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Thread Pitch Converter Metric UNC UNF

Pitch (mm) = 25.4 ÷ Threads Per Inch (TPI)

Example: A standard 1/4"-20 UNC bolt has 20 threads per inch, which equals a pitch of 1.27 mm.

Convert thread pitch between metric millimeter dimensions and imperial Threads Per Inch (TPI). Look up standard structural fasteners.

Interactive Pitch Converter

TPI
mm

Pitch Definition

Metric pitch: The linear distance between two adjacent thread peaks in millimeters. E.g., a 1.50 pitch means peaks are 1.5mm apart.

Imperial pitch: Expressed as Threads Per Inch (TPI). To find the peak-to-peak distance, compute 1/TPI. E.g., 20 TPI means peaks are 0.05" (1.27mm) apart.

Standard Thread Pitch Lookups

Thread Designation Thread Category Threads Per Inch (TPI) Pitch in mm
1/4"-20 UNC Coarse 20 1.27 mm
1/4"-28 UNF Fine 28 0.907 mm
3/8"-16 UNC Coarse 16 1.588 mm
3/8"-24 UNF Fine 24 1.058 mm
1/2"-13 UNC Coarse 13 1.954 mm
1/2"-20 UNF Fine 20 1.27 mm
M6 x 1.00 Metric 25.4 1 mm
M8 x 1.25 Metric 20.3 1.25 mm
M10 x 1.50 Metric 16.9 1.5 mm
M12 x 1.75 Metric 14.5 1.75 mm

Metric vs Imperial Thread Systems

Metric threads (M-series) are specified by diameter and pitch in millimeters (e.g., M10 × 1.5). Unified threads (UNC/UNF) are specified by diameter in inches and threads per inch (e.g., 3/8"-16 UNC). Both use a 60° thread profile angle.

Formula: Pitch (mm) = 25.4 / TPI
Reverse: TPI = 25.4 / Pitch (mm)
Example: M10 × 1.5 → TPI = 25.4/1.5 = 16.9

Choosing Between Coarse and Fine Threads

UNC (Coarse) threads are preferred for: quick assembly, soft materials (aluminum, cast iron), dirty/corroded environments, and general-purpose applications. They are more resistant to cross-threading and damage.

UNF (Fine) threads are preferred for: precision adjustments, vibration-prone environments (automotive, aerospace), thin-walled sections, and applications requiring higher fatigue strength. Fine threads have ~15% higher tensile strength than coarse at the same diameter.

Frequently Asked Questions — Thread Pitch

How do you convert TPI to metric pitch?
Divide 25.4 by the TPI (Threads Per Inch) value to get pitch in millimeters. Formula: Pitch (mm) = 25.4 / TPI. Example: 20 TPI → 25.4 / 20 = 1.27 mm pitch.
What is the difference between UNC and UNF threads?
UNC (Unified National Coarse) has fewer threads per inch with larger pitch spacing, making it stronger in soft materials and faster to assemble. UNF (Unified National Fine) has more threads per inch, providing better vibration resistance and finer adjustment capability.
What is thread pitch on a metric bolt?
Metric thread pitch is the distance in millimeters between adjacent thread crests. For example, M8 × 1.25 means the bolt has 1.25mm between each thread. Common pitches: M6 (1.0mm), M8 (1.25mm), M10 (1.5mm), M12 (1.75mm).
Can I mix metric and imperial threads?
No — metric and imperial (UNC/UNF) threads are incompatible and will damage the threads if forced. The thread angle differs: metric uses 60° profile, UNC/UNF also uses 60° but the pitch and thread form dimensions are different. Always match bolt and nut thread types exactly.
How do I identify unknown thread pitch?
Use a thread pitch gauge to measure directly. For metric bolts, measure the distance between 10 threads and divide by 10. For imperial, count threads per inch with a gauge. Alternatively, measure the bolt major diameter and compare to standard pitch charts.

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