Estimate relatively safe days in your menstrual cycle using the calendar/rhythm method. Note: this method is not reliable contraception.
How the Calendar Method Works
The calendar (rhythm) method estimates a fertile window from your shortest and longest cycle lengths over the past 6 months: the earliest fertile day is set at 18 days before the end of the shortest cycle, and the latest fertile day at 11 days before the end of the longest cycle. Days outside that window, within the current cycle, are labelled relatively safe.
Why This Method Is Less Reliable Than Other Options
The calendar method only works reliably for women with consistent cycle lengths, since it projects future ovulation from past cycle history rather than measuring the current cycle directly. Illness, stress, travel, and normal biological variation can all shift ovulation outside the predicted window, which is why typical-use failure rates for calendar-based methods run around 24% per year — far higher than hormonal or barrier contraception.
What This Calculator Does Not Do
This tool is a planning aid, not a contraception method, and does not track basal body temperature, cervical mucus, or ovulation test results — the three signals that improve on calendar-only predictions. Anyone relying on cycle tracking to prevent pregnancy should combine it with one of those methods or speak to a healthcare provider about more reliable options.
How accurate is the calendar (rhythm) method?
Typical-use failure rates for calendar-based methods run around 24% per year, since the method projects future ovulation from past cycle history rather than measuring the current cycle directly.
Why does the calculator ask for 6 months of cycle history?
The calendar method needs your shortest and longest cycle lengths over several months to set the fertile window: 18 days before the end of the shortest cycle to 11 days before the end of the longest cycle.