Who knows forty?

Please cite/link your sources, if possible. At some point after Shabbat, I will:

  • Upvote all interesting answers.

  • Accept the best answer.

  • Go on to the next number.

18 Answers 18


40 generations from Moshe Rabbeinu to Rabbeinu HaKadosh (Rabi Yehudah HaNasi).

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    Checkmark for fitting with after Shavuot. – Isaac Moses May 23 '10 at 12:09

40 years that Rabi Akiva was a boor, 40 years he learned Torah, and 40 years he served Klal Yisrael.

Bamidbar Rabbah, Parshasa 100

  • The Gemara Sanhedrin says something similar for Raban Yochanan ben Zakai. 40 years a businessman, 40 years a student, 40 years a teacher. – Shalom May 18 '10 at 15:48
  • A boor is another term for Am HaAretz, so I hope you did not mean to equate the two! (I know you didn't!) :-) – Yahu May 18 '10 at 23:08
  • These terms aren't really interchangeable. See Avos 2:5: אין בור ירא חטא, ולא עם הארץ חסיד - implying that the Am Ha'aretz, at least, can be ירא חטא. – Alex May 18 '10 at 23:25
  • Good point! It seems according to this medrash that when Rabi Akiva was an am haAretz he was also a boor – Yahu May 21 '10 at 16:28
  • When I said that "I hope you did not mean to equate the two" I meant businessman and boor. – Yahu May 21 '10 at 16:29

(age) 40 for wisdom -Avot 5:24


Forty days before a baby is formed, it is determined in Heaven who the baby will marry.

(Sotah 2a)

  • If I'm not mistaken, it's "40 days before the baby is fashioned", which is interpreted as the time the baby is conceived. – Shalom May 18 '10 at 20:45
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    What Shalom meant is that 40 days before a baby is fashioned is actually the moment of conception. – Yahu May 18 '10 at 23:09
  • related judaism.stackexchange.com/q/2447/759 – Double AA May 1 '17 at 18:21
  • Edited the question to match what our commenters have been saying here, and also added a source. :) – ezra May 1 '17 at 18:43

40 Sa'ah (usually given as 200 gallons) is equivalent to 3 cubic Amah (cubits), and is the minimum volume of a mikvah.

A mikvah must be big enough to immerse an average person (estimated at 1 Amah wide x 1 Amah deep x 3 Amah tall).

Knowing those facts, the Gemara in Arvei Pesachim computes the Reviis-Halog (from units of volume to cubic length) as 2 x 2 x 2.7 etzbaos. The computation is pretty straightforward, but many Rishonim do it in all sorts of convoluted ways -- as they try to avoid decimals!

Per Mishna Mikvaos 7:1 and this table of salt solubility, 40 lbs of salt is what you can add to a 198-gallon mikva of pure water to get it up to 200 gallons. (Rounding slightly.)


40 are the days Moshe Rebbeinu was on Har Sinai receiving the Torah (Shemos 24:18) "Moshe arrived in the midst of the cloud and ascended the mountain; and Moses was on the mountain forty days and forty nights."

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    He also prayed for forgiveness for another 40, then ascended the mountain for a 3rd set of 40... (This gives us a few troikas of 40, as well. Ad meiah ve'Esrim!) – RavingRabbi Feb 5 '14 at 17:07

Forty days from an egg's fertilization at which point it's Halachically called a fetus. (Before that, it's "just water.")

Some interesting applications of this:

  • Some poskim use this source to allow some cases of abortion before this point, though R' Moshe Feinstein vehemently disagrees.

  • R' Ahron Soloveichik uses the above source to argue that Halachic motherhood is determined by womb and not egg, "as before 40 days the Halacha treats the egg no differently than a synthetic substance made in Japan."

  • When the Mishna in Avos talks about decrees made "40 days before the fetus is fashioned", that's usually interpreted as the moment of conception = (fashioning minus 40).

  • The Flood waters rage for 40 days and nights, as the world needs to be re-born, so to speak. Similarly, Moses is on Mount Sinai 40 days and nights, at which point he's reborn at a higher spiritual level (so to speak).

  • The Gemara (Nida 30b, see Rashi) records the results of some human experimentation conducted by non-Jews that involved impregnating women and later cutting them open to see whether a fetus' gender is visible by the 40-day-point. This seems to imply that Judaism is willing to make use of scientific knowledge even if it was obtained unethically. (Heard on a Rabbi Frand tape.)

  • Re the last point: I assume you're referring to the Gemara in Niddah 30b, about Cleopatra's maids who were sentenced to death? If so, then I don't know that this would count as "a hideous experiment involving killing lots of pregnant women"; it seems that they were anyway going to be killed, so Cleopatra (or, more likely, Greek scientists from the Library of Alexandria) took the opportunity to dissect them and draw conclusions. – Alex May 18 '10 at 16:45
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    Thank you. See Rashi; if I understand correctly, yes they were going to be killed anyhow, but to go purposely impregnate them for the sake of human experimentation? I'd call that unethical. – Shalom May 18 '10 at 16:57
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    Aha, I hadn't noticed what Rashi says, so I thought that it just so happened that they were pregnant when they were sentenced. In that case, you're right. – Alex May 18 '10 at 17:35

40 cubits is the radius of a circle, centered on a vegetable or grain planted in a vineyard, in which (in some cases) all of the plants within the circle become forbidden for use and must be burned. (Rambam, Hil. Kilayim 6:2. Very timely, incidentally; this was the daily portion of Rambam, according to the one-chapter-a-day cycle, two days ago.)


40 "years" per "generation" (see Psalms 95:10).

Rabbi Isaac of Akko used this factor to calculate the age of the Universe as ~15 billion years. (He lived ~650 years ago.)


Rabbi Tzadok fasted for 40 years (days only) to stave off the destruction of Jerusalem (Bavli, Gittin 56b). (Interestingly, the Ostrovtzer Rebbe is reputed to have fasted 40 years before the Holocaust, as well!)

40 years wandering in the Desert. (It's so obvious, that nobody said it!)

At 40 days, the fetus is considered to have a human form, and for some purposes it becomes like murder to abort it at this point. It also has ramifications for Ritual Purity (Mapelet Shilya). Hence, the moment of conception is sometimes referred to as "40 days before the formation of the child".

The Maharal considers 40 as the number of formation, so he considers the Jewish people as having been "formed" during their desert experience.


40 days and 40 nights of rain for the Flood ("'Let it rain for 40 days and 40 nights and wait for the sewers to back up.' 'RIGHT.'"-Bill Cosby)


Each stone that they set up in the Jordan River (in Yehoshua) weighed 40 Seah.

Sotah 34a

  • Was that weight or volume? – Shalom May 18 '10 at 17:26
  • It says that each one's "burden" was 40 Sa'ah. That means weight. – Yahu May 18 '10 at 23:11
  • Isn't the sa'ah a unit of volume? Does that mean the weight of 40 sa'ah in water? (In which case the rocks themselves didn't actually have a 40-sa'ah volume?) – Shalom May 21 '10 at 1:52
  • I've seen sa'ah used in the form of "the weight of a sa'ah of wheat". I do not remember at the moment where I saw it used this way. It could be what is meant here. – Yahu May 21 '10 at 3:08

40 years before the destruction of the Beis Hamikdash the Sanhedrin moved from the Hewn Chamber to a place called Chanus. (Sanhedrin 41a)


The torah says a person gets 40 lashes per transgression of a typical prohibition. Rabi Yehudah takes this literally. (Makkos 22a) The Tana Kamah holds that a man can only get 39 per sin.

  • Besides lashes listed as 40 minus 1, Av Melachot of Shabbat are also listed as 40 minus 1. – RavingRabbi Feb 5 '14 at 16:51

40 are the days on which Moshe was on Har Sinai asking for forgiveness for the Jewish people, and 40 are the days that Moshe was on Har Sinai to get the second set of luchos, which are the days from Rosh Chodesh Elul to Yom Kippur.


40 are the days Yechezkel lay on his right side for the sins of the Southern Kingdom (Eze. 4:6).


40 are the days Goliath presented himself for battle with Israel, before David arrived and ultimately slayed him (Shmuel I 17:16).


Esav married when he was 40 years old Bereshit Rabbah 68:5

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