AV - cubit 242, measure 1, post 1, not translated 1; 245. 1) cubit - a measure of distance (the forearm), roughly. 18 in (.5m). There are several cubits used in the OT, the. cubit of a man or common cubit (Dt 3.11), the legal cubit. or cubit of the sanctuary (Eze 40.5) plus others.
http://blogs.christianpost.com/the-pastors-eye/how-long-is-a-cubit-8782/
Noah’s Cubit. Though the original cubit length is uncertain, it was most likely one of the long cubits (about 19.8–20.6 in). If so, the Ark was actually bigger than the size described in most books today, which frequently use the short cubit.
https://arkencounter.com/noahs-ark/cubit/
The royal cubit was 523 to 525 mm (20.6 to 20.64 inches) in length: and was subdivided into 7 palms of 4 digits each, for a 28-part measure in total. The royal cubit is known from Old Kingdom architecture dating from at least as early as the construction of the Step Pyramid of Djoser around 2,700 BCE [13–15].
https://www.hindawi.com/journals/janthro/2014/489757/
How Long is a Cubit? The cubit (normally defined as the length from elbow to fingertip) is the most widespread unit of measure from the ancient world. However, ancient cubits vary and not all cubits are defined as the distance from elbow to fingertip. Investigation reveals the cubit length to be anywhere from 17 to 24 inches.
http://www.dancingwithwater.com/articles/rings-and-tensor-fields/how-long-is-a-cubit/
From late Antiquity, the Roman ulna, a four-foot cubit (about 120 cm) is also attested. This length is the measure from a man's hip to the fingers of the outstretched opposite arm. The English yard could be considered to be a type of cubit, measuring 12 palms, ~90 cm, or 36 inches (3.00 ft). This is the measure from the middle of a man's body to his fingers, always with outstretched arm. The English ell is essentially a kind of great cubit of 15 palms, 114 cm, or 45 inches (3.75 ft).
https://en.wikipedia.org/wiki/Talk%3ACubit
The copper bar cubit of Nippur identifies the Sumerian cubit as 517mm (20.4 inches). In Egypt, both a common (short) cubit and a longer Royal or Sacred cubit emerged. The common Egyptian cubit was the length of the forearm from the elbow to the tip of the middle finger (about 18 inches).
http://www.dancingwithwater.com/articles/rings-and-tensor-fields/how-long-is-a-cubit/
The first is the common cubit which was about 17.5 inches long, while the less common long cubit was about 20.4 inches long. This latter cubit is mentioned in Ezek 40:5. If you are interested in Noah's Ark, using the 17.5 inch cubit, it would have been around 437.5 ft long, 72.9 inches wide, and 43.8 feet high. It had a surface area of 95700 sq ft, a cubic volume of 1,396,000 cu ft, and an approximate displacement of 20,000 tons.
https://answers.yahoo.com/question/index?qid=20081023223732AAcWGjP
Taking 21 inches for the cubit, the ark Noah built would be 525 feet in length, 87 feet 6 inches in breadth, and 52 feet 6 inches in height. Using the standard 20′′ cubit and 9′′ span, Goliath’s height would be 6 cubits plus a span for about 10 feet and 9 inches. With a cubit of 18′′ his height is 9 feet 9 inches.
https://www.hindawi.com/journals/janthro/2014/489757/
A cubit is an ancient measurement of length based on the distance from the elbow to the tip of the longest finger. The actual length of a cubit varied between different ancient groups of people. Here are some samples from Egypt, Babylon, and ancient Israel: Hebrew Short: 17.5in. Egyptian: 17.6in.
https://arkencounter.com/noahs-ark/cubit/
Standard/Biblical cubit: 6 palms x 4 fingers = 24 units, Egyptian Royal cubit: 7 palms x 4 fingers = 28 units. I tweaked the following with an excellent reference... The cubit is a traditional unit of length, based on the length of the forearm: from the elbow to the tip of the middle finger.
https://en.wikipedia.org/wiki/Talk%3ACubit
how long is a cubit?
A cubit is 7.5 inches long.   A cubit is 17.5 inches long.