Your cycle
A calendar alone is a poor way to avoid pregnancy
Calendar-based rhythm methods have a high real-world failure rate — some estimates put typical-use failure over 20% a year. Ovulation shifts with illness, stress and sleep, so a predicted date can easily miss the real one. If avoiding pregnancy matters, pair this with a reliable method such as condoms or hormonal contraception.
What the Standard Days Method calculates
Enter the shortest and longest cycle you have had in the last six months. The fertile window is estimated to start on day (shortest cycle − 18) and end on day (longest cycle − 11), counting from the first day of your last period. Days outside that window are the relatively lower-risk ones.
It only works for 26–32 day cycles
The Standard Days Method is validated only for cycles that fall consistently between 26 and 32 days. Outside that range the estimate becomes unreliable, and this tool should not be used.
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Frequently asked questions
QIs it safe to rely on this for contraception?
No. Calendar-based rhythm methods, including the Standard Days Method, have a high failure rate because ovulation timing can shift unpredictably. Combine this with a reliable contraceptive method if avoiding pregnancy matters.
QMy cycle length changes every month — can I still use this?
The Standard Days Method is only considered reliable when your cycles have stayed between 26 and 32 days over the last six months. Outside that range, this calculation is not accurate enough to use.