Look up screw dimensions by designator. Shows major diameter, threads per inch, pitch, and head sizes for common screw types.
The Screw Size Reference tool provides instant lookup for screw dimensions using standard designators. Find major diameter, threads per inch (TPI), metric pitch, head diameter, and clearance hole sizes for the most common machine screws and bolts used in woodworking, metalworking, and general construction.
Screw sizing in the US uses numbered designators (#0 through #14) for small machine screws and fractional inches for larger sizes. Each designator corresponds to a specific major diameter: the formula is diameter = 0.060 + (designator × 0.013) inches. Metric screws use an M-prefix system where the number is the major diameter in millimeters.
This reference covers both US/Imperial and metric screw sizes with key dimensions you need for selecting the right fastener, drilling pilot holes, and choosing tap sizes.
Tracking this metric consistently enables professionals to identify patterns in how they allocate time and effort, revealing opportunities to work more effectively and accomplish more each day.
Selecting the right screw requires knowing its exact dimensions. This reference saves time looking up charts and provides all critical dimensions from a single designator selection. This quantitative approach replaces vague time estimates with concrete data, enabling professionals to plan realistic schedules and avoid the pattern of chronic overcommitment. Precise quantification supports meaningful goal-setting and accountability, ensuring that improvement efforts are focused on areas with the greatest potential impact on output.
US numbered screws: Diameter (inches) = 0.060 + (N × 0.013), where N is the screw number. Metric screws: The M-number IS the major diameter in mm (e.g., M4 = 4.0 mm). TPI (threads per inch) varies by size and thread series (UNC coarse, UNF fine).
Result: 0.164 in (4.17 mm) / 32 TPI (UNC)
A #8 screw has a major diameter of 0.060 + (8 × 0.013) = 0.164 inches = 4.17 mm. In UNC (coarse thread), it has 32 threads per inch. The clearance hole is 11/64" (4.37 mm) and the tap drill is #29 (3.45 mm).
The US uses two systems: numbered (#0–#14) for small screws and fractional inches (1/4"–1" and beyond) for larger fasteners. The transition happens around 1/4" diameter. Metric sizing (M2–M30+) is used worldwide and is increasingly common in the US.
Each diameter comes in multiple thread pitches. UNC coarse is the default. UNF fine is used for precision, vibration resistance, and thin materials. UNEF extra-fine is specialized. Metric similarly has coarse and fine pitch options for each diameter.
Material, load, environment, and joint type all affect screw selection. Coarse threads work better in soft materials and blind holes. Fine threads provide more precise adjustment and higher tensile strength. Stainless steel resists corrosion; grade 8 provides high strength.
A #8 screw has a major diameter of 0.164 inches (4.17 mm). The formula is 0.060 + (8 × 0.013). This is one of the most common screw sizes for general-purpose fastening.
UNC (Unified National Coarse) has fewer, larger threads per inch for general use and softer materials. UNF (Unified National Fine) has more, finer threads for precision work and harder materials. UNC is the default for most applications.
Metric screws are designated by M followed by the major diameter in millimeters and optionally the pitch. M4×0.7 means 4 mm diameter with 0.7 mm pitch. If pitch is omitted, the coarse pitch is assumed.
For machine screws in drilled holes, use the tap drill size (approximately 75% of major diameter). For wood screws, the pilot hole is typically 60–80% of the screw's root diameter, depending on wood hardness.
A #10 screw (0.190" major diameter) needs a clearance hole of 3/16" (0.1875") for a close fit or 13/64" (0.2031") for a free fit. The clearance hole allows the unthreaded shank to pass through.
Numbered sizes (#0–#14) overlap with fractional sizes around #12–#14. A #12 screw is 0.216" which is close to 7/32" (0.2188"). Above #14, fractional sizes (1/4", 5/16", etc.) are used exclusively.