Use the first day of full flow.
Record the first day of menstrual bleeding, not light spotting. A few saved starts help you see whether your typical cycle length is stable.
A private planning calendar
Dates are signals,
not certainties.
Map your own cycle pattern as an expected range. Save starts on this device, adjust variability, and keep medical decisions separate from calendar estimates.
NEXT EXPECTED RANGE—
RANGE WIDTH—
Month
Expected periodRange marginEstimated fertile daysToday
Three-cycle view
Plan with a window,
not a single date.
Longer-range estimates accumulate uncertainty. If your cycles vary widely, use the calendar as a personal log rather than a prediction.
!
Calendar estimates are not contraception or diagnosis.
Ovulation timing can vary even in regular cycles. Do not use this tool to prevent pregnancy. A late or missed period, unusual bleeding, severe pain or other concerning symptoms deserves advice from a qualified clinician. Pregnancy tests and clinical guidance are more reliable than a calendar estimate.
Widen the range when life changes.
Stress, illness, travel, postpartum changes, perimenopause and medications may affect timing. Increase variability instead of forcing an exact date.
No health account is created.
Saved dates use browser local storage. They do not sync across devices. Clear them before using a shared device.
Questions about cycle estimates
How is the next period range calculated?
The center date is the most recent start plus your typical cycle length. The range extends earlier and later by the variability you choose.
How is the estimated fertile window calculated?
The display uses a simplified calendar estimate around 14 days before the next expected period. Biology varies, so the highlighted days are not a reliable measure of fertility or pregnancy risk.
Where are saved dates stored?
Only in local storage in this browser. The site has no member account or health-record database.
Is this a medical device?
No. Cycle Atlas is a personal organizer and educational calendar, not a medical device. It does not diagnose, treat, or prevent any medical condition.
Can I use this for birth control?
No. Calendar-based methods are not reliable contraception. Ovulation can vary cycle to cycle. Consult a healthcare professional for contraception options.