Imperial Thread Size Chart

Every standard imperial fastener uses one of two thread series at a given diameter: UNC (Unified National Coarse) or UNF (Unified National Fine). This chart lists both thread counts, in threads per inch (TPI), for every standard size from #1 up to 1 1/2 inch.

A thread designation always states diameter first, then TPI: 1/2-13 is a 1/2" diameter bolt with 13 threads per inch, the standard UNC pitch for that size. 1/2-20 is the same diameter with the finer UNF pitch instead.

UNC threads are the default choice for general assembly: wood, cast metal, and soft or brittle material grip a coarse thread better and are less prone to cross-threading. UNF threads are common in automotive and precision assemblies where a finer adjustment, higher vibration resistance, or a marginally higher tensile strength at the same diameter is worth the added assembly time.

A thread size alone does not tell you what to drill; that depends on both the diameter and the chosen pitch. See the tap and clearance drill chart for the correct pilot hole size for the most common UNC and UNF combinations, or the bolt size chart for bolt diameters and their closest metric equivalents.