Safe Period Calculator
Next period, estimated ovulation and the fertile window from the Ogino calendar method — plain arithmetic, nothing leaves your browser.
How to use
- Enter the first day of your last period (YYYY-MM-DD).
- Set your average cycle length and period length.
- The estimated dates appear instantly — recalculate each cycle.
Frequently asked questions
How does the calendar method work?
The Ogino rhythm method estimates ovulation as 14 days before the next expected period, then flags a fertile window of roughly 6 days around it (sperm survive up to 5 days; the egg about 24 hours). Days outside that window are the traditional “safe days”.
How reliable is this as contraception?
Not very — with typical use the calendar method fails for more than 1 in 5 users per year, and cycles shift with stress, illness and travel. Treat this tool as a period tracker and cycle-learning aid, never as your only contraception.
Which cycle lengths work with this calculator?
Regular cycles between roughly 21 and 35 days. The calculator clamps inputs to 20–45 days; if your cycles vary by more than 7–9 days month to month, calendar predictions will be unreliable by design.
Why is ovulation set exactly 14 days before the period?
The luteal phase (after ovulation) is fairly fixed at about 14 days in most women, while the first half of the cycle varies. Counting backwards is therefore more stable than counting forward from day 1.
What about cycle-tracking apps and devices?
Apps use the same calendar math plus symptoms; devices add basal body temperature or LH tests, which are meaningfully more accurate. This page deliberately does the plain calendar arithmetic only — no data ever leaves your browser.