Minimum Thread Engagement Equation and Calculator ISO

Fastener and Thread Design Menu | ISO Hardware Data
ANSI Hardware Data
Mechanics / Strength of Materials Menu

The screw should fail before the thread strips. For this, it is necessary the shear area of the threaded feature be at least 2 times the tensile area. (Assuming the threaded feature is of the identical material).

Where:

L_e = Minimum Thread Engagement Length
A_t = Tensile Stress Area
D = Major Diameter of fastener (screw)
p = 1 / Number of threads per inch (n)

Open Minimum ISO Thread Engagement Calculator

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Threaded Hole / Fastener Ratio:

Open - ISO Thread / Fastener Ratio and Length Calculator

This assumes that the male and female thread materials have the same strength. If the Female Material strength is lower i.e. J as calculated below is greater than 1 then the length of engagement must be increased to prevent the female thread stripping

Where:

J = Thread Engagement Ratio

If the value of J is greater than 1 then the length of engagement must be increased to at least  

Where:
l_e = Thread Engagement Length