How the fertile window is estimated
The calculator uses the calendar method. It assumes the luteal phase, the time from ovulation to the next period, is about 14 days, so ovulation falls on cycle length − 14 days after the first day of your period. The fertile window runs from five days before that date to one day after it. Sperm can survive several days in the reproductive tract, whereas an egg lives roughly a day, which is the reason the window starts well before ovulation.
The page repeats the calculation for three consecutive cycles by adding your cycle length each time, and lists the fertile window, ovulation date and the date the next period is due for each. If the period date you enter is old, the table starts at the cycle that contains today, so the period due next and any fertile window still in progress stay visible. The Next ovulation figure is the first ovulation date on or after today, and the fertile window shown is the one in progress or coming up. The cycle length must be a whole number from 20 to 45 days.
Worked examples
| Period starts | Cycle | Ovulation | Fertile window | Next period |
|---|---|---|---|---|
| 10 Mar 2026 | 28 days | 24 Mar | 19–25 Mar | 7 Apr |
| 10 Mar 2026 | 32 days | 28 Mar | 23–29 Mar | 11 Apr |
In a 28-day cycle, the day your period starts is cycle day 1, so ovulation lands on cycle day 15 (14 days later) and the window covers seven calendar dates. Because the luteal phase is held constant, a longer cycle moves ovulation later, not the days before the period.
Why treat it as a rough guide
- The 14-day assumption varies. Luteal phases of about 10 to 16 days are commonly reported, and ovulation timing shifts from month to month even in regular cycles.
- Later cycles are less certain. Each additional cycle compounds any error in your cycle length, so the third row of the table is a weaker prediction than the first.
- Irregular cycles. If your cycle length changes by a week or more, a single average will not capture it. Methods based on physical signs, such as ovulation test strips or basal temperature, directly detect the event rather than predicting it.
- Old start dates are projected forward. If the period date is months ago, the tool adds your cycle length repeatedly to reach the next upcoming cycle, which assumes every cycle in between was the same length. Use your most recent period start for the best estimate.
- Not contraception. The calendar method is far too imprecise to prevent pregnancy.
These dates are estimates for planning, not medical advice. Once a pregnancy is confirmed, the due date calculator converts the last period date into an estimated due date and trimester.