Editing Calendar-based contraceptive methods (section)

===Rhythm method (Knaus–Ogino method)===
To find the estimated length of the pre-ovulatory infertile phase, eighteen (18) is subtracted from the length of the woman's shortest cycle. To find the estimated start of the post-ovulatory infertile phase, eleven (11) is subtracted from the length of the woman's longest cycle.<ref name="kippleycalendar" /> A woman whose menstrual cycles ranged in length from 30 to 36 days would be estimated to be infertile for the first 11 days of her cycle (30-19=11), to be fertile on days 12–25, and to resume infertility on day 26 (36-10=26). When used to avoid pregnancy, such fertility awareness-based methods have a typical-use failure rate of 25% per year.<ref name="hatcher"/>