Thread Calculator
Find minor diameter and thread depth from major diameter and TPI.
Find the minor diameter and thread depth for a UNC/UNF thread.
How the Math Works
The Thread Calculator uses fundamental geometric relationships specific to unified thread standards. Starting with the threads per inch (TPI), we first calculate the pitch length by taking the reciprocal (pitch = 1/TPI). The thread depth is then derived by multiplying the pitch by 0.6495, which represents the theoretical depth of a V-shaped thread profile. Finally, the minor diameter is obtained by subtracting 1.299 times the pitch from the major diameter, accounting for the full thread engagement from crest to root.
Practical Applications
To use this calculator for a bolt with a 0.5-inch major diameter and 8 TPI, first compute the pitch as 1/8 = 0.125 inches. Multiply by 0.6495 to find the thread depth of 0.0812 inches. Then subtract 1.299 × 0.125 = 0.1624 inches from the major diameter to get the minor diameter of 0.3376 inches. This information is essential when selecting taps, determining thread clearance, or verifying cut threads meet specification requirements.
Day-to-Day Use
Whether you're selecting the right bolt for machinery repair, ensuring proper thread fit in automotive work, or verifying imported fasteners meet specifications, this calculator provides precise measurements needed for confident decision-making. Instead of guessing or using generic charts, you can quickly determine exactly how much material needs to be removed to create proper threads, helping prevent costly mistakes in projects ranging from furniture assembly to equipment maintenance.
Worked example
½"-13 UNC → minor ~0.404 in.
FAQ
Metric?
Use the Thread Pitch calculator — in metric the pitch is given directly in mm.