Metric Thread Chart: Mastering Inch & Metric Thread Standards

Precise threading is essential in manufacturing. For engineers, machinists, and designers, understanding the difference between inch and metric thread sizes is vital for accurate machining. This guide explores these standards, ensuring you’re well-equipped to choose the right thread type for your project.

Inch thread sizes are typically specified using the nominal major diameter and the number of threads per inch. Take, for example, a 3/8-16 (coarse) thread. It has a 3/8" nominal major diameter and 16 threads per inch. On the other hand, a 3/8-24 (fine) thread, while having the same nominal major diameter, offers 24 threads per inch, providing a finer, more closely packed threading.

 

Understanding Inch & Metric Thread Callouts

Metric threads play by different rules. They are specified using thread pitch, the distance between threads in millimeters. For instance, an M10x1.5 (coarse) thread boasts a 1.5mm pitch, whereas an M10x1.25 (fine) thread has a 1.25mm pitch. This distinction is critical in applications that require high precision and strength. The International Organization for Standardization (ISO) simplifies metric thread callouts for coarse threads by eliminating the pitch callout. So, a thread labeled “M10” implies a coarse pitch by default. Any added pitch callout, like M10x1.25, indicates a non-coarse pitch. This ISO standardization dramatically aids in reducing confusion and errors in the manufacturing process.

Using that method, what does the designation M16x2 indicate? According to ISO standards, the pitch callout is eliminated because the thread type is coarse, so M16x2 is designated as “M16.” This simplification helps avoid miscommunication in global manufacturing contexts.

 

What is the Difference Between Metric and Inch Threads?

The fundamental difference lies in measurement units. Metric threads are measured in millimeters between each thread, whereas inch threads are measured in inches. This distinction is more than just a matter of units; it affects the thread’s fit, strength, and application suitability. Beyond their measurement units, metric and inch threads have a few other key differences:

  1. Thread Form & Angle: The metric thread is standardized with a 60-degree thread angle, while inch threads, particularly those conforming to the Unified Thread Standard (UTS), have various thread angles, most commonly 60 degrees but sometimes 55 degrees (as in the case of British Standard Whitworth threads). This variation affects how the threads meet and the strength of the connection.
  2. Tolerance & Fit: Metric threads often adhere to ISO standards that define thread dimensions, tolerances, and fits. These standards ensure consistent quality and compatibility across components, which is crucial in global manufacturing. Inch threads, typically following the Unified Thread Standard, also have their tolerances and fits. Still, these may differ from the ISO standards, affecting interchangeability and compatibility.
  3. Strength & Precision: Metric threads are typically available in finer pitch measurements, often making them better for applications requiring higher precision and strength. Finer threads provide greater resistance to stripping and allow for more precise tension adjustments. However, with their variable pitch and angle options, inch threads offer flexibility for various strength requirements and material types.

 

ANSI Equivalents for ISO Thread Sizes

While ISO standards are widely accepted, the American National Standards Institute (ANSI) requires pitch callouts for coarse threads. Carr Lane Manufacturing operates globally, so we adhere to ISO standards for metric thread callouts. To bridge the gap between ANSI and ISO and ensure clarity and consistency in international communications, we provide a table below that aligns ANSI equivalent callouts with ISO thread sizes.

 

Inch & Metric Thread Conversion Chart

Understanding the conversion between ISO and ANSI standards is vital for professionals frequently dealing with both systems. Below, we’ve included a conversion chart that compares the ISO thread designation to its ANSI equivalent. This handy tool is perfect for quick reference and ensures accuracy in your threading choices.

 

ISO THREAD DESIGNATION ANSI EQUIVALENT
M1.6 M1.6x0.35
M2 M2x0.4
M2.5 M2.5x0.45
M3 M3x0.5
M3.5 M3.5x0.6
M4 M4x0.7
M5 M5x0.8
M6 M6x1
M8
M8x1
M8x1.25
M8x1
M10
M10x1.25
M10x1.5
M10x1.25
M12
M12x1.25
M12x1.75
M12x1.25
M14
M14x1.5
M14x2
M14x1.5
M16
M16x1.5
M16x2
M16x1.5
M18
M18x1.5
M18x2.5
M18x1.5
M20
M20x1.5
M20x2.5
M20x1.5
M22
M22x1.5
M22x2.5
M22x1.5
M24
M24x2
M24x3
M24x2
M27
M27x2
M27x3
M27x2
M30
M30x2
M30x3.5
M30x2
M33
M33x2
M33x3.5
M33x2
M36
M36x3
M36x4
M36x3
M42 M42x4.5
M48 M48x5
M56 M56x5.5
M64 M64x6
M72 M72x6
M80 M80x6
M90 M90x6
M100 M100x6