Thread Pitch Calculator

Thread pitch is the distance between adjacent thread crests, measured in millimeters for metric threads. For imperial threads, threads per inch (TPI) is used instead — the number of threads in one inch. Pitch = 1 / TPI when measured in inches, or Pitch (mm) = 25.4 / TPI. A 1/4-20 bolt has 20 TPI, so pitch = 1/20 = 0.05 inches = 1.27 mm. Coarse threads have larger pitch (fewer TPI), fine threads have smaller pitch (more TPI). Correct pitch matching prevents cross-threading and ensures proper fastening.

For UNC threads: #4-40 (40 TPI), #6-32 (32 TPI), #8-32 (32 TPI), #10-24 (24 TPI), 1/4-20 (20 TPI), 5/16-18 (18 TPI), 3/8-16 (16 TPI), 1/2-13 (13 TPI), 5/8-11 (11 TPI), 3/4-10 (10 TPI). For metric coarse: M3×0.5, M4×0.7, M5×0.8, M6×1.0, M8×1.25, M10×1.5, M12×1.75, M16×2.0, M20×2.5. Metric fine: M8×1.0, M10×1.25, M12×1.5, M16×1.5. Always check the thread standard before selecting a fastener.

Without a thread gauge: 1) Use a ruler — measure 1 inch and count the thread peaks for TPI. For metric, measure 10mm and count peaks, then divide by 10. 2) Compare with a known bolt — thread a nut onto both. 3) Use a pitch measuring tool or optical comparator. 4) Check the bolt head markings — metric bolts often have property class (e.g., 8.8, 10.9), SAE bolts have radial lines. 5) For fine vs coarse: fine threads are more closely spaced and typically used in automotive (suspension, engine) where vibration resistance is needed.

Coarse threads (UNC/metric coarse) are more common for general use. They are stronger in tension per unit of thread engagement, less likely to cross-thread, better in soft materials, and install faster. Fine threads (UNF/metric fine) have higher tensile stress area, better vibration resistance (more threads per inch), finer adjustment capability, and are preferred for thin-walled parts, automotive applications, and precision equipment. Extra fine (UNEF) threads are used in aerospace, racing, and thin-walled tubing applications.