'ammāh [Hebrew, אַמָּה]

An ancient Hebrew unit of length which belongs to the cubit family of units, ones based on the distance from the human elbow to the fingertips. Ammāh is usually translated into English as cubit. = 6 tophach. chart symbol The most probable value is 444 millimeters (17.47 inches), although many scholars prefer  450 mm. See below for a longer (= 7 tophach), older, cubit later used for religious purposes, such as sizing altars.

How do we know the length of one 'ammāh?

The most concrete evidence is a tunnel dug by Hezekiah, the King of Judah (late 8th early 7th centuries bce) and described in 2 Kings 20:20 and 2 Chronicles 32:30.  This tunnel still exists, and an inscription on the wall, discovered in 1880 and now in the Istanbul Museum, states in part:

...and the water flowed from the spring toward the reservoir for 1,200 cubits, and the height of the rock above the head(s) of the quarrymen was 100 cubits.¹

Several modern measurements of the length of the tunnel have been made, but dividing these by 1200 does not necessarily give a precise length for the 'ammāh. First, “1200” sounds like a measurement that has been rounded off. As the tunnelers seem to have followed a natural cleft in the rock, working from both ends, and the tunnel is serpentine rather than straight, it is highly unlikely that the tunnel's length could have been preplanned to such a neat figure. The “1200” is an approximation, and we have no way of knowing how approximate it is. Second, we have no way of knowing where the ancient measurement began and ended. Even the modern measurements vary. 

A second line of evidence relies on the dimensions of ancient buildings. Especially where they were not constrained by adjacent structures, ancient builders tended to build in round numbers.

The ammah is derived from earlier models. The word itself is clearly related to the Akkadian ammatu (cubit).

1. W. F. Albright, translator.
The Siloam Inscription.
in James B. Pritchard, editor.
Ancient Near Eastern Texts: Relating to the Old Testament. 3rd ed. with supp.
Princeton, NJ: Princeton University Press, 1969.

examples: cubits in the Old Testament

For only Og king of Bashan remained of the remnant of giants; behold, his bedstead was a bedstead of iron; is it not in Rabbath of the children of Ammon? nine cubits was the length thereof, and four cubits the breadth of it, after the cubit of a man.

Deuteronomy 3:11 (King James)

The Hebrew phrase the King James version translated as "cubit of a man" was translated as "the common cubit" in the Revised Standard Version. But mightn't the author be distinguishing the natural cubit, the distance from the elbow to fingertips, from a standardized measure?

Fifteen cubits upward did the waters prevail; and the mountains were covered.

Genesis 7:20 (King James)

And Harbonah, one of the chamberlains, said before the king, Behold also, the gallows fifty cubits high, which Haman had made for Mordecai, who had spoken good for the king, standeth in the house of Haman. Then the king said, Hang him thereon.

Esther 7:9 (King James)

And there went out a champion out of the camp of the Philistines, named Goliath, of Gath, whose height was six cubits and a span.

1 Samuel 17:4 (King James)

And Jehoash king of Israel took Amaziah king of Judah, the son of Jehoash the son of Ahaziah, at Bethshemesh, and came to Jerusalem, and brake down the wall of Jerusalem from the gate of Ephraim unto the corner gate, four hundred cubits.

2 Kings 14:13  (King James)

And concerning the pillars, the height of one pillar was eighteen cubits; and a fillet of twelve cubits did compass it; and the thickness thereof was four fingers: it was hollow.

Jeremiah 52:21 (King James)

The valley gate repaired Hanun, and the inhabitants of Zanoah; they built it, and set up the doors therof, the locks thereof, and the bars thereof, and a thousand cubits on the wall unto the dung gate.

Nehemiah 3:13 (King James)

The “cubit and a handbreadth”

And behold a wall on the outside of the house round about, and in the man's hand a measuring reed of six cubits long by the cubit and a handbreadth: so he measured the breadth of the building, one reed; and the height, one reed.

Ezekiel 40:5. (King James)

And these are the measures of the altar after the cubits: The cubit is a cubit and an hand breadth; even the bottom shall be a cubit, and the breadth a cubit, and the border thereof by the edge thereof round about shall be a span: and this shall be the higher place of the altar.

Ezekiel 43:13 (King James)

These are Solomon's measurements for building the house of God: the length, in cubits of the old standard, was sixty cubits, and the breadth twenty cubits.

2 Chronicles 3:3 (Revised Standard Version)

for further reading

R. B. Y. Scott.
The Hebrew Cubit.
Journal of Biblical Literature, vol. 77, no. 3, pages 205-214 (September 1958).

And see Scott, Postscript on the Cubit, Journal of Biblical Literature, vol. 79, no. 4, page 368 (December 1960).

A number of web sites have photographs of Hezekiah's tunnel, including:

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