Estimate your ovulation day, fertile window and next period from your last period and cycle length.
Ovulation is estimated for the day that falls 14 days before your next period, which for a 28-day cycle is 14 days after the first day of the last one. The NHS says ovulation usually happens about 14 days before the start of the next period but depends on how long the cycle is. The page uses that calendar method: ovulation = start of the period + cycle length − luteal phase. If your last period began on 1 October 2026 and your cycle is 28 days, the next period is due on 29 October and ovulation is estimated for Thursday 15 October 2026. With a 35-day cycle and the same 14-day luteal phase, ovulation falls 21 days after the start, not 14, which is why the cycle length matters more than the textbook 14.
The fertile window on this page is the 5 days before the estimated ovulation day, the day itself and the day after, which is 7 days. In the example that is 10 to 16 October 2026. The window starts early because sperm can survive in the body for several days, and the egg itself is thought to last about a day, so the days just before ovulation matter most. Sources give different lengths for how long sperm survive, from about 5 to about 7 days, so treat the first and last days of the window as the least certain. The table lists the next six cycles with their period start, window, ovulation and next period. A cycle whose window has already ended is skipped, so the first row is always the one that is coming up or under way.
Cycle length is counted from the first day of one period to the first day of the next, and the page accepts 21 to 45 days. The luteal phase is the time from ovulation to the next period, 14 days by default and adjustable from 10 to 16. A shorter luteal phase moves the estimate later in the cycle: with 28 days and a luteal phase of 12, ovulation is 16 days after the start instead of 14. If your cycle changes from month to month, the calendar method becomes unreliable, and the page adds a note for cycles over 35 days. If you have no date, it shows an example where the last period began 10 days ago.
This is a calendar estimate. It does not measure a hormone, temperature or cervical change, so it cannot confirm that you ovulated. It is not contraception and must not be used to avoid pregnancy: ovulation can come earlier or later than the calendar says, even with regular cycles. It does not allow for breastfeeding, recent hormonal contraception, polycystic ovary syndrome, perimenopause or other conditions that change timing. A doctor or midwife can advise on irregular cycles or on trying to conceive. Dates are shown in English and counted in days in this browser tab. Nothing you type is stored or sent.
Add your cycle length to the first day of your last period and subtract the luteal phase, which is usually 14 days. For a 28-day cycle starting 1 October that is 1 October + 28 − 14 = 15 October. Enter the date and cycle length and the page does it for you.
On this page it is 7 days: the 5 days before ovulation, ovulation day and the day after. For ovulation on 15 October that is 10 to 16 October. Sperm can live in the body for several days and the egg for about a day, so the days before ovulation count most.
Only as a rough guide. The calendar method assumes each cycle is the same length. If your cycles vary, for example between 26 and 38 days, the estimate can be off by days, and a cycle over 35 days gets a note on the page. A doctor or midwife can suggest better ways to find out.
It is the time from ovulation to your next period, 14 days by default. Change it only if you know yours is different, for instance 12 days from tracking. Moving it from 14 to 12 shifts the estimate from cycle day 15 to day 17 on a 28-day cycle.
No. It is an estimate, not contraception. Ovulation can come earlier or later than the calendar says even with regular cycles, and sperm can survive for days. Use a method of contraception recommended by a doctor or nurse if you want to avoid pregnancy.
No. The text is processed in your browser and not uploaded, and this page does not save your numbers or dates, so a refresh brings back the example.