Nishmat's Women’s Health and HalachaIn memory of Chaya Mirel bat R' Avraham

  • Hebrew
  • English
  • Espnaol
  • Francais

Can I calculate onot from ovulation?

2 November, 2016


I have a question about the Onos. My cycles are 28–29 days, usually giving me 3 days of Onos in a row (29–30–31). I know that my period comes 14 days after I ovulate and most of the time I know exactly when that happens. Once in a while my cycle will be 30 days long but because of previous month's shorter cycle I have to still keep the onos on day 29. But if I know my period won't come before day 30, do I have to observe the earlier onos? In other words, the "official" days of onos may not even be a day I'm expecting my period.


Unfortunately, a calculation based on ovulation two weeks previously, which is largely but not 100% reliable, cannot release a woman from observing onot. Even if you expect your period later based on your ovulation calculation, the halachic requirement to observe the prior interval stands.

Based on your description, however, we are not certain why you wind up keeping three days of onot in a row.  If you would like to share more complete information about your onot and calculations over the past few cycles and your customs regarding onah beinonit, we would be happy to review it.

This internet service does not preclude, override or replace the psak of any rabbinical authority. It is the responsibility of the questioner to inform us of any previous consultation or ruling. As even slight variation in circumstances may have Halachic consequences, views expressed concerning one case may not be applied to other, seemingly similar cases. All health and health-related information contained within Nishmat's Women's Health & Halacha Web site is intended to be general in nature and should not be used as a substitute for consulting with your health care professional. The advice is intended to offer a basis for individuals to discuss their medical condition with their health care provider but not individual advice. Although every effort is made to ensure that the material within Nishmat's Women's Health & Halacha Web site is accurate and timely, it is provided for the convenience of the Web site user but should not be considered official. Advice for actual medical practice should be obtained from a licensed health care professional.

For further questions or comments: 

The Nishmat Women's Health and Halacha Site is a public service of Nishmat, The Jeanie Schottenstein Center for Advanced Torah Study for Women. This project and others like it are made possible by contributions from people like you. If you have benefited from the service, and wish to enable us to help others, click here to donate.

Users of Internet filtering services: This site discusses sensitive subjects that some services filter without visual indication. A page that appears 100% complete might actually be missing critical Jewish-law or medical information. To ensure that you view the pages accurately, ask the filtering service to whitelist all pages under

Accessibility Toolbar