Tell you an indigenous method:
Diameter× diameter× length×123 = steel weight (kg)
20? ×250× 123= 12300000
Answer: The weight of this round steel123kg = 61.5kg..
Where 123 is a constant, 1233 is more accurate and sufficient for daily use.
As for the unit, for the time being. Look at the real thing and you will know that it is 123 kg, but it can't be 12.3 kg, let alone.
It may be 1230 kg,
This calculation is very convenient, and the constant 123 is also easy to remember, and the error is not worse than that of table lookup, and there is no size gauge.
This is a local method, so we might as well verify it.
Good luck, goodbye.