Before even costing the materials, knowing the overall weight of a steel structure with reasonable accuracy affects several parts of the quote — from material cost to transport, through to the lifting equipment needed on site.
Theoretical weight from standard sections
Every standard section — box, angle, flat — has a published theoretical weight per linear metre for its size and thickness. Adding the length of each section used in the structure multiplied by its unit weight gives a reasonably precise estimate of total material weight.
Why weight affects transport
Above a certain weight, transport requires specific vehicles and in some cases permits for abnormal loads if dimensions exceed standard limits. Underestimating the weight means discovering at despatch that a different vehicle is needed, at a cost that was never in the original quote.
Weight and lifting requirements on site
A structure of significant weight may need a crane or dedicated lifting equipment for installation — an additional cost to be assessed in advance, not discovered when the structure is ready and cannot be moved with what is available on site.