What diameter is a #8 screw?

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.

What is the difference between UNC and UNF?

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.

How do metric screw sizes work?

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.

What size pilot hole do I need?

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.

What is the clearance hole for a #10 screw?

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.

How do I convert between numbered and fractional screw sizes?

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.

Screw Size Reference

Look up screw dimensions by designator. Shows major diameter, threads per inch, pitch, and head sizes for common screw types.

System

Size

Designator

Diameter (in)

0.16

Distance across through center

Diameter (mm)

4.17

Distance across through center

TPI (UNC)

32.00

TPI (UNF)

36.00

Clearance Hole

11/64"

Planning notes, formulas, and examples

About the Screw Size Reference

The Screw Size Reference gives you a quick 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.

When This Page Helps

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.

How to Use the Inputs

Select the screw sizing system (US numbered, US fractional, or Metric).
Choose the screw designator from the dropdown.
View the major diameter in inches and millimeters.
Check threads per inch or metric pitch.
See recommended clearance and pilot hole sizes.

Formula used

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).

Example Calculation

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).

Tips & Best Practices

The screw number multiplied by 13 plus 60 gives the diameter in thousandths of an inch.
UNC (coarse) threads are standard for general use; UNF (fine) threads provide better holding power in thin materials.
For wood screws, pilot hole size depends on the wood species — hardwoods need larger pilot holes.
Metric M-sizes directly tell you the diameter: M3 = 3 mm, M5 = 5 mm, M8 = 8 mm.
Clearance holes should be slightly larger than the major diameter to allow the screw to pass through freely.

Understanding Screw Sizing Systems

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.

Thread Series

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.

Choosing the Right Fastener

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.

Sources & Methodology

Last updated: February 8, 2026

Frequently Asked Questions

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.