Safe period calculator (calendar method)
The method uses at least six months of cycle records to mark the days to avoid for anyone trying to lower the chance of pregnancy without other methods.
Loading the tool…
Processing and privacy: The result is calculated inside your browser and your data is not sent to any server.
How to use it
- Enter first day of your last period, shortest cycle in the last 6 months and longest cycle in the last 6 months.
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
First fertile day = shortest cycle - 18. Last fertile day = longest cycle - 11. Days are counted from the first day of the period (day 1). Days outside this range are the so-called safe days.
Example
Inputs: First day of your last period = 2026-09-01, Shortest cycle in the last 6 months = 26 days, Longest cycle in the last 6 months = 32 days. Result: Fertile cycle days: Day 8 to day 21; Fertile days start: Tuesday, September 8, 2026 (2026-09-08); Fertile days end: Monday, September 21, 2026 (2026-09-21).
Limits and caveats
- The calendar method is one of the least effective forms of birth control in real use; pregnancy can happen on so-called safe days.
- It is not suitable with irregular cycles, after childbirth or while breastfeeding.
- It gives no protection against sexually transmitted infections.
Questions about this tool
Are safe days guaranteed?
No. Ovulation can come early or late in any month, so no day is completely free of pregnancy risk.
Why do I need the shortest and longest cycle?
The method covers the earliest ovulation in a short cycle and the latest in a long one, so the fertile range widens as cycles vary more.
What is more reliable?
Ask your doctor about more effective methods, or about fertility awareness methods that combine body temperature and cervical mucus.