About the ovulation calculator
Ovulation is the release of an egg, and it usually happens about 14 days before your next period starts, not 14 days after the last one began. The luteal phase after ovulation is fairly constant for each person, while the first half of the cycle varies. That is why the calculator counts back from the end of the cycle using your own cycle and luteal phase lengths.
The fertile window is the six days that end on ovulation day: sperm can survive for up to five days, while the egg lives for about 24 hours. The two or three days up to and including ovulation are the most fertile.
The table projects the next six cycles. It assumes that your cycles are regular; stress, illness and travel can all move ovulation. For better accuracy you can combine the estimate with ovulation test strips or basal body temperature tracking. The calculator is not a method of birth control.
Formula
- Ovulation day = last period start + (cycle length − luteal phase) days
- Fertile window = 5 days before ovulation through 1 day after
- Next period = last period start + cycle length
- Due date if conceived = ovulation + 266 days
Example
With a last period starting on February 29, 2024 and a 28-day cycle with a 14-day luteal phase, ovulation is expected on March 14, 2024, the fertile window runs from March 9 to March 15 and the next period is due on March 28.