Estimate your next periods, ovulation day, and fertile window from your last period and average cycle length. These are calendar estimates for education — not a contraceptive method.
How the estimate works
The calculator uses the standard calendar method. Your next period is your last period plus your average cycle length. Ovulation is estimated by counting back the length of your luteal phase (the gap between ovulation and your period, usually about 14 days) from that next period. Your fertile window spans the five days before ovulation and ovulation day itself.
Because ovulation timing varies from cycle to cycle — and becomes unpredictable in perimenopause — these dates are estimates only. Do not rely on them to prevent or plan a pregnancy without talking to a clinician.
Common questions
How is the fertile window calculated?
Ovulation is estimated as your next period minus the luteal phase (about 14 days). The fertile window is the five days before ovulation plus the day of ovulation, because sperm can survive several days and the egg lives about a day.
Is a period tracker reliable during perimenopause?
No. Calendar tracking assumes regular cycles. In perimenopause, cycle length varies and ovulation becomes unpredictable, so estimates are unreliable. The tool is educational and is not a contraceptive method.
Can you still get pregnant in perimenopause?
Yes, until you have gone twelve consecutive months without a period. Fertility falls but does not stop, ovulation becomes unpredictable rather than absent, and contraception is generally advised until 55 or for a year after the last period, whichever comes first.
Why has my cycle suddenly got shorter?
It is usually the first sign of perimenopause. The follicular phase shortens as the ovarian reserve falls, so cycles of 24 to 26 days replace 28-day ones several years before periods become erratic. It is often the earliest change and the least noticed.
Do ovulation tests work during perimenopause?
Poorly. They detect an LH surge, and LH rises erratically in perimenopause without ovulation following, so false positives are common. They also cannot be used the other way round as contraception — a negative test does not mean a cycle is safe.