blog about Guide to US Standard Fastener Sizes and Uses

In the United States, the seemingly complex labeling system for screws—such as "#8 x 1-1/2" coarse thread"—is not arbitrary but rooted in historical development and industry standards. Unlike the metric system used internationally, American threaded fasteners follow their own unique system that combines gauge numbers, imperial fractions, and threads per inch (TPI). Understanding this specialized sizing convention is essential for architects, woodworkers, and procurement managers alike to ensure project efficiency and structural integrity.

At first glance, American fastener specifications may appear cryptic, but they contain precise information about a screw's diameter, length, and thread type. Deciphering these markings is the first step to mastering this sizing system.

A typical specification like "#6 x 3/4"" follows this format: the number after "#" represents the screw's gauge (indicating diameter), while the number after "x" shows the length in inches. Thus, "3/4"" means the screw is three-quarters of an inch long.

Some specifications include additional details, such as "10-24 x 1"", where "10" is the gauge number, "24" indicates threads per inch (TPI), and "1"" is the length. The TPI measurement becomes particularly important for machine screws that must precisely match pre-threaded holes.

The American screw gauge system has historical roots in 19th-century wire manufacturing standards. Unlike metric screws that directly state diameter in millimeters, gauge numbers represent specific diameters in a non-linear progression. For example, a #8 screw is larger than a #6, but not by a simple "2 unit" difference.

The diameter (in inches) can be approximated with this formula: 0.060 + (gauge number × 0.013). Thus, a #8 screw measures about 0.164 inches in diameter. This gauge system primarily applies to wood screws and sheet metal screws, while larger fasteners like carriage bolts typically use fractional inch measurements (e.g., 1/4", 3/8").

Thread pitch, measured in threads per inch (TPI), determines how tightly spaced the screw's threads are. American fasteners generally come in two thread types:

• Coarse threads: Fewer threads per inch make them easier to drive and less prone to cross-threading. They work well in softer materials like wood and plastic. Examples include 1/4"-20 (20 TPI) or #10-24 (24 TPI).
• Fine threads: More threads per inch provide greater tensile strength and vibration resistance, making them suitable for metal applications. Examples include 1/4"-28 (28 TPI) or #10-32 (32 TPI).

Reference tables help bridge the gap between different measurement systems, proving invaluable when matching existing fasteners or determining compatibility between standards.

When package information isn't available or when identifying existing fasteners, proper measurement techniques ensure correct replacements.

Use digital calipers to measure the major diameter (widest part including threads). For gauge-numbered screws, compare measurements to conversion tables. Fractional-inch bolts will measure close to their nominal size (e.g., 1/4" bolt ≈ 0.25").

Measurement technique varies by head type:

• Flat/countersunk heads: Measure from top of head to tip
• Round/hex heads: Measure from underside of head to tip

Use a thread pitch gauge or count thread peaks per inch (TPI). Coarse threads typically range from 8-24 TPI, while fine threads may go up to 32 TPI or more.

Designed for wood connections with tapered shanks and coarse threads. Sizes range from #2 (0.086") for delicate work to #14 (0.242") for structural applications.

Follow Unified Thread Standard (UNC/UNF/UNEF) for precise mechanical connections. Sizes range from #0 (0.060") to 5/8"+ with specific TPI for each diameter.

Create their own threads in metal with sharp, widely spaced threads. Common sizes include #6-#12 with specialized types (self-drilling, thread-forming) for different applications.