==Food or Drink==
#According to Ashkenazim, all sizes are measured in volume and not weight.<ref>Mishna Brurah 456:3, Vezot HaBracha (pg 6) </ref> However, according to Sephardim the actual halacha is that measures are in volume but the minhag is to follow the measures in weight. <ref>See Yalkut Yosef (Klalei [[Brachot]] pg 195-202) who discusses this at length. However, see Or Letzion (Vol. 3, Introduction) who argues that the custom to use weight is only for foods which have similar density to water, but for other foods (like Matza) one would use volume. Halacha Brurah 210:2 rules that the kezayit is measured by volume and not weight. See Otzrot Yosef 8:9. </ref>
#There’s a dispute in the Rishonim whether the [[Kezayit]] (olive size) is a half or third of a [[KeBaytzah]] (egg size), and regarding [[Bracha Achrona]] and Deoritta Mitzvot, the halacha follows the opinion that a [[Kezayit]] is half a KeBeytzah. <ref>*Mishna Brurah 486:1 writes that the Tosfot holds a [[Kezayit]] is half of a KeBeitzah with the shell and the Rambam holds it is a third of a KeBeitzah (see Tosfot (Yoma 80b s.v.
Agav), Rambam (Eiruvin 1:9)). S”A 486 rules like Tosfot. Rav Avraham Chaim Noeh (Shiurei Torah 3:12) argues that Tosfot holds half a KeBeitzah without the shell.
*What's the halacha? Mishna Brurah 486:1 rules that for Deoritta Mitzvot and [[Bracha Achrona]], one should eat the size of half a KeBeitzah, but for Derabbanan Mitzvot, one third suffices. He adds that since one has to make a Bracha on [[maror]], one should eat half a KeBeitzah.
*Chazon Ish 39:17 decided that we could be lenient like the Rambam.
see below for an exact amount according to Chazon Ish</ref>
#According to Rav Chaim Noeh, the [[Kezayit]] for Deoritta measures (eating [[Matzah]]) and [[Bracha Achrona]] is 27cc. The Chazon Ish’s opinion is that the [[Kezayit]] is 33.3 grams (1.3 oz). <ref>*The Tzlach (Pesachim 116b) holds that the modern day eggs are half the size of those in the days of Chazal. However, Rav Noeh (Shiurei Torah Shaar 3) argues that the modern day eggs have not changed from the days of Chazal. Mishna Brurah 486:1 writes that for the mitzvah deoraitta of [[matza]], one should follow the stringent view of the Tzlach. see Rav Shlomo Wahrman (Orot Haesach 28) who explains why there is no need to say that the eggs have shrunk since the times of the Gemara. see also Maaseh Rav 74, where it says that the Gra accepted the position of the Tzlach.
*Practically, how much is a [[Kezayit]]?
#On Mitzvot Deoritta such as eating [[Matzah]] one must be strict to hold a [[Kezayit]] is a half a [[KeBaytzah]]. However, regarding מצות Derabbanan it’s preferable to hold that a [[Kezayit]] is a half a kebaytzah, however, one may rely on the opinion that a [[Kezayit]] is a third a kebaytzah. <ref>Mishna Brurah 486:1 </ref>
#One needs to make sure that he does not count air in the food as part of the shiur of kezayis. If one has a food that is hollow, a kezayis of that food is considered when the food would be pressed.<ref>Rama 486:1 </ref>
===Kedi Achilat Pras===
===Kedei Shtiyat Reviyit===
#See [[Bracha_Achrona#Drink|Kedei Shtiyat Revi'it
|]] (the time in which a Revi'it is considered drunk together).
##Among Ashkenazim, some are strict to hold like the Magen Avraham, unless there is an serious need, in which case one may hold like the Gra. <ref>Rabbi Yechiel Michel Tukachinsky in Sefer Eretz Yisrael (pg 18#3) writes that the minhag of [[Israel]] is to follow the magen avraham and only under pressure may one rely on the Gra.</ref> However, some are lenient to rely on the Gra. <ref>[http://www.yutorah.org/lectures/lecture.cfm/728553/Rabbi_Hershel_Schachter/Shiur_on_Zmanim_in_Halacha Rav Hershel Schachter on yutorah.org] (min 77-79) says that many follow the opinion of the Gra in calculating the time for Sof [[Kriyat Shema]]. </ref>
#Many rishonim hold that [[
Olot HaShachar]] is 72 minutes before sunrise on a "perfect day" (when sunrise and sunset are exactly 12 hours apart) in Jerusalem which is equivalent to when the sun is 16 degrees below the horizon for everywhere in the world, however, others hold that [[ Olot HaShachar]] is 90 minutes on a perfect day which is 19.78 degrees below the horizon. <ref>*The Rosh (Brachot 4:1) writes that Olot for the korban tamid and Shacharit is after the rays of the sun begin to show and also the Eastern part of the sky began to become light. This is also the opinion of the Rabbenu Yerucham cited by Bet Yosef 89:1. Shulchan Aruch 89:1 codifies their opinion. Magen Avraham 89:3 argues that many rishonim hold that Olot is once the rays of the sun begin to show even though the Eastern part of the sky isn't light. Torat Yoma 28a siman 11 writes that the Rambam Pirush Mishnayot Yoma 3:1 and Ritva Yoma 28a hold like the Rosh and Rabbenu Yerucham, while Rashi sides with the Magen Avraham. Practically, the Mishna Brurah (Biur Halacha 89:1 s.v. v'im) writes that one shouldn't be lenient for the Magen Avraham as the Eliya Rabba, Mateh Yehuda, and Gra argue with the Magen Avraham. Therefore, Olot is once the rays of the sun began to show and also the Eastern part of the sky became light.*Sefer Zmanim KeHilchatam (Rabbi Berstein, pg 347) writes that 72 minutes on a perfect day in Jerusalem is equal to when the sun is 16 degrees below the horizon and 90 minutes is equal to 19.78 degrees below the horizon. He personally rules that one should be strict for both of these opinions for Deoritta halachas and those derabbanan halachas where safek is lechumra. He tries to prove that this is the opinion of the Mishna Brurah (92:3, Beiur Halacha 163:1 s.v. Berachok). However, he totally rejects the opinion that [[ Olot HaShachar]] is 120 minutes before sunrise. [It is noteworthy that astronomical dawn is the time when the Sun is at 18 degrees below the horizon and before then the sky is absolutely dark ([timeanddate.com https://www.timeanddate.com/astronomy/different-types-twilight.html]).]
*He also quotes Rabbi Tukachinsky who held like the opinion of 90 minutes.
*[http://www.yutorah.org/lectures/lecture.cfm/728553/Rabbi_Hershel_Schachter/Shiur_on_Zmanim_in_Halacha Rav Hershel Schachter on yutorah.org] (min 65-7) says that we assume that [[
Olot HaShachar]] depends on degrees below horizon and the specific times vary according to one’s location. Similarly, [http://www.ou.org/webcast_kosher Rav Yisrael Belsky] (OU Kosher Webcast, 2011, min 19-21) mentioned in passing that [[ Olot HaShachar]] is assumed to be 72 minutes before sunrise, varying according to the longitude and latitude.</ref>
# According to Sepharadim, Alot HaShachar is 72 minutes before sunrise.<ref> Yalkut Yosef, Kitzur Shulchan Aruch, Siman 58, Seif 3. </ref> These minutes are proportional to the length of the day,<ref> Yalkut Yosef, Pesuke DeZimra and Keriat Shema, pages 432-433 based on Rambam, Perush Mishnayot, Masechet Berachot, Perek 1.</ref> and do not depend on current location.<ref>Zmanim KHalacha, page 43, quoting Hacham Ovadia zt"l. Hence, 72 minutes would be 1.2 multiplied by the day's shaa zmanit. Rabbi Yehuda Levi, author of Zmanim KHalacha, argues with Hacham Ovadia on the same page. He writes that the 72 minutes are not calculated based on the shaot zmaniot and do depend on current location. Rabbi Levi's difficulties stem from the Rambam's language and astronomic calculation. In the Berachot (1:1), Rambam uses a unique Arabic term when referring to alot hashachar, one that is not used in reference to zman Kriyat Shema (Berachot 1:5) or a the time of a mil (Pesachim 3:2). Furthermore, in the winter, it starts to become light earlier than it starts to become light the spring and the fall, despite having shorter shaot zmaniot in the winter; given this reality, it is difficult to justify calculating alot hashachar strictly according to shaot zmaniot. Therefore, Rabbi Levi maintains that alot hashachar's calculation is determined by degrees below the horizon.</ref>
'''Degrees below Horizon'''
# Some hold that
Olot is determined degrees below horizon. There are three opinions about the measure of the mil and therefore, there are three opinions of when Olot is:
## 120 minutes by degrees is 25.9 degrees below horizon.<ref>Zmanei Hahalacha Lmaaseh p. 18</ref>
## 90 minutes by degrees is 19.75 degrees below horizon.<ref>Zmanei Hahalacha Lmaaseh p. 18</ref>
## 72 minutes by degrees is 15.99 degrees below horizon.<ref>Zmanei Hahalacha Lmaaseh p. 18</ref>
# Some hold that
Olot is determined degrees below horizon. There are three opinions about the measure of the mil and therefore, there are three opinions of when Olot is 120, 90, and 72 zmaniot minutes.<ref>Zmanei Hahalacha Lmaaseh p. 18</ref>
# Some hold that
Olot is determined by 4 mil by fixed minutes. There are three opinions as to the measure of the mil and therefore, there are three opinions of when Olot is: ## 120 minutes fixed before Netz.<ref>Zmanei Hahalacha Lmaaseh p. 18 writes that 120 minutes fixed is based on the rishonim that a mil is 24 minutes and there is 5 mil between Olot and Netz.</ref>## 90 minutes fixed before Netz.<ref>Zmanei Hahalacha Lmaaseh p. 18 writes that 90 minutes fixed is based on the rishonim that a mil is 22.5 minutes and there is 4 mil between Olot and Netz. He writes that Rav Tukachinsky used that for his luach, though he argues that we shouldn't follow it since Shulchan Aruch held of the 18 minute mil.</ref>## 72 minutes fixed before Netz.<ref>Zmanei Hahalacha Lmaaseh p. 18 writes that 72 minutes fixed is based on the rishonim that a mil is 18 minutes and there is 4 mil between Olot and Netz.</ref>