What this tool does
Estimate the next period, likely ovulation date and a six-day fertile window from the first day of the last period and usual cycle length.
How to use it
- Enter the first day of the last menstrual period.
- Enter the usual cycle length from 21 to 45 days.
- Review the estimated dates as a calendar aid and track actual cycle signs rather than treating one result as certain.
Formula and method
Next period = last-period start + rounded cycle length. Estimated ovulation = next period − 14 days. The displayed fertile window runs from 5 days before estimated ovulation through 1 day after it. This follows the calendar pattern described by ACOG but cannot confirm that ovulation occurred.
Worked example
For a last-period start of January 1, 2026 and a 28-day cycle, the next period is estimated for January 29. Ovulation is January 15 and the displayed fertile window is January 10–16.
Common uses
- Planning when to observe cycle signs
- Creating an approximate trying-to-conceive calendar
- Comparing estimates as cycle length changes
Limitations
Ovulation varies even in regular cycles, and irregular periods, postpartum changes, perimenopause, illness, stress and hormonal medicines reduce calendar accuracy. This tool does not diagnose fertility, confirm ovulation or provide contraception. Use reliable birth control if avoiding pregnancy and consult a clinician about irregular cycles or fertility concerns.
FAQ
Why subtract 14 days from the next period?
The post-ovulation part of the cycle is often more consistent, and ACOG describes ovulation as typically about 14 days before the next period; it remains an estimate.
Can I use these dates to avoid pregnancy?
No. Calendar estimates can be wrong and should not replace an evidence-based contraceptive method or professional advice.