In 'real' engineering, every engineering calculation has a safety factor attached to it. You figure out how much you need, and then you multiply it by the chose

In 'real' engineering, every engineering calculation has a safety factor attached to it. You figure out how much you need, and then you multiply it by the chosen safety factor to get what you need. The choice of safety factor varies. In some cases, it may be regulated/required. In other cases it is based on the engineer's judgement. And of course, it will vary from field to field. In aerospace, safety factors are typically much lower than in civil engineering, since weight is a much greater concern. Which brings back to why we have a safety factor at all. The only reason why we have it is because of uncertainty. The real why we have safety factor at all is because we don't know enough.