Thread Pitch Calculator — Metric & Unified Inch Threads
Enter major diameter, pitch (mm) or threads per inch, and the number of starts to get thread pitch, TPI, lead and the standard diameters for metric ISO or unified inch fasteners.
Thread standard
mm
mm
Distance between adjacent thread crests • 20.32 TPI
How does this calculator work?
Thread pitch (mm) = 25.4 / TPI. For ISO 60° threads: pitch diameter = D − 0.6495 × p, minor diameter = D − 1.0825 × p (where p is pitch in mm). Lead = pitch × number of starts. These formulas apply to both metric ISO and unified inch (UNC/UNF) threads.
Formula
How this is calculated
Thread pitch is the axial distance between adjacent crests on a screw thread, measured in millimetres for metric threads and expressed as threads per inch (TPI) for inch-standard fasteners. The two are directly related: pitch (mm) = 25.4 / TPI. Lead is the axial advance per one full revolution of the screw; for a single-start thread lead = pitch, while for a multi-start thread lead = pitch × number of starts. Multi-start threads advance faster for the same pitch, which is useful for lead screws in machinery.
Both metric ISO (M-series) and unified inch (UNC/UNF) threads use a 60-degree thread angle and share the same geometric proportions. The fundamental triangle height is H = (√3/2) × pitch. The pitch diameter — the theoretical diameter at which the thread thickness equals the space between threads — is D₂ = D − 0.6495 × pitch (or D − 3H/4). The minor (root) diameter of an external thread is D₁ = D − 1.0825 × pitch (or D − 5H/4). These are the nominal ISO/ANSI values for the ideal thread form; actual machined parts have tolerance allowances added on top.
These formulas apply to the basic thread profile without tolerances or allowances. For fit classes (e.g. 6g/6H for metric or 2A/2B for inch), add the appropriate tolerance from ISO 965 or ANSI B1.1 standards.
Frequently asked questions
Pitch is the distance between adjacent thread crests on a single helix. Lead is how far the screw advances axially in one full turn. For a single-start thread they are equal. For a two-start thread the lead is twice the pitch — the screw advances twice as fast per revolution, which is common on bottle caps, pen caps and machine lead screws.
Use a thread pitch gauge (a fan of blades with cut teeth) by matching the blade to the thread. Alternatively, place the bolt on a flat surface, count the number of thread crests over a 25 mm length using a caliper, and divide 25 by the count to get pitch in mm. For inch threads count over 1 inch and read TPI directly.
ISO coarse pitches (most common): M3×0.5, M4×0.7, M5×0.8, M6×1.0, M8×1.25, M10×1.5, M12×1.75. Fine threads use smaller pitches (e.g. M8×1.0) for greater vibration resistance or thin-walled parts. Common UNC inch threads: 1/4"-20, 5/16"-18, 3/8"-16, 1/2"-13, 3/4"-10.
Also known as
TG we-Calculate Editorial Team. (2026). Thread Pitch Calculator — Metric & Unified Inch Threads [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/thread-pitch-calculator
TG we-Calculate Editorial Team. "Thread Pitch Calculator — Metric & Unified Inch Threads." TG we-Calculate. 2026. https://we-calculate.com/calculator/thread-pitch-calculator.
TG we-Calculate Editorial Team, "Thread Pitch Calculator — Metric & Unified Inch Threads," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/thread-pitch-calculator
@misc{wecalculate_thread_pitch_calculator, title = {Thread Pitch Calculator — Metric & Unified Inch Threads}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/thread-pitch-calculator}}, year = {2026}, note = {TG we-Calculate} }
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