Density of Mild Steel

i want to calculate the actual weight for my project....but this project have a big size. so i take a sample of mild steel to compare the volume then convert to actual size..as i know density = mass/volume(kg/m3). then i know the actual size for my project. so, if i know the mass and volume for sample then i can calculate the density. but the value of density is not same with standard density? should i calculate the density or just take the standard density for calculate the actual weight.