How it is calculated
The fertile window on the screen starts five days before the estimated ovulation and ends the following day. A living person’s cycle drifts, so the date moves easily.
Ovulation = first day + cycle length − 14
Where the numbers come from
- Cycle length — unit in the field: days
- First day of the period
Example
A 28-day cycle from 1 January 2026 puts ovulation on 15 January.
Questions
Is this birth control?
No. The calendar method is unreliable for that.
Why minus 14, and not the middle of the cycle?
The luteal phase is usually close to two weeks. Cycle length changes mostly in the first half.
A short cycle or a long one?
You can enter from 20 to 45 days. A cycle that is completely irregular is not described by this formula.