How to use the Ovulation Calculator
- Enter the first day of your last period.
- Enter your average cycle length (count from the first day of one period to the day before the next).
- See your predicted fertile window and ovulation date for upcoming cycles.
- Optional: compare shortest and longest cycle lengths to illustrate date uncertainty.
When does ovulation happen?
Ovulation usually happens about 14 days before your next period — not necessarily on day 14. With a 28-day cycle that is day 14; with a 32-day cycle it is around day 18.
The fertile window
Sperm can survive up to 5 days and an egg about 24 hours, so the fertile window is the 5 days before ovulation plus ovulation day. The two days before ovulation have the highest chance of conception.
How accurate is it?
Calendar predictions work best for regular cycles. Ovulation tests (LH strips), basal body temperature and cervical mucus tracking are more precise. This calculator should not be used as contraception.
This calculator gives general estimates for healthy adults and is not medical advice. Talk to a doctor or registered dietitian for personal guidance.
Frequently asked questions
Can I get pregnant outside the fertile window?
It is less likely but possible, especially with irregular cycles, because ovulation can shift.
What if my cycles are irregular?
Use your average cycle length, but consider ovulation tests for better accuracy.