How it is calculated
When cycle length varies, so does ovulation. The calendar (Ogino) method gives a wide window covering the possibilities:
First fertile day = shortest cycle − 18 Last fertile day = longest cycle − 11Days count from the first day of your period (day 1). The bigger the gap between shortest and longest cycle, the wider the window.
Worked example
Shortest cycle 26 days, longest 35: possible fertile days run from cycle day 8 to day 24.
Frequently asked questions
Why is the window so long?
It covers every possibility between your shortest and longest cycle. Ovulation tests narrow it a lot.
What causes irregular cycles?
PCOS, thyroid problems, stress and weight changes among others. See a doctor if it persists.
Can I use this for contraception?
No — calendar methods are unreliable for avoiding pregnancy, especially with irregular cycles.