The exact lenth of Aaron's rod is unknown.
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(All quotations from the King James translation unless stated otherwise).
How long was Aaron’s rod because I wonder how it fit in the ark of the covenant?
Hebrews 9:4 Which had the golden censer, and the ark of the covenant overlaid round about with gold, wherein was the golden pot that had manna, and Aaron’s rod that budded, and the tables of the covenant;
NKJV Exodus 25:10 "And they shall make an ark of acacia wood; two and a half cubits shall be its length, a cubit and a half its width, and a cubit and a half its height.
A cubit (six hand’s [each 4 fingers wide] breadths or the distance from the tip of the extended middle finger to the elbow) is approx 18 inches (46 cms). The length of the Ark was 2.5 cubits or 45 inches (114cms). The diagonal across corners would be 2.9 cubits or 52.5 inches (133 cms). Allowing for an unknown wall thickness and a depth of 1.5 cubits or 27 inches (69 cms) we could guess that the Ark could accommodate a rod of approx 2.6 cubits or 48 inches (122 cms). In old measurements that would be a maximum length of four feet.
That Aaron’s rod did fit into the Ark as you say seems clear in Hebrews 9. However, we are not told how it long was, so the correct answer is that no one knows.
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