Rhythm Method Calculator

Calculate your fertility phases with the Rhythm Method (also known as Knaus-Ogino Method), a calendar based method. As a conception tool, Rhythm can be highly effective for getting pregnant as it identifies a more expansive fertility window by using your shortest and longest cycles.

Member? Login and your tracking history fills in the last period, average cycle; your recent shortest and longest cycles, if calculable, are filled in from your recent cycle length chart (on right or below).


(if no, average used)

(samples at right/below)

Important: The information on this rhythm calendar should not and cannot be used to prevent pregnancy.

Is the Rhythm Method the same as an Ovulation Calculator?

No, the calculations are very different. MyMonthlyCycles Ovulation Calculator is a conception tool which uses your cycle statistics, including your luteal phase length which plays an important role in forecasting ovulation. Based on these, it displays your projected ovulation date and likely fertile days. Many MyMonthlyCycles moms report it's been highly accurate for them when trying to conceive.

How effective is Rhythm Method at Preventing Pregnancy?

With perfect use, the Knaus-Ogino Rhythm Method is 91% effective at preventing pregnancy (9 in 100 women will get pregnant); With typical use, it is only 75% effective at preventing pregnancy (25 in 100 women will get pregnant). Perfect use includes regularly tracking your cycles so that you know your recent shortest and longest cycles, and that Periodic Abstinence (for Natural Family Planning), or a Barrier Method of birth control (for Fertility Awareness Method) is used during the fertile phase.

Related: Birth Control Calculator - Compare Rhythm Method to Condoms, the Diaphragm, and more.

O-K Method, Rhythm Method

Prior to the 1920s, while there was much speculation (and many a myth) as to when women ovulate, it was two gynecologists who made tremendous breakthroughs toward understanding the menstrual cycle. They were Kyusaku Ogino, of Japan, and Hermann Knaus, of Austria, working half a world away, yet both were arriving at similar conclusions: that women ovulate once per cycle, that there were fertile and infertile periods (thereby being "safe periods"), and that the ovum only survives for a small amount of time. Sometimes referred to as the O-K Method (after each doctor's initials), it was Leo Latz, a doctor in Chicago and a devout Catholic, who promulgated their work when he published "The Rhythm of Sterility and Fertility in Women" in 1932, which ultimately would sell hundreds of thousands of copies; By the early 1950s, Pope Pius XII sanctioned it as a natural method of family planning.

Related MyMonthlyCycles Cycle Tools:

Disclaimer: The information on this calculator should not and cannot be used to reliably prevent pregnancy. This calculator is not intended for those having irregular cycles; if your cycle is irregular, talk to your healthcare provider regarding methods that can help determine your fertility cycles. Even if your cycles are regular, we make no guarantee that your period and/or fertile days will occur on the predicted dates shown on the calculated fertility calendar, since each person's menstrual and fertility cycles can vary in any given month for a variety of reasons.