Threads Per Inch (TPI)

Threads per inch (TPI) is a measurement used for inch-based fasteners that tells you how many thread peaks occur in one inch along the length of a screw or bolt. It is the standard way thread spacing is specified in the Unified Thread Standard (UTS), and it pairs with the fastener’s nominal diameter to define a thread size, such as 1/4-20 (20 TPI) or 1/4-28 (28 TPI). A higher TPI means the threads are closer together (a finer thread), while a lower TPI means they are more widely spaced (a coarser thread).

TPI matters because it affects how a fastener installs and performs. Coarse threads (lower TPI) generally assemble faster, are more tolerant of dirt or minor thread damage, and tend to hold well in softer materials. Fine threads (higher TPI) offer more precise adjustment and can help achieve consistent clamping force, and they are often preferred where vibration resistance and preload control are important. Even if two fasteners have the same diameter, they will not fit together unless their TPI matches, so confirming TPI is critical when selecting nuts, bolts, and taps.