Safety factors for ductile materials based on yield strength
Safety factor	Knowledge of loads	Knowledge of stress	Knowledge of material	Knowledge of environment
1,2-1,5	Accurate	Accurate	Well known	Controllable
1,5-2	Good	Good	Well known	Constant
2-2,5	Good	Good	Average	Ordinary
2,5-3	Average	Average	Less tried	Ordinary
3-4	Average	Average	Untried	Ordinary
3-4	Uncertain	Uncertain	Better known	Uncertain