Calculator
Fertile-window calculator — calendar scenario
Even a regular cycle cannot reliably predict ovulation. This model shows how assumed cycle length shifts an illustrative six-day window. It does not establish your fertility or pregnancy-free days. Do not use it for contraception.
Calculator
Enter data
First day of the last period and the range of recent regular-cycle lengths. Fields marked with an asterisk are required.
How to use the calculator
- Count a cycle from the first bleeding day through the day before the next period. Enter the shortest and longest observed lengths.
- Treat dates as an illustration only. For irregular cycles the model declines to calculate rather than output apparently certain dates.
Calculation details
Formula and calculation method
Modelled ovulation cycle day = cycle length − 14. The first bleeding day is day 1, so the date offset is cycle length − 15. Each window includes the 5 days before modelled ovulation and ovulation day.
Worked example
For a period starting 1 January 2026 and an assumed 28-day cycle, modelled ovulation is cycle day 14, or 14 January, and the six-day window is 9–14 January. This is an illustration, not confirmation.
Limitations
- The fixed 14-day assumption does not reproduce cycle-phase variation. The date envelope is not a confidence interval; actual ovulation can fall outside it.
- Not for contraception, infertility diagnosis, irregular cycles, postpartum use, breastfeeding or hormonal treatment. The result does not confirm ovulation occurred.
Sources
Engine version: 2.0 (2026-09-22) · Last reviewed: 22 September 2026
Frequently asked questions
Is pregnancy impossible outside the displayed range?
No. This is only a mathematical scenario; actual ovulation and fertile dates can differ. Do not treat any day as contraceptively safe on this basis.
Why is each window six days long?
ASRM describes the five days before ovulation plus ovulation day. The calendar model’s unresolved limitation is that the actual ovulation date is unknown.