Before you order it, before you lift it, before you work out what the trailer can carry. Choose the shape and the metal, enter the dimensions, and get the weight per metre, the total, and the cost if you know the rate.
Notes
Where the densities come from
These are standard handbook densities in kg per cubic metre: mild steel 7850, stainless 8000, aluminium 2700, brass 8500, copper 8960, cast iron 7200, lead 11340. Alloys vary by a percent or two, which is well inside the tolerance of the stock itself.
Tube weight is the difference of two cylinders
A tube weighs the same as the solid bar of its outside diameter minus the bar of its bore. Which is why doubling the wall thickness of a thin tube nearly doubles its weight, but barely changes the outside size.
Why the merchant weight differs
Rolling tolerances, mill scale and paint all add a little. Structural sections are also sold to nominal sizes that are not quite the real ones. Expect a couple of percent either way.
What this does not do
It weighs geometry. It does not check whether a section is strong enough, which depends on span, load and how it is fixed.