Menstrual Cycle Calculator

A menstrual cycle calculator tracks the timing of periods, ovulation, and fertile windows based on cycle length and period duration. The average cycle is 28 days (range 21–35 days), with ovulation typically occurring 14 days before the next expected period

The calculator uses the standard published formula for menstrual cycle. Results are accurate to the precision of the inputs you provide. For financial, medical, or legal decisions, always verify with a qualified professional.

This calculator works with inches. You can enter values in the units shown — the calculator handles all conversions internally.

The core formula is: Next period = Last period start + Cycle length. Each step in the calculation is shown so you can verify the result manually.