The words used for linear measurement in the OT are the qaneh, "reed" or "rod," the 'ammah, "cubit," the zeret, "span," the tepah or topah, "palm," and the 'etzba, "finger." Generally speaking, these units are related like this:
1 reed = 6 cubits.
1 cubit = 2 spans.
1 span = 3 palms.
1 palm = 4 fingers.
The most common measure of length mentioned in the OT is the 'ammah, cubit, which is defined as the length of the arm from the elbow to the tip of the middle finger, making it an average of 44.5cm or 17½inches. "This is confrimed by information in the Siloam Inscription stating that the tunnel (which measures 1749 feet) was 1200 cubits long. This would make the cubit then used (in Hezekiah's day) approximately 17½ inches" (TWOT). Eze 40:5, also speaks of a "long cubit" (sometimes also called the "royal cubit") which is a standard cubit plus a palm, making it 51.8cm or 20.4in.
The span is defined as the distance between the tips of the small finger and the thumb with the hand spread out—this averages about 22.5cm or 9 inches (see, e.g., Exo 28:16; 39:9, Eze 43:13). The palm is the width of the hand at the base of the four fingers, while the finger, naturally, is a quarter of the palm.
Such measurements were not, of course, universal in ancient times. The Babylonian cubit, e.g., was 50.3cm, while the Egyptian cubit was 52.45cm, both longer than the Hebrew unit by more than 10%.
Other units of measurement are also mentioned in the OT. The gomed is mentioned only in Judg 3:16 as the length of the sword with which Ehud killed fat Eglon; no one knows for certain what length it referred to. Longer distances, where precision is not important, are referred to by such expressions as "a bowshot" (Gen 21:16), "a journey of three days" (Gen 30:36, Exo 3:18).
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