Enter your cycle information
Use the first full day of bleeding as day 1. Enter a typical cycle length rather than a single unusually short or long cycle.
Your estimated dates
These results are educational estimates based on the information entered.
Do not use these dates to diagnose a condition, confirm pregnancy, or replace contraception or medical care.
How the period calculation works
The calculator treats the entered date as cycle day 1. It adds your average cycle length to find the next estimated period start, then continues that interval for future predictions. The estimated ovulation date is placed about 14 days before the next predicted period, and the fertile window covers the five days before that estimate through the estimated ovulation day.
The expected period range adds a two-day margin on either side of the central date. That range is a planning aid, not a promise that bleeding will begin within those dates. Even people with usually regular cycles can have occasional changes.
Example calculation
If a period began on July 1 and the average cycle is 28 days, the next period estimate is July 29. Using the 14-day luteal-phase assumption, the ovulation estimate is around July 15 and the fertile window begins roughly five days earlier. A different luteal phase or a delayed ovulation can move all of these dates.
Accuracy and limitations
Calendar predictions work best as a rough pattern for people whose cycle lengths stay fairly similar. They cannot confirm that ovulation occurred. Estimates may be less reliable with irregular cycles, PCOS, recent pregnancy, breastfeeding, perimenopause, illness, significant stress, major weight changes, or after starting or stopping hormonal contraception.
If your period is late and pregnancy is possible, use a pregnancy test according to its instructions. Seek medical advice for very heavy bleeding, severe pain, fainting, pregnancy concerns, or repeated major cycle changes.
Related calculators
Period calculator FAQ
Can a period calculator predict the exact day?
No. It projects dates from your previous period and average cycle length. Ovulation and bleeding can shift even in a generally regular cycle.
What if my cycle is shorter than 21 days or longer than 45 days?
This tool limits entries to a common planning range. Repeated cycles outside that range are worth discussing with a qualified healthcare professional.
Can I use this as birth control?
No. A calendar estimate alone is not a reliable contraceptive method and does not confirm when ovulation occurs.
References and editorial note
- U.S. Office on Women’s Health: Your menstrual cycle
- ACOG: Methods for estimating the due date
- NHS: Doing a pregnancy test
Last updated: July 22, 2026. Written and reviewed for conservative health wording by the Ovucal Editorial Team. This page has not been individually reviewed by a physician.