Calculator
Fertile window calculator
Produce only an indicative fertile window from cycle length assumptions. Data is processed only in your browser, and the page explains the formula and limitations of the result.
Calculator
Enter data
Cycle start, typical cycle length and assumed luteal phase. Fields marked with an asterisk are required.
How to use the calculator
- Enter the required data: cycle start, typical cycle length and assumed luteal phase.
- Check the values and select “Calculate”.
- Review the primary result and the detailed calculation components.
Calculation details
Formula and calculation method
Estimated ovulation = cycle start + cycle length − luteal phase. The window covers 5 days before and 1 day after.
Worked example
For a 28-day cycle and 14-day luteal phase, ovulation is estimated around day 14 and the window spans days 9 to 15.
Limitations
- Ovulation can shift; this calculator does not confirm fertility and should not be used as contraception.
Sources
Engine version: 1.0 · Last reviewed: 3 August 2026
Frequently asked questions
Is the entered data sent to a server?
No. The calculation runs locally in your browser and the values are not saved by default.
Is the result definitive?
Not always. Ovulation can shift; this calculator does not confirm fertility and should not be used as contraception.