Menstrual Cycle Calculator

How the calculation works

Next period = start + cycle length. Ovulation = next period − luteal phase length.

Inputs

• First day of last period
• Average cycle length, days

Calculation details

Calendar predictions are unreliable for irregular cycles and must not be used for contraception. Even regular cycles can vary.

Cycle day one is the first day of bleeding. The approximate fertile window runs from five days before ovulation through the following day; a calendar does not confirm ovulation.