Bad math steel weight estimation

Ok, here is a quick n dirty system we use at the steel yard to estimate the weight on a given bar of steel (this works for hot or cold roll as well as carbon steels). Since we often buy by the pound, not per piece, knowing about how much you have in the truck before you hit the scale helps to keep the sticker shock down to a minimum at check out. This system is really for getting random steel for playing with or so you have various stock on hand for what ever comes up, not for a given detailed project.

For those who have not purchased steel (or coal) in large quantity, I should explain quickly; You weigh your vehicle before you go get what you want, load it up and weigh it again on the way out and pay for the difference. Yes, you could fill your back seat with sand and pour it out before you load up to get more “free” stuff, but be advised if you get caught (and you will) its a whole lot of jail time.

Most steel weighs 489 pounds per cubic foot. All you have to figure out is what part of a cubic foot your bar weighs, right? Heck no! I am way to lazy for that. So some math later, we find that a cubic inch of steel weighs .28 pounds. This is a lot handier... round up just to make the numbers easier; a cubic inch guesstimated weight is .3 pounds. Therefore every 4” section of 1/2” square is .3 pounds, so 3 of those equals just under a pound. Remember, round up. Hey, if you are 20 pounds under at the scale, that's less you have to pay on the way out the door!

So 1 foot of 1/2” square weighs a pound. Ok, a “bad math pound”.

Therefore, if you have a 1” x 1/2” x 1 foot bar, it's going to weigh 2 pounds (since there is twice as much steel there) and a 1” square bar 1 foot long should weigh under 4 pounds. (It's actually more like 3.4 pounds per foot, but round up for bad math.)

you could round down to a quarter pound per cubic inch, but I would rather not accidentally overload the truck and break an axle or some dang thing, not to mention I often hit the steel yard with a given amount of money on hand and I want to be sure I come in under budget for random steel. On a given project, I tend to buy what I need plus 10% for screw ups, but this system is for random stuff to make what ever out of.

Buying thick plate is similar. A 12” by 12” section of 1/4” plate weighs almost exactly 10 pounds, so if you can do some quick math based on this you can guesstimate the weight of a given section of plate. 1/8” thick is half as heavy, 1/2” plate is twice as heavy. Simple! Ok, simplish. You may be able to envision the bar you are looking at as a section of plate, cut into strips and laid end to end to make up a bar or vice versa. It all depends on how you happen to think about volumetric objects, as different peoples brains work in different ways.

Ok, so that works for square or rectangular stock, what about rounds? Since a circle fills about 80% of the volume of a square that is the same width, you can take your formula above to figure out what a square bar that dia/across would weigh, then multiply that by .8 or just call it close enough and have a little money left over at the check out to go grab lunch. Again, this is already rounded up a bit, so your actual weight will be under the guesstimate.

Most other shapes can be though of as made up of parts that you can already easily figure out with the above formula. Angle iron is two sections of flat bar stuck together down one edge, channel is three sections. You can think of pipe as a flat bar that has been rolled into a tube, so here take the thickness then wrap the tape measure around the pipe to figure out what a given section of pipe would be as flat stock (unless you can do circumference of a circle in your head easily – some people can, some can't).

And there you have it, a lazy, bad math way to guesstimate the weight of a given bar of steel!

Or just write down 1/2” square is a pound a foot and 1/4” plate is 10 pounds per square foot and work off that.

Either way.