Estimate your next period start date, ovulation date, and fertile window from your latest period start.
In days, usually between 20 and 45
In days, usually between 1 and 10
This calculator provides estimates for reference only. Actual ovulation, fertile window, and period dates can vary by individual health conditions and lifestyle factors.
Estimates next period, ovulation, and fertile window.
From last period start and average cycle length, it estimates next period start, ovulation date, and fertile days.
These are estimates built from past cycles. Understanding why ovulation is counted backwards from the next period makes the results far easier to interpret.
A cycle runs from the first day of one period to the day before the next period starts. It is not measured from the day bleeding stops.
Anything from 21 to 35 days is generally considered typical, with an average near 28. Variation between people, and from month to month in the same person, is normal.
The luteal phase — from ovulation to the next period — is relatively consistent at around 14 days. The phase before ovulation varies much more between people and cycles.
So ovulation is better estimated as 'next expected period minus 14 days' than as 'cycle start plus 14 days'. In a 35-day cycle, ovulation falls around day 21, not day 14.
Sperm can survive in the reproductive tract for up to about five days, while an egg survives roughly 12 to 24 hours after ovulation.
That gives a fertile window of about seven days: the five days before ovulation, the day itself, and the day after, since the egg is still viable for part of it. This calculator marks the window the same way. The chance drops sharply after that.
If cycles are irregular, the predicted next period moves, and the ovulation estimate moves with it.
Stress, significant weight change, poor sleep, travel across time zones, illness and heavy training can all delay ovulation or cause a cycle without it. If you use hormonal contraception, cycle-based prediction does not apply at all.
Calendar-based methods have a high failure rate in typical use. Ovulation can shift by several days, and with sperm surviving up to five days, a day the calendar calls safe may not be.
If you are trying to avoid pregnancy, do not rely on this. Speak to a clinician about a method with a reliable failure rate. Seek medical advice if cycles are consistently shorter than 21 or longer than 35 days, if you miss periods for three months or more, or if bleeding or pain changes markedly.
| Stage | Day |
|---|---|
| Period | Days 1–5 (3–7 is normal) |
| Fertile window opens | Around day 9 |
| Estimated ovulation | Around day 14 (next period − 14) |
| Fertile window closes | Around day 15 |
| Next period due | Day 29 |
