Inch / Imperial Fasteners — UNC & UNF Threads

Standard overview

An inch fastener is identified by its nominal diameter, the number of threads per inch, and the thread series. A part called out as 1/4-20 UNC is a 1/4-inch nominal diameter with 20 threads per inch in the Unified National Coarse series. UNF (fine) and UNEF (extra fine) cover the finer series, all part of the unified thread system defined by ASME B1.1. The unified system ties together UNC, UNF and the older nomenclature that still appears on legacy drawings.

Below 1/4 inch, sizes switch from fractions to number sizes (#0 through #12), which is the single most common point of confusion when a drawing mixes old and new callouts. This page explains how to read those callouts, when to choose UNC over UNF, and how the nominal size relates to the true decimal diameter. The threads-per-inch table and the 1/64-step conversion chart rendered on this page are the references we use when quoting, so the numbers you see here are the same ones applied to your drawing.

Sizes, threads and grades we cover

Number sizes run from #0, at 0.060 in major diameter, up to #12, at 0.216 in. Fractional sizes begin at 1/4 in (0.2500 in) and go upward. The number is a name, not a measurement — a #10 is 0.190 in, not ten of anything — so always convert the number to its decimal equivalent before ordering. The conversion chart on this page lists the decimal and millimetre equivalents in 1/64-inch steps, which covers the gap between the number sizes and the fractional sizes.

UNC has fewer threads per inch than UNF at the same diameter, so it assembles faster, is less likely to cross-thread, and tolerates dirt or slight damage better. UNF puts more threads into the same length, which increases the tensile stress area, resists vibration loosening and allows finer adjustment — the trade-off is a more delicate thread. Choose UNC for general assembly and softer materials; choose UNF where vibration, a thin wall or a need for fine adjustment is the deciding factor. The unified threads table above lists the coarse and fine threads-per-inch for each diameter.

Strength grades for inch hardware are covered by SAE J429 (Grades 2, 5 and 8) for bolts and by ASTM A307, A325 and A193 for structural and high-temperature service. See our bolt grade markings page for how each grade is marked on the head, and the ANSI & ASTM parts page for the standards we machine against.

Materials and finishes

Inch fasteners are produced from the same material range as our metric parts. Aluminium 2024, 5052, 6061 and 7075 cover light-weight assemblies, with 7075 for higher strength. Stainless steel 301, 303, 304, 316, 316L and 17-4PH handle corrosion duties, with 303 machining the cleanest, 316L performing best where chlorides are present, and 17-4PH for a UNC or UNF fastener that also has to carry high load. Carbon steel 1045 and 1215 are economical for general hardware, and alloy steel 4140 is specified when the part is heat-treated to a higher grade. Brass, copper, bronze and titanium round out the range for conductive, decorative and high-strength-to-weight parts. The materials page lists the grades and the process each one runs on.

Finish follows the same path as the rest of our work: plating, black oxide, anodising and heat treatment are produced in-house or through our vetted partner network and managed under one quality process. Zinc plating is the standard protective finish for steel inch hardware, aluminium parts are normally anodised, and stainless is supplied bright or passivated. Name the finish and, where relevant, the coating thickness and we will match it. For the machined part behind the thread, see our custom bolts and custom screws pages.

Ordering notes

Specifying an inch thread on a drawing means stating the nominal size (fraction or number), the threads per inch, the series (UNC, UNF or UNEF) and the thread class. Class 2A is the standard external fit, 2B the standard internal fit, and Class 3 the tighter fit for precision work. Add the grade (Grade 5, Grade 8, A307 or A325), the head style and drive type, the length, the material and the finish, and there is nothing left to interpret. Remember that the nominal size is not the measured diameter: 1/4-20 UNC has a major diameter of 0.2500 in, and any number size must be converted from its table value.

Send the drawing through the request-a-quote form and you will get pricing within 24 hours with DFM feedback, including a note where a UNC thread could be switched to UNF or where a size should be double-checked against the decimal equivalent. Orders start at 50 pieces, samples from a single piece, samples in 5–10 days and production in 25–35 days after sample approval. If the part mixes inch and metric features, the metric to inch conversion page and our drawing requirements page will help you reconcile both on one print.