How to measure a period
Use the first and last bleeding days plus an average cycle length.
Enter the dates
Use the first and last day of bleeding for the period you want to measure.
Add the cycle length
Use an average of several cycles rather than a single unusual one.
Read the projections
The next period and fertile window are estimates, not scheduling guarantees.
What does this tool do?
Period length is counted from the first bleeding day to the last, inclusive. This calculator also projects the next period, ovulation and a fertile window from the cycle length you enter.
The Formula
Bleeding days = end date − start date + 1; next period = start + cycle length; ovulation ≈ next period − 14 daysFrequently Asked Questions
How is period length counted?
Count the first day of bleeding as day one and the last day of bleeding as the final day, inclusive. Spotting before the main flow is usually counted separately.
Can I use this to avoid pregnancy?
No. Calendar estimates are unreliable for contraception because ovulation timing varies and cycles change with illness, stress and travel.