How to use this calculator
Enter the first day of bleeding as cycle day 1, the average start-to-start cycle length and optional variation. The highlighted calendar shows the estimated fertile intercourse interval. The ovulation date is a midpoint estimate, assuming a 14-day luteal phase; real ovulation can differ. Later cycles project the same average length and become increasingly uncertain.
- Enter first day of last menstrual period, average cycle length (days), cycle variation (± days) and review the remaining options.
- Select Calculate or change a value to update the result. Read the result label and units before copying.
- Use Reset to restore the worked starting values.
Formula and method
Calendar estimates cannot identify exact fertile or safe days and are not reliable contraception. Irregular cycles and variable luteal phases reduce accuracy; the displayed range is not a medical confidence interval. Consult a qualified healthcare professional for fertility or contraception decisions. No conception guarantee is made.
Worked example
With a period starting 1 November 2026 and a 28-day cycle, the next period is estimated for 29 November. Estimated ovulation is 15 November (cycle day 15), and the illustrative fertile intercourse window is 10–16 November.
Questions about ovulation calculator
How many days after my period can I get pregnant?
Count from the first day of bleeding, not the day bleeding ends. A 28-day calendar model places ovulation around cycle day 15 with a fertile intercourse interval around days 10–16, but real timing varies. Calendar predictions do not establish safe days and must not be relied on for contraception.
What assumptions should I check?
Calendar estimates cannot identify exact fertile or safe days and are not reliable contraception. Irregular cycles and variable luteal phases reduce accuracy; the displayed range is not a medical confidence interval. Consult a qualified healthcare professional for fertility or contraception decisions. No conception guarantee is made.
Are my inputs saved?
Calculations run in your browser. We store only tool IDs for favorites and recent tools. Sharing inputs in links is disabled for this category.
Health tools provide general estimates, not diagnoses or personal medical advice. BMI offers a labeled CDC child/teen reference; adult screening bands must not be applied to children. Discuss health and exercise decisions with a qualified professional. Read the tool-specific sources below for background on the method.