Safe Days Calendar
This uses the classic calendar, or rhythm, method. It needs your shortest and longest cycle lengths from at least 6 months of records.
General information only, not medical advice. Ask your doctor or midwife.
How this works
Formula: First fertile day = shortest cycle − 18. Last fertile day = longest cycle − 11 (counting the first day of your period as day 1).
- Take the shortest cycle you have recorded and subtract 18. That is your first likely fertile day.
- Take the longest cycle you have recorded and subtract 11. That is your last likely fertile day.
- Days outside that range are the lower-chance days. Lower chance does not mean no chance.
- The calendar method alone is one of the less reliable ways to avoid pregnancy. It does not protect against infections.
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Questions
Are 'safe days' really safe?
No day is completely safe. Ovulation can come early or late, and sperm can live several days. If avoiding pregnancy matters, talk to your doctor about more reliable birth control.
Why do I need my shortest and longest cycles?
The method widens the fertile window to cover the earliest and latest you are likely to ovulate. More months of records make it fairer.
Does this work with irregular cycles?
The more your cycles vary, the longer the fertile window becomes and the less useful this method is.