How do I calculate the depth of a thread?

The depth of a thread is the difference between the major and minor diameters of a screw. Thread depths and diameters are standardized, and these values are available in charts like this one.

If screw threads were perfect triangles, the depth of thread would be sqrt(3)/2 x the pitch (the number of inches between two threads), or about 0.866 x pitch, thanks to the geometry of equilateral triangles. However, threads are actually truncated, meaning the top of the triangle is cut off, leaving a trapezoidal cross section. As a result, the actual depth of thread is less. A standard full thread (100%) is closer to 0.65 x the pitch.

However, the more the triangle is truncated, the smaller the fraction gets. Some screws are labeled as being 60% or 75% thread, and in this case, the depth would be 0.60 x 0.65 x pitch, or 0.75 x 0.65 x pitch, respectively.