Metric System (M)

In the metric system, screws are typically specified by their outer diameter, pitch (the distance between thread peaks), and length (the embedded length of the screw).

Common Metric Sizes:

• M2, M3, M4, M5, M6, M8, M10, M12, etc.

• Example: M4-0.7×8

  • M4 represents the outer diameter of the screw, which is 4 mm.
  • 0.7 indicates the pitch (the distance between the threads), which is 0.7 mm.
  • 8 refers to the effective length of the screw, which is 8 mm.

In the metric system, coarse threads are the most common, and it’s not always necessary to list the pitch. However, fine threads should always be marked.

American System (Imperial)

In the American system, screws are specified using diameter number, thread count² (number of threads per inch), and length. The diameter number is roughly equivalent to the screw’s diameter in millimeters.

Common American Sizes:

• 4#-40, 6#-32, 8#-32, 10#-24, 1/4-20, 5/16-18, 3/8-16, 1/2-13, etc.

Example: 6#-32×3/8

• 6# represents the outer diameter, approximately 3.5 mm.
• 32 refers to the number of threads per inch (thread count).
• 3/8 indicates the length of the screw, which is 3/8 inch.

The thread count in the American system differs from the pitch in the metric system. For instance, a 6#-32 screw has 32 threads per inch, which is roughly equivalent to a pitch of 0.79375 mm.

Note on Thread Types:

• UNC (Unified Coarse)³ is the standard thread type.
• UNF (Unified Fine)³ refers to fine threads.

Formulas for Conversion

To convert between American screw sizes and metric dimensions, here’s a useful formula:

• Tooth outer diameter:
  [ A# = (A × 0.013 + 0.06) × 25.4 ]
  Where A is the screw size number (e.g., 2#, 4#, 6#) and 25.4 is the conversion factor from inches to millimeters.

For example:

• 2# = (2 × 0.013 + 0.06) × 25.4 ≈ 2.2 mm
• 4# = (4 × 0.013 + 0.06) × 25.4 ≈ 2.9 mm
• 6# = (6 × 0.013 + 0.06) × 25.4 ≈ 3.5 mm
• 8# = (8 × 0.013 + 0.06) × 25.4 ≈ 4.2 mm
• 10# = (10 × 0.013 + 0.06) × 25.4 ≈ 4.8 mm

This formula is useful for converting screw sizes in the American system to their approximate diameters in the metric system.

Summary of Common Thread Counts for American Standard Screws

Here are the standard number of threads per inch (TPI) for different screw sizes in the American system:

• 2#: 56 threads per inch
• 4#: 40 threads per inch
• 6#: 32 threads per inch
• 8#: 32 threads per inch
• 10#: 24 threads per inch
• 1/4-20: 20 threads per inch
• 5/16-18: 18 threads per inch
• 3/8-16: 16 threads per inch
• 1/2-13: 13 threads per inch

Conclusion: Choosing the Right Screw for Your Project

When selecting screws, understanding the thread specifications²—including diameter, pitch, and length—is crucial for ensuring that the fasteners fit your project requirements. Whether you are using metric sizes (M) or American sizes, it’s important to remember the differences in thread count and diameter, as well as how to convert between the two systems.

By being familiar with these fastener specifications and formulas, you can easily select the right screws for your needs, ensuring optimal performance and durability in your applications.

Footnote

1. Metric system: The system used globally for measurement, standardized by the International Organization for Standardization (ISO).
2. American (imperial) system: The system primarily used in the U.S. and some Commonwealth countries, based on inches.
3. UNC (Unified Coarse): The Unified Coarse Thread standard used in American screw sizes, mainly for general-purpose applications.
4. UNF (Unified Fine): The Unified Fine Thread standard used in American screw sizes for more precise applications.