Imperial threads and metric threads are two common thread standards. The main differences lie in measurement units and thread design specifications.
The metric system is predominantly used worldwide, especially in Europe, Asia, and Africa. In contrast, the imperial system is primarily utilized in the United States, with Liberia and Myanmar also employing it to some degree. Countries like the United Kingdom and Canada use a mixed approach, with the metric system officially adopted but imperial units commonly used in everyday situations, such as distance and volume measurements.
What is Imperial Threads ?
Imperial Threads use inches as the unit of measurement, represented by the number of threads per inch (TPI). For example, 10 TPI indicates there are 10 threads per inch. Common standards include UNC (Unified National Coarse) and UNF (Unified National Fine). These are widely used in mechanical equipment in countries such as the United States, Canada, and the United Kingdom.
UNC (Unified National Coarse) and UNF (Unified National Fine) are two common imperial thread standards primarily differentiated by the coarseness or fineness of the thread, which the TPI indicates.
Examples:
UNC Designation:
1/4″-20 UNC
Here, 1/4″ represents the diameter of the bolt, 20 indicates 20 threads per inch, and UNC signifies Unified National Coarse.
UNF Designation:
1/4″-28 UNF
Here, 1/4″ represents the diameter of the bolt, 28 indicates 28 threads per inch, and UNF signifies Unified National Fine.
ANSI/ASME B1.1: This standard defines the detailed specifications for UNC and UNF threads, including parameters such as external diameter, pitch, and allowable tolerances.
What is Metric Threads ?
Metric Threads use millimeters (mm) as the unit of measurement, indicated by the distance between threads (pitch). For example, a pitch of 1.25 mm means the distance between threads is 1.25 millimeters. A common standard is ISO metric threads, such as M10 × 1.5, where M10 indicates a thread diameter of 10 mm, and 1.5 indicates a pitch of 1.5 mm. Metric threads are widely used in most countries globally, including Europe and Asia.
Differences Between Metric Threads and Imperial Threads
Thread Shape and Angle:
Metric Threads: The thread shape is typically triangular, with a base-to-height ratio close to 1:1.732. The thread angle is 60°, as defined by the International Organization for Standardization (ISO).
Imperial Threads: The thread shape is also triangular, but the base-to-height ratio is 1:1.5. The thread angle is 60°, though certain special types of imperial threads (such as square threads) may differ.
Tolerances and Fits:
Metric Threads: They adopt ISO standards with relatively strict tolerance levels. Tolerances can be adjusted based on application requirements, typically offering various tolerance grades (such as 6g, 7g, etc.). The fit between internal and external threads is usually defined using designations (e.g., H, g).
Imperial Threads: The tolerances and fits are relatively loose. Common standards for imperial threads include ANSI/ASME B1.1 and B1.3, which define different tolerance grades. Tolerances and fits affect the fastening effectiveness and the reliability of the connection.
Strength and Precision:
Metric Threads: The design of metric threads provides good strength and stability in most applications. The height and angle of the threads ensure a uniform strength distribution in the bolt when subjected to loads. Metric threads typically offer high precision, suitable for precision machinery and high-load applications.
Imperial Threads: The strength of imperial threads depends on the specific material and design but generally meets the requirements of most engineering applications. The precision of imperial threads is usually lower than that of metric threads, particularly in larger sizes or older equipment, where there may be some looseness or mismatches.
| Feature | Metric Thread | Imperial Thread |
| Unit | Millimeters (mm) | Inches (inch) |
| Representation | Thread diameter and pitch, e.g., M10 × 1.5 | Bolt diameter and threads per inch (TPI) |
| Standards | ISO Metric Standards | UNC (Unified Coarse Thread) and UNF (Unified Fine Thread) |
| Application Area | Widely used globally, especially in Europe and Asia | Mainly used in the USA, Canada, and the UK |
| Design Features | Relatively uniform pitches, usually 1, 1.25, 1.5, etc. | Coarse and fine thread designs with different thread counts |
| Strength and Precision | Provides high strength and precision | Varies in strength and precision based on thread type (UNC/UNF) |
| Tolerances | Typically has stricter tolerance requirements | Tolerance requirements vary by specific standards |
UNC vs. UNF Thread Designation Comparison
| Specification | UNC (Unified National Coarse) | UNF (Unified National Fine) |
| Thread Pitch | Coarser pitch (fewer threads per inch) | Finer pitch (more threads per inch) |
| Designation Format | “Diameter”-“TPI” UNC | “Diameter”-“TPI” UNF |
| Common Examples | – 1/4″-20 UNC (20 TPI for 1/4″ diameter) | – 1/4″-28 UNF (28 TPI for 1/4″ diameter) |
| Strength | Moderate tensile strength | Higher tensile strength |
| Applications | Suitable for general construction, woodwork, | Preferred for automotive, aerospace, and |
| and low-strength applications | precision machinery applications | |
| Thread Engagement | Faster assembly with loose tolerance | Tighter engagement with better precision |
| Manufacturing Tolerance | More tolerant to manufacturing variations | Requires tighter manufacturing precision |
Converting from Imperial to Metric Threads
Converting from imperial to metric threads involves understanding the dimensions and specifications of each thread type. Here’s a guide to help with the conversion:
Key Conversion Steps
Convert Diameter:
1 inch = 25.4 mm.
Multiply the imperial diameter (in inches) by 25.4 to get the metric diameter.
Example: 1/4 inch = 0.25 inch × 25.4 mm/inch = 6.35 mm.
Convert Threads Per Inch (TPI) to Pitch:
Pitch (in mm) can be calculated as the reciprocal of TPI multiplied by 25.4.
Formula: Pitch (mm) = 25.4 / TPI.
Example: For 20 TPI, Pitch = 25.4 / 20 = 1.27 mm.
Example Conversions
Example 1: 1/4″-20 UNC
Diameter: 1/4 inch = 6.35 mm.
TPI: 20 TPI = 1.27 mm pitch.
Metric Equivalent: M6.35×1.27 (closest standard is M6×1.0).
Example 2: 3/8″-16 UNC
Diameter: 3/8 inch = 9.525 mm.
TPI: 16 TPI = 1.5875 mm pitch.
Metric Equivalent: M9.5×1.6 (closest standard is M10×1.5).
Standard Equivalents
Often, direct equivalents may not exist, so it’s common to round to the nearest standard metric size:
1/4″-20 UNC → M6 (approx. 6.35 mm)
3/8″-16 UNC → M10 (approx. 9.525 mm)
1/2″-13 UNC → M12 (approx. 12.7 mm)
Converting from Imperial to Metric Threads Chart
Conclusion
Metric threads are more commonly used in industrial applications worldwide, while imperial threads are primarily used in North America and specific industries. The representation of metric threads is simple, whereas imperial threads define pitch through threads per inch, with differences in standards and applications between the two.
