation shown moves to the following cycle so the date is always useful for planning.
A textbook 28-day cycle with ovulation on day 14 is a useful reference, not a universal rule. Cycle length is driven mainly by the follicular phase, which can stretch or shrink with stress, travel, illness, sleep disruption, significant weight change, and intense training. The luteal phase is usually more consistent within one person, which is why this calculator anchors ovulation to the next period instead of assuming day 14. Conditions such as polycystic ovary syndrome, thyroid disease, and perimenopause can make cycles unpredictable, and postpartum or breastfeeding cycles may take time to settle. If your cycles are consistently shorter than 21 days, longer than 35 days, or very irregular, a clinician can help interpret what is happening rather than relying on calendar math alone.
Calendar prediction is the starting point; direct signs add evidence. Cervical mucus tends to become clearer, wetter, and stretchier as estrogen rises before ovulation. Basal body temperature rises slightly after ovulation, which confirms it happened but does not predict it in advance without several cycles of data. Ovulation predictor kits detect the luteinizing hormone surge roughly one to two days before ovulation, though they can be misleading in PCOS and do not guarantee that an egg was released. None of these methods is a contraceptive. If you are trying to conceive and cycles are irregular, or if you have been trying for a year without success (six months if you are over 35), professional evaluation is more useful than increasingly precise guesses from a calendar.
It is an estimate, not a measurement. The calculation assumes a regular cycle and a stable luteal phase, which is roughly true for many people but shifts with stress, illness, travel, sleep loss, and conditions such as PCOS. Use the predicted window as a planning guide and confirm ovulation with signs or tests if timing matters clinically.
The fertile window is generally the five days before ovulation plus ovulation day, with a possible extra day afterwards. Sperm can survive several days in the reproductive tract, while the egg survives roughly 12–24 hours. Conception is most likely in the two days before ovulation and on ovulation day.
No. Cycle-based predictions cannot prevent pregnancy because ovulation can happen earlier or later than estimated, and sperm can survive for days. If you do not want to conceive, use a method your clinician recommends rather than relying on calendar timing.
Use your average over the last three to six cycles, but expect wider uncertainty. If cycles vary by more than about a week, keep getting shorter, or stop altogether, the estimate becomes much less reliable — and those changes are worth discussing with a healthcare professional. Ovulation test strips or basal body temperature tracking can add direct evidence.
If conception occurs around ovulation, the estimated due date is ovulation plus 266 days (38 weeks). The more common clinical rule adds 280 days to the first day of the last period, which assumes a 28-day cycle; the two estimates line up when ovulation happens around day 14.
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