Formula and calculation method
Next period = last period + cycle length. Estimated ovulation = next period − luteal-phase assumption.
Cycle timing varies even with regular periods. Not reliable contraception, a fertility diagnosis or confirmation of ovulation.
How to use period calculator
- Set First day of last period. The example below uses 2026-10-01.
- Set Typical cycle length (days). The example below uses 28.
- Set Luteal-phase assumption (days). The example below uses 14.
- Select Calculate to update the result. Reset restores the illustrated inputs. Copy, print or download your own result if you need a record.
Worked example
Example inputs
- First day of last period
- 2026-10-01
- Typical cycle length (days)
- 28
- Luteal-phase assumption (days)
- 14
- Next expected period
- 2026-10-29
- Estimated ovulation
- 2026-10-15
- Approximate fertile window starts
- 2026-10-10
- Approximate fertile window ends
- 2026-10-16
Factors affecting your result
Cycle timing varies even with regular periods. Not reliable contraception, a fertility diagnosis or confirmation of ovulation.
Changing first day of last period changes the scenario being evaluated. Adjust one input at a time when comparing results, keep a copy of the assumptions, and compare values using the same unit and period. A precise arithmetic result does not make an uncertain assumption precise.
Frequently asked questions
How does the period calculator work?
Next period = last period + cycle length. Estimated ovulation = next period − luteal-phase assumption.
What inputs does Period Calculator need?
Enter first day of last period, typical cycle length (days), luteal-phase assumption (days). Choose values that correspond to the same situation and measurement convention.
How should I interpret the period calculator result?
Cycle timing varies even with regular periods. Not reliable contraception, a fertility diagnosis or confirmation of ovulation.
Are my period calculator inputs uploaded?
No. The calculation runs in this browser. Favorites, recent tool names, theme and formatting preferences may be saved locally, but calculation inputs are not sent to a calculation server or stored by this website. Shared links do not include entered values.
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