How can you calculate the weight of steel?
Calculate the volume of steel in your part or assembly. Find the density of your particular alloy; regular carbon steel is around 7.85g/cm^3. Multiply the volume by the density to get the weight. If you have a small, complex shape you can weigh it directly.
If your steel parts are of a standard section like RSA, RSC or CHS, you can look up the weight per meter from the manufacturer’s tables. Multiply this by the length in metres, and you have your weight.
Model the whole thing in Solidworks or similar, set the material properties to the alloy of your choice and use the evaluation tools to find the weight, volume and centre of mass.