Next period may start
Calculated only from the cycle range you entered.
Calendar estimate with uncertainty
Cycle window estimator
Use recent shortest and longest cycle lengths to make a broad calendar estimate. The output cannot detect whether or when ovulation happened, whether you are fertile, or whether pregnancy occurred.
No fertility confirmationNo pregnancy claimNo saved dates
Do not use these dates to decide unprotected sex is safe
A calendar estimate cannot identify guaranteed “safe days” and is not contraception. If avoiding pregnancy matters, use a contraceptive method you have chosen with reliable guidance. Condoms can also reduce STI risk.
Your recent pattern
Enter a cycle range
Calendar output
Your broad estimate
Possible ovulation estimate
This does not show whether ovulation occurred.
Pregnancy may be possible within this wider span
Pregnancy can still be possible outside a calendar estimate. Do not treat excluded dates as safe.
Why the range is wide
Cycle dates alone cannot confirm ovulation, fertility, infertility, pregnancy, or contraceptive safety.
Privacy
Cycle dates stay off storage
No dates are saved to localStorage, sessionStorage, accounts, URLs or analytics. TXT and PDF files are created locally only after you request them. They contain sensitive cycle information, so store and share them carefully.
Source check
Official guidance used
- ACOG fertility awareness methods - cycle definition, pregnancy-possible timing and contraception limits.
- NHS periods guidance - ovulation often 12-16 days before the next period and cycle variability.
- CDC contraception methods - fertility-awareness methods vary in typical-use effectiveness and do not protect against STIs.
Sources checked: 26 July 2026.
Use the estimate carefully
Does this confirm ovulation?
No. It applies date ranges to a cycle history. It does not measure hormones, temperature, cervical mucus, ultrasound findings or egg release.
Can this tell me whether I am pregnant?
No. A calendar cannot confirm pregnancy. Follow pregnancy-test instructions and contact a qualified service if you need individual guidance.
Can I use dates outside the span as contraceptive “safe days”?
No. Biological timing and cycle length vary. The tool deliberately does not mark any day safe for unprotected sex.