How to use this tool
- Enter the first day of your last period.
- Enter your shortest and longest cycle lengths from at least the last six months.
- Avoid unprotected sex on the highlighted fertile days, and re-enter the date when each new period starts.
How the calendar method works
An egg survives about a day after ovulation, and sperm can survive up to five days. Because ovulation happens around 14 days before the next period, and cycles vary, the calendar (Ogino-Knaus) rule allows a margin on both sides: subtract 18 from your shortest cycle for the first fertile day and 11 from your longest cycle for the last fertile day. For example, with cycles between 27 and 30 days, days 9 to 19 are fertile.
The Standard Days Method
Developed at Georgetown University, this simpler method applies to women whose cycles are always between 26 and 32 days long. Days 8 to 19 are treated as fertile. In its clinical trial it had a pregnancy rate of about 5% a year with correct use and about 12% with typical use.
How reliable is it?
Calendar-based methods are less effective than most modern contraception. The CDC reports that fertility awareness methods have typical-use pregnancy rates ranging from about 2% to 23% a year, depending on the method and how consistently it is followed. Combining the calendar with tracking cervical mucus and basal body temperature (the symptothermal method) is more reliable. Condoms add protection and are the only option here that also protects against sexually transmitted infections.
When not to rely on it
The method doesn't work well if your cycles are irregular or shorter than 26 days, after childbirth, while breastfeeding, approaching menopause, after stopping hormonal contraception, or with conditions such as polycystic ovary syndrome. A doctor, family planning clinic or pharmacist can help you choose a method that suits you. Emergency contraception is available if you have unprotected sex on a fertile day.
Frequently asked questions
Which days are safe to avoid pregnancy?
No day is completely safe. Days before the first fertile day and after the last fertile day carry lower risk; this calculator shows them from your cycle history.
Is the week after my period safe?
Not always. With short cycles, ovulation can come soon after a period, and sperm can survive five days.
How effective is the rhythm method?
About 88 in 100 women using the Standard Days Method avoid pregnancy in a year with typical use. Modern methods such as implants or IUDs are over 99% effective.
Can I use this if my periods are irregular?
It is not recommended. If your cycles vary by more than about a week, talk to a clinician about other contraception.
Sources and references
- Arevalo M, Jennings V, Sinai I. Efficacy of a new method of family planning: the Standard Days Method. Contraception 2002;65:333–8
- ACOG: Fertility awareness-based methods of family planning
- CDC: Contraception, effectiveness of family planning methods
- NHS: Natural family planning (fertility awareness)