One of the most useful numbers to keep in your head is: one square foot of 1" steel weighs 40#
(really 40.8#, but I don't keep the small stuff upstairs well)
So, quick math says a square 1/4" thick being on quarter the dimension, is 10# ,1/8" is 5#, etc.
A 1" square bar 12' long is 40#, or 3.3# foot x 20'= 66#
A 3" square tube with 1/4" wall one foot long is nothing but a one square foot piece folded up, it's 10# also.
No, for the purist, this figures aren't accurate, but if you find yourself without a chart, and want a quick guesstimate, this works well.