How to find your fertile window
- Pick the First day of your last period: the day bleeding started.
- Enter your usual Cycle length, counted from the first day of one period to the first day of the next. Leave Luteal phase at 14 unless you know yours.
- Read your fertile window, the estimated ovulation day and the dates for the next six cycles.
How the dates are worked out
Take a last period that started on Thursday 1 October 2026 and a 28-day cycle. The next period is due 28 days later, on 29 October. Ovulation is estimated 14 days before that, on Thursday 15 October, and the fertile window runs from 10 to 15 October.
| Cycle length | Ovulation (day of cycle) | Fertile window (days of cycle) |
|---|---|---|
| 24 days | Day 11 | Days 6 to 11 |
| 28 days | Day 15 | Days 10 to 15 |
| 32 days | Day 19 | Days 14 to 19 |
| 35 days | Day 22 | Days 17 to 22 |
Days are counted with the first day of your period as day 1, so ovulation 14 days after the start of a 28-day cycle falls on day 15.
If the date you enter is more than one cycle ago, the calculator moves forward in steps of your cycle length to the cycle you are in now, and says so.
Irregular cycles
If your cycle length changes from month to month, use your shortest recent cycle for the earliest likely fertile window and your longest for the latest. Very irregular cycles, or cycles shorter than 21 or longer than 35 days, are worth mentioning to a doctor. Not a method of birth control: ovulation can move in any cycle.