Ovulation Calculator
Estimate ovulation day, fertile window, and next period from cycle length and last period.
days
About this calculator
Ovulation is estimated as 14 days before the next period. The fertile window is treated as the five days before ovulation through ovulation day. These dates are averages, not a guarantee of fertility.
How to calculate
Enter the first day of the last period and typical cycle length in days. Most cycles fall between 21 and 35 days; 28 is the default example.
Next period = LMP + cycle length
Ovulation = next period − 14 days
Fertile window = ovulation − 5 days through ovulation
Examples
28-day cycle, LMP on the 1st
Next period is the 29th. Ovulation is estimated on the 15th. The fertile window is about the 10th through the 15th.
FAQ
Why subtract 14 days?
The luteal phase after ovulation is often close to 14 days. Cycle-length differences usually come from the days before ovulation.
Can I use this as contraception?
No. Cycle timing is not reliable enough on its own to prevent pregnancy.