Calculate the minor and pitch diameter of an internal thread from its major diameter and pitch or TPI, or pick a common metric or UNC thread size.
Internal Thread Formula
The following formula estimates the basic minor diameter of a 60-degree internal thread before tolerance-class limits. It is not a tap-drill size or a fit/strength check.
Variables:
- d is the minor diameter of the internal thread (mm)
- D is the major diameter of the internal thread (mm)
- P is the pitch of the thread (mm); for TPI input, P = 25.4 / TPI
To calculate the minor diameter of the internal thread, multiply the pitch of the thread by 1.08253. Then, subtract this result from the major diameter of the internal thread.
What is an Internal Thread?
An internal thread refers to a helical or spiral structure inside a hole, typically in materials like metal, wood, or plastic. This structure is designed to align and engage with an external thread, such as a screw or bolt, to hold pieces together. The creation of internal threads involves processes such as tapping or thread milling. The size and shape of the internal thread must match the external thread for proper function.
How to Calculate Internal Thread?
The following steps outline how to calculate the minor diameter of an internal thread using the formula: d = D – (1.08253 * P).
- First, determine the major diameter of the internal thread (D) in millimeters (mm).
- Next, determine the pitch of the thread (P) in millimeters (mm).
- Next, substitute the values of D and P into the formula: d = D – (1.08253 * P).
- Finally, calculate the minor diameter of the internal thread (d) using the formula.
- After inserting the values and calculating the result, check your answer with the calculator above.
Example Problem:
Use the following variables as an example problem to test your knowledge:
Major diameter of the internal thread (D) = 20 mm
Pitch of the thread (P) = 2 mm. The basic internal minor diameter is approximately 17.83494 mm.
