How this calculator works
The calendar method assumes a 14-day luteal phase and estimates ovulation before the next period. The displayed fertile window extends from five days before to one day after estimated ovulation. It applies only to regular cycles without hormonal contraception, pregnancy or breastfeeding. Even regular cycles vary by several days. This calendar estimate must not be used as contraception. The result is an estimate for information only and is not medical advice.
Formula
Ovulation = LMP + cycle - 14 days. Fertile window = ovulation - 5 through ovulation + 1 day. Next period = LMP + cycle days.
Worked example
With LMP 2026-01-08 and a 28-day cycle, ovulation is estimated on January 22, the window is January 17 to 23, and the next period is February 5.
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Frequently asked questions
Does this confirm ovulation?
No. A calendar cannot confirm that ovulation happened or identify its exact date.
Can it prevent pregnancy?
No. This simple fixed-window estimate is not a contraceptive method.
What if my cycle is longer or shorter than 28 days?
The estimate moves with the cycle length. Ovulation is cycle length minus 14 days after the first day of the last period.