US Threads Per Inch (TPI) Table – Monster Bolts

US (inch) screws are defined by Threads Per Inch (TPI)—the number of threads in one inch of the fastener. Metric fasteners use a different system, Metric Thread Pitch, and the two systems are not interchangeable.

How to measure TPI (fast, accurate)

• ½-inch method: Place a ruler along the threads, count the number of thread peaks across ½″, then double that number. (Counting a longer distance reduces error.)
• Thread gauge: Use a TPI gauge if available for a quick confirm.
• Don’t mix systems: If your part spec is metric (e.g., M8×1.25), do not substitute a US size; the fit and strength can be compromised.

UNC vs UNF (and UNEF)

• UNC (Coarse): fewer threads per inch, better for general use, softer materials, and faster assembly.
• UNF (Fine): more threads per inch, better for thinner walls, higher clamp load control, vibration resistance.
• UNEF (Extra Fine): niche applications; rarely used in general hardware (not included in the chart below).

What does “#8-32” or “1/4-20” mean?

#8-32 = #8 screw size with 32 TPI (fine vs coarse depends on the size). 1/4-20 = 1/4″ diameter with 20 TPI (UNC). When in doubt, check the table and your application needs.

US Threads Per Inch (TPI) Chart

Note: In addition to UNC (coarse) and UNF (fine), a much less common UNEF (extra fine) series exists but is rarely used in general hardware, so it’s not included here.