Safe Period Calculator
Estimate lower-fertility days in your cycle with the calendar rhythm method. This is not a reliable form of contraception.
About the Safe Period Calculator
The calendar rhythm method uses your shortest and longest recent cycles: the first fertile day is the shortest cycle minus 18, and the last is the longest cycle minus 11. With typical use, calendar-based methods fail for about 1 in 4 people a year, so they shouldn't be relied on to prevent pregnancy.
The formula
First fertile day = shortest cycle − 18; last fertile day = longest cycle − 11
Worked example
Example: Likely fertile days this cycle = Day 11 to 24
- Inputs: First day of your last period: 15 March 2026; Shortest cycle in the last 6+ months: 29 days; Longest cycle in the last 6+ months: 35 days.
- Formula: First fertile day = shortest cycle − 18; last fertile day = longest cycle − 11
- Result: Likely fertile days this cycle = Day 11 to 24
- Fertile window length: 14 days
Things to keep in mind
- Not reliable contraception. It doesn't protect against sexually transmitted infections.
- Irregular cycles make the method unusable.
Questions people ask
How effective is the rhythm method?
Around 76–88% with typical use, much lower than the pill, implant or IUD.
Do I need 6 months of data?
Yes, at least 6 cycles, ideally 12, to find your shortest and longest.