Zokilos
Zokilos
Estimate your fertile window, ovulation date & next period
Most people can leave luteal phase length at the default of 14 — it varies far less than cycle length.
An ovulation calculator estimates when you're likely to ovulate and which days give the best chance of conception, based on the first day of your last period and how long your cycle usually runs.
Enter the first day of your last period.
Enter your average cycle length (leave luteal phase at 14 unless you know otherwise).
Your fertile window, ovulation date, and next three cycles update instantly.
Ovulation is estimated by counting back from your next expected period, not forward from your
last one: next period date − luteal phase length. This is more reliable because the
luteal phase (ovulation to next period) is fairly consistent at around 14 days, while the first half of
the cycle is what actually varies between people and between cycles.
Sperm can survive in the body for up to about 5 days, and a released egg is viable for roughly 12–24 hours, so the fertile window runs from about 5 days before ovulation through the day after it — a 6-day span in total. The two days right before ovulation and ovulation day itself are generally the highest-probability days.
Time intercourse around the days with the best chance of pregnancy.
Get a general sense of where you are in your cycle and when your next period is due.
Have estimated dates ready ahead of a fertility or gynecology appointment.
Ovulation is estimated as your next expected period minus your luteal phase length. Cycle length varies more from person to person than the luteal phase, which is why counting back from the next period is more reliable than counting forward from the last one.
It's the span of days when pregnancy is possible: sperm can survive up to about 5 days, and the egg is viable for roughly 12-24 hours after ovulation, so the fertile window runs from about 5 days before ovulation through the day after it.
The two days immediately before ovulation and ovulation day itself are generally considered the highest-probability days for conception.
It's the time from ovulation until the next period starts. It's typically close to 14 days and far more consistent than the first half of the cycle, so it's used to work back from your next period to estimate ovulation.
Less accurate. This calculator assumes a fairly regular cycle length; if your cycles vary a lot month to month, the estimate is a rough guide at best — tracking basal body temperature or using ovulation predictor kits will be more reliable.
No. Nothing is saved or sent anywhere — everything runs in your browser and clears when you refresh or close the page.