Factor Of Safety

factor of safety is very importan thing for engineers and all engineers use this factor for their projects.But what is factor of safety ?
engineers use very useful calculation for their projects but everytime they have to neglect somethings for example transverse shear for strength of materials or air resistance for some aerodynamics calculation and sometimes they use some materials but the material!s properties can be changeble because of this reasons mechanical engineers use factor of safety. but how ve can calculate factor of safety it is so easy first of all the calculations done and then make a decision for factor of safety value for example 1.8 is food value for strength of materials applications if we made calculation and we have found for example 100 mpa then we will divide it with 1.8 and we can find new value
new value=calculated value/f.s