We have now seen that in order to fulfill the mitzva of eating matza, as well as any other Torah commandment governing eating, one must eat at least a kezayit. One condition must now be added: it is only considered a single act of eating if it is completed within the amount of time it takes to eat half a loaf of bread, or a shi’ur akhilat pras. If one eats half a kezayit, waits ten minutes, and then eats another half a kezayit, it is as if he has only eaten half a kezayit, and hence he has not fulfilled the mitzva (SA 475:6).
In the past, people would bake loaves of bread that would feed one person for a day – half a loaf (a pras) at each of the two main meals.
Once again, our long exile has caused disagreement among poskim over the precise size of a pras: according to Rambam, it is the volume of three eggs, and according to Rashi, four eggs. Several leading Aĥaronim tried to measure how long it takes to eat a pras, and many opinions have been offered: nine minutes (Ĥatam Sofer), seven and a half minutes (Arukh La-ner), seven minutes, six minutes, five minutes, and four minutes (R. Ĥayim Naeh and Kaf Ha-ĥayim).
However, all these opinions relate only to a be-di’avad situation, because le-khatĥila one should eat the matza continuously, and whoever does so certainly fulfills his obligation. In addition, there is no need to look at the clock while eating the matza, because any ordinary person who eats casually but without interruption will certainly finish eating in time. Even a slow eater, if he does not stop to do other things, will assuredly finish eating a kezayit within a shi’ur akhilat pras, because a pras is eight or nine times the size of a kezayit, and it is inconceivable that one who eats a kezayit without stopping will not finish in the time it takes to eat eight or nine times that amount.
. As we have seen, according to Rashi, a pras is the size of four eggs, and according to Tosafot, a kezayit is half an egg. Thus, a pras is eight kezeytim. According to Rambam, a pras is the size of three eggs, and each egg is a bit more than three kezeytim. Thus, a pras is just over nine kezeytim. (If we combine Rashi’s pras with Rambam’s kezayit, a pras would come out to be twelve kezeytim. If we combine Rambam’s pras with the kezayit of Tosafot, a pras would be six kezeytim.)
Although some individuals measured and discovered that they could not eat a kezayit of matza within a shi’ur akhilat pras, they only reached this conclusion because they used conflicting measurements. On the one hand, they measured a kezayit according to the most stringent opinion possible – a Ĥazon Ish shi’ur calculated by weight – and on the other hand, they calculated a shi’ur akhilat pras based on the smallest possible measure – Rambam’s shi’ur of three eggs, calculated by volume. These two measurements are contradictory: a Ĥazon Ish kezayit by weight is c. 50 grams, while the three eggs’ volume of matza is c. 54 grams. Since matza is difficult to chew, and a shi’ur akhilat pras is calculated based on the time it takes to eat bread that is easy to chew and swallow, these individuals discovered that they could not finish that “kezayit” in time. According to this, the Sephardic custom to eat two weight-based kezayit-sized pieces, or c. 58 grams of matza, in the time it takes to eat three eggs, or c. 54 grams, is certainly impossible under normal circumstances. Rather, one must calculate the size of a kezayit and the shi’ur akhilat pras using the same standard: if one is stringent regarding the size of a kezayit (and measures it by weight), he must allow himself more time to eat it by calculating the size of an egg based on weight as well, meaning that a shi’ur akhilat pras would be fifteen minutes or more.
Therefore, anyone who eats casually, provided that he does not pause or excessively procrastinate in his eating, will certainly be able to finish the kezayit within a shi’ur akhilat pras; thus, there is no need to glance at one’s watch while eating.
Nevertheless, if one paused while eating and did not finish one third of a matza within four minutes, he should act stringently and eat another piece in that time. Logic dictates that since the determination of shi’ur akhilat pras is based on the average person, and since there are so many measurements offered, we should calculate it based on the median of the opinions of the Aĥaronim (6-7 minutes). However, since the mitzva of eating matza is from the Torah, one should follow the strictest opinions. See Peninei Halakha: Berakhot 10:7, 8.
Additionally, according to Minĥat Ĥinukh and Responsa Torat Ĥessed OĤ 32, the shi’ur akhilat pras is determined separately for every food, based on the ease or difficulty of eating it. Accordingly, the shi’ur akhilat pras for matza would be longer than usual, since matza is difficult to chew and swallow. Nevertheless, most poskim who determined a shi’ur akhilat pras understood that it is a fixed standard based on the eating of regular bread.