Predict your fertile window and ovulation day from your cycle — with next 3 cycles shown.
Count from the first day of one period to the first day of the next. The average is 28 days, but 21–35 days is common.
The luteal phase runs from ovulation to your next period and is usually 14 days. Leave it blank unless you track yours and know it differs.
Estimated ovulation day
Enter your last period to see your fertile window.
Next 3 cycles
| Cycle | Period starts | Fertile window | Ovulation | Next period |
|---|
This is an estimate for regular cycles only, and it must not be used as a contraceptive method — ovulation can shift from cycle to cycle, and sperm can survive several days. If your cycles are irregular, tracking signs like cervical mucus or ovulation tests is more reliable, and a clinician can help.
How to use this tool
Pick the first day of your most recent period from the calendar.
Count from the first day of one period to the first day of the next, averaged over a few cycles. Leave it at 28 if you are unsure.
Your estimated ovulation day, fertile window and next period appear instantly, with the next 3 cycles in the table.
If you track your luteal phase and know it is not 14 days, open Advanced and enter your own figure.
Formula & worked example
Last period started 1 October 2026, with a 28-day cycle and the usual 14-day luteal phase: ovulation is estimated 14 days later, on 15 October 2026, and the fertile window runs 10–15 October 2026. The next period is due 29 October 2026, with the following cycle ovulating around 12 November 2026.
Limitations — read this before you rely on it
This is an estimate for regular cycles only, not a diagnosis and not a contraceptive method. Ovulation can shift with stress, illness, travel, breastfeeding or hormonal conditions, and cycle length varies naturally. It cannot confirm ovulation happened, detect pregnancy or replace medical advice — see a healthcare professional if your cycles are irregular, if you have not conceived after 12 months of trying (6 months if over 35), or for any fertility concern.
Frequently asked questions
Enter the first day of your last period and your average cycle length. Ovulation is estimated as cycle length minus the luteal phase (usually 14 days) after your period starts.
The fertile window is the 5 days before ovulation plus ovulation day itself — the days when conception is possible, because sperm can survive for several days while the egg lives about 24 hours.
Yes — change the cycle length to your own average. A 28-day cycle is only the average; cycles of 21–35 days are common, and the prediction moves with your number.
No. Ovulation can shift from cycle to cycle even with regular periods, so a calendar estimate must never be relied on to avoid pregnancy. Use barrier contraception or talk to a clinician about reliable methods.
Calendar predictions assume regular cycles. With irregular cycles, polycystic ovary syndrome, breastfeeding or perimenopause, the estimate is much less reliable — ovulation predictor kits or tracking cervical mucus give a better signal, and a clinician can investigate persistent irregularity.