Ovulation Window Calculator
Estimate fertile window from cycle length.
Enter your values
ⓘ Live results
Enter a valid last period start date.
Quick answer
Ovulation typically occurs about 14 days before the next period, with the fertile window spanning roughly 5 days before to 1 day after.
ovulation day = cycle length - 14 days from last period start
How to use the Ovulation Window Calculator
- 1Enter the first day of your last period and your typical cycle length.
- 2Ovulation timing is estimated, not measured — actual timing varies cycle to cycle.
- 3For family planning, combine with ovulation predictor kits or basal body temperature tracking, and consult a clinician for personalised guidance.
Frequently asked questions
Is this reliable for irregular cycles?
No — irregular cycles make calendar-based estimates much less accurate; consider tracking methods like OPKs.
Can I get pregnant outside this window?
It's less likely but not impossible, since sperm can survive several days and ovulation timing can shift.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.