Estimate your upcoming period dates, your likely ovulation day, and your fertile window from your last period and average cycle length.
Your next six estimated cycles:
| Period starts | Fertile window |
|---|
These dates are estimates based on a regular cycle and the averages you enter; real cycles vary, so actual dates can differ. This tool is not a form of birth control and should not be relied on to prevent or achieve pregnancy. It does not diagnose any condition. If your cycles are irregular, very painful, or you have concerns, please consult a healthcare provider. This tool is for educational purposes only and is not medical advice.
The calculator starts from the first day of your last period and adds your average cycle length to project when the next ones are likely to begin. Ovulation is estimated at about 14 days before the next period starts, which is the typical length of the luteal phase, and the fertile window is the days around ovulation when conception is most possible.
Because it assumes a regular cycle, the projections are most accurate for people whose cycles are fairly consistent. The more your cycle varies month to month, the more the dates are rough estimates.
A cycle is counted from the first day of one period to the first day of the next. While 28 days is the textbook average, anything from about 21 to 35 days is common and normal in adults. The cycle has phases: menstruation, the follicular phase leading up to ovulation, ovulation itself, and the luteal phase afterward.
Tracking the first day of each period over a few months gives you a personal average cycle length, which makes these predictions more accurate than relying on the 28-day default.
Ovulation is when an ovary releases an egg, and it usually happens roughly midway through the cycle. The fertile window spans the few days before ovulation through about a day after, because sperm can survive several days while the egg is viable for about a day.
This calculator estimates that window for general awareness. It isn't a precise fertility test — methods like ovulation predictor kits or basal body temperature tracking pinpoint ovulation more reliably.
It's important to be clear: a calendar estimate cannot reliably prevent pregnancy. Cycles shift with stress, illness, travel, and many other factors, so the fertile window can move. Conception is possible at times the calculator marks as low risk.
If you're trying to avoid pregnancy, use a proven contraceptive method and talk to a healthcare provider about what fits you. Use this tool for awareness, not protection.
Some variation is normal, but certain patterns deserve medical attention: cycles consistently shorter than 21 or longer than 35 days, periods that stop unexpectedly, very heavy or very painful periods, or bleeding between periods. These can have manageable causes worth checking.
A provider can look at your full history and, if needed, investigate further. A calculator can flag what's typical, but it can't diagnose why your cycle behaves the way it does.
Add your average cycle length to the first day of your last period. Enter those details above and the calculator projects your next six cycles, plus your estimated ovulation and fertile window.
It's most accurate for regular cycles. Since cycles naturally vary with stress, illness, and other factors, treat the dates as estimates rather than guarantees.
Generally in the days just before and including ovulation, about midway through your cycle. The calculator estimates this window, but it isn't a precise fertility test.
No. A calendar estimate can't reliably prevent pregnancy because cycles shift. Use a proven contraceptive method and consult a healthcare provider.
In adults, roughly 21 to 35 days is common. Consistently shorter or longer cycles, or sudden changes, are worth discussing with a doctor.