How to use this calculator
Enter the start date of a recent period and your typical cycle length to view estimated cycle dates. Day 1 is the first day of that period. Check that the dates describe the cycle you intend to inspect.
Method and interpretation
The calendar estimate assumes ovulation occurs about 14 days before the next expected period. It uses the selected cycle length to estimate that next period and a nearby fertile interval. The interval represents an assumption about timing, not evidence that ovulation occurred.
Assumptions and limits
Cycles can vary and calendar predictions may be inaccurate, particularly with irregular periods. This tool is not reliable contraception and cannot diagnose fertility or confirm pregnancy. Physiological measurements and a clinician can provide information this simple calendar model does not have.
Worked example
For a typical 28-day cycle, the model places ovulation around cycle day 14. If the cycle length changes, the estimated date shifts. A real cycle may still ovulate earlier or later than that calendar estimate.