Factors of Safety - FOS

Calculate the Factor of Safety (FOS) for structural and mechanical engineering applications. FOS is expressed as the ratio of failure load or stress to allowable load or stress, helping engineers ensure safe design margins.

Factors of Safety - FOS Calculator

Factors of Safety are used in engineering design to ensure structural integrity and safety. The basic formulas are:

Formula 1 - Load Based:

FOS = Ffail / Fallow

Formula 2 - Normal Stress Based:

FOS = σfail / σallow

Formula 3 - Shear Stress Based:

FOS = τfail / τallow

Where:

FOS = Factor of Safety
Ffail / σfail / τfail = failure load/stress
Fallow / σallow / τallow = allowable load/stress

Calculator

Inputs

Calculation Type

Failure Load/Stress (N or N/m²)

Allowable Load/Stress (N or N/m²)

Results

Factor of Safety (FOS)--

Equipment	Factor of Safety - FOS
Aircraft components	1.5 - 2.5
Boilers	3.5 - 6
Bolts	8.5
Cast-iron wheels	20
Engine components	6 - 8
Heavy duty shafting	10 - 12
Lifting equipment - hooks	8 - 9
Pressure vessels	3.5 - 6
Turbine components - static	6 - 8
Turbine components - rotating	2 - 3
Spring, large heavy-duty	4.5
Structural steel work in buildings	4 - 6
Structural steel work in bridges	5 - 7
Wire ropes	8 - 9

General Recommendations for FOS

Application Type	Material Reliability	Conditions	Recommended FOS
Highly reliable materials	Highly reliable	Not severe, weight-critical	1.3 - 1.5
Reliable materials	Reliable	Not severe	1.5 - 2
Ordinary materials	Ordinary	Not severe	2 - 2.5
Less tried/brittle materials	Less reliable	Not severe	2.5 - 3
Unreliable properties or reliable materials	Variable	Severe or difficult	3 - 4

Note: FOS values are often specified in technical standards and design codes, particularly for statutory items such as cranes and pressure vessels.