Using the Period Calculator
Cycle length means the interval from the first day of one period to the first day of the next. Entering the number of bleeding days instead will produce the wrong projection. The additional ovulation and due-date outputs follow the same simplified cycle assumptions.
Worked example
Use the example inputs below to reproduce this result. These are the form's starting values, not recommended targets. Changing your inputs updates your result above; this worked example stays fixed for comparison. Results are rounded for display.
Example inputs
- First day of last period
- 2026-01-01
- Typical cycle length (days)
- 28
Example result
- Estimated next period
- 2026-01-29
- Estimated ovulation
- 2026-01-15
- Estimated fertile window
- 2026-01-10 to 2026-01-16
- Estimated due date
- 2026-10-08
Method and assumptions
Assumes ovulation about 14 days before the next period and a regular cycle. Dates can vary considerably; this is not a reliable contraception method or confirmation of conception.
Treat the dates as calendar estimates
This model starts from a recorded period date and a chosen cycle length. It does not observe the biological events shown in the results. A neat date on the calendar can look more certain than the assumptions justify, especially when cycles vary. The projections are not confirmation of pregnancy and are not a reliable contraceptive method.
Common question
What if my cycles vary from month to month?
The form accepts one cycle length and cannot model a distribution of cycle lengths. Trying a shorter and a longer observed cycle can show how the calendar changes, but neither result guarantees the next date or explains why a cycle varies.
Guide updated .