Mishna Berura Source Sheet 160:13 - 15

The term reviis is short for "reviis halog", a quarter of a log.

1 log is 6 beitzim. (Source: Manachos 89a)

1 beitzah is 1.9 fl. oz.

1 log is this 11.4 fl. oz. (1.9*6)

A quarter of a log (reviis) is 2.85 fl.oz.

Let's get a picture as to how much a reviis is in practical terms.

Based on Biur Halacha (271:13) in a different context in Hil. Kiddush, the shiur is 3.8 fl. oz.

Other opinions put a reviis between 3 and 5.5 fl. oz.

For a point of reference:

- That's a little less than a half full average styrofoam cup (8 oz.)

- The standard measurement of a "cup" (as found in recipes or on measuring cups) is 8 fl. oz. Therefore, a revi’is is approximately one half of a cup.

Mishna Berura here begins by mentioning the correct procedures to wash netilas yadayim with the different amounts of water available to him.

1. In Mishna Berura he explains the way to wash min hadin with a reviis.

2. In Shaar Tzion he says the recommended way to wash lechatchila with a reviis.

3. Mishna Berura ends off the most recommended way to wash with an abundance of water.

1. The way to wash min hadin with a reviis

Mishna Berura says that min hadin one do one act of pouring of a reviis of water on both hands.

This is inferred from a halacha from later on:

2. The recommended way to wash lechatchila with a reviis

But lechatchila one should pour the reviis in two pourings over each of his hands.

This is inferred from the next seif in the above quoted siman.

The reason this is more recommended than the first method is because although when one washes once upon each hand, his hands become tahor, however, the water that is now on his hands has become tameh and there still exists the possibility that this water may splash onto terumah and make it tameh, and thus inedible for Kohanim to eat (which is itself an issur, see (Bamidbar 18:8) and Sanhedrin 83a-b)

Therefore, by washing a second pouring onto that original water, one purifies that tameh water and eliminates that second concern of water contaminating the terumah.

3. The most recommended way to wash with an abundance of water

But if a person has abundant water, he should pour even more than a reviis in the prescribed manner of alternating between both hands with two pourings.

This is inferred from a halacha from a few simanim ago.

Additional halachos concerning the need for shiur reviis in washing

1. From the simple reading of Shulchan Aruch, one needs a reviis in total for both hands.

However, some hold that one needs to pour a reviis on each hand.

Shaar Tzion brings proof that the first opinion seems to be more correct, however in Mishna Berura he ends off that one who wants to avoid any issues should pour a reviis on each hand to keep all shittos.

2a. Since the halacha is that you need a reviis for two hands, one could have thought that if one only has one hand or if one hand was previously tahor*, that you only need 1/8 of a reviis, this is not the case because the minimum "shiur tahara" is a reviis.

2b. Similarly, one could have thought that someone who has small hands doesn't need a full reviis, this is not the case, even he will need a full reviis.

3. Because of this "shiur tahara" one needs to wash with a full reviis at once; he cannot split two washings into 2 pourings of an with of a lug one after the other. (seif 14 in the name of Rashba in Toras HaBayis HaKatzar)

There are a couple discussions in this unusual washing going on here.

1. The need for the water to be continuously falling between each person washing & the intention to have this "doubles" washing.

2. The distance between each person's hands.

3. The differences in the measurements based on the amount of people (next section)

1. The need for the water to be continuously falling between each person washing and the intention to have this doubles washing

According to this shitta, even if the water is continuous, it is still necessary that they are both having kavanna for this one reviis to work for both of them.

To fully understand the requirement according to these Rishonim (Rashba and Ritva) for why the water cannot stop, we need to first understand a Klal called chibbur nitzok.

By Mikva, the of chibbur nitzok allows the second person to become tahor since the first person was still in the mikvah as the second person was going in, thus connecting the tevilla of both of them to be a tevilla in a full 40 seah mikvah.

Here too, since although they aren't washing at the exact same time (the second person is coming afterwards), we can apply the klal of chibbur nitzok to say that the second person was also washing from a full reviis cup.

Although the comparison is slightly different, nevertheless, since netilas yadayim is DeRabbanan, we were maykel to generously apply the din here too.

There are those who argue (brought in Rashba's work Toras HaBayis HaKatzar) and say that the need for the water to be flowing continuously between both of them is unnecessary. Even if they are washing one after the other in the most literal sense, it is okay for them tot share a single reviis as long as they both had in mind to do this "doubles" washing from the start.

2. The distance between each person's hands

The wording of the Shulchan Aruch seems to imply that they need to position their hands close together in order to consider them one unit in regards to this Netilas Yadayim. (If ones hands would be further away, it would be like washing from ones friends Netila which we said was passul at the end of seif 11, ayin sham)

However, Beis Yosef has doubt as to the necessity of close proximity between both of their hands.

Although their hands should be close together, they should not be pressed to closely together in order for the water to reach the surface of each of their hands. The likeliness of this being overlooked is more prevalent to a case of numerous people washing together (as this is mentioned in that context at the end of the seif), but it applies to all cases.