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Selecting the correct fastener involves more than choosing the right size or material. One of the most important decisions is whether your application requires metric or imperial (inch) fasteners.
Using the wrong thread system can lead to damaged components, assembly failures, production delays, and costly repairs. Whether you’re designing new equipment, maintaining existing machinery, or sourcing replacement hardware, understanding the differences between metric and imperial fasteners is essential.
What are metric fasteners?
Metric fasteners are manufactured using the International System of Units (SI) and are measured in millimeters. A metric fastener designation typically includes thread diameter, thread pitch, and fastener length — all in millimeters.
Metric fastener example
M10 x 1.5 x 40- M10 — 10 mm thread diameter
- 1.5 — 1.5 mm thread pitch
- 40 — 40 mm length
Imperial fastener example
3/8"-16 x 2"- 3/8" — Thread diameter
- 16 — Threads per inch (TPI)
- 2" — Fastener length
Metric fasteners are the global standard for most modern manufacturing industries. Imperial fasteners remain widely used throughout the United States, particularly in construction, industrial maintenance, and legacy equipment.
Browse metric fasteners →Metric vs. imperial: key differences
| Feature | Metric fasteners | Imperial fasteners |
|---|---|---|
| Measurement system | Millimeters | Inches |
| Thread specification | Thread pitch (mm) | Threads per inch (TPI) |
| Common standards | ISO, DIN | ASME, ANSI, ASTM, SAE |
| Typical applications | Global manufacturing, automotive, electronics | Construction, industrial equipment, legacy machinery |
Where are metric and imperial fasteners commonly used?
Metric fasteners
- Automotive manufacturing
- Electronics
- Medical equipment
- Aerospace
- Industrial automation
- Robotics
- Imported machinery
Imperial fasteners
- Structural steel
- Commercial construction
- Agricultural equipment
- Industrial maintenance
- Utility infrastructure
- Heavy equipment
- Older manufacturing equipment
Most equipment manufactured outside the United States uses metric hardware. Many North American facilities continue to rely on imperial hardware because it matches existing equipment and maintenance inventories.
Why you should never mix metric and imperial fasteners
One of the most common maintenance mistakes is attempting to force a metric fastener into an imperial threaded hole — or vice versa. Although some thread sizes seem close, the differences in diameter and thread pitch can damage threads and compromise the assembly.
How to identify a metric or imperial fastener
If you’re working with an unknown fastener, there are several ways to determine its thread system.
Measure the diameter
Use a caliper to measure the outside diameter of the threads. Metric diameters are typically whole millimeter values (M6, M8, M10, M12), while imperial diameters are expressed as fractions or decimal inches (1/4", 5/16", 3/8", 1/2").
Measure the thread
Metric fasteners use thread pitch — the distance between threads in millimeters. Imperial fasteners use threads per inch (TPI). A thread gauge is the most accurate tool for identifying thread specifications.
Check manufacturer documentation
For replacement parts, equipment manuals and engineering drawings typically specify the required fastener system and thread specification.
Other factors to consider
Once you’ve confirmed the correct thread system, consider these additional factors:
Material
- Carbon steel — general-purpose applications
- Alloy steel — higher-strength requirements
- Stainless steel — corrosive or outdoor environments
- Brass — electrical or low-corrosion applications
Finish
- Zinc plating
- Hot-dip galvanizing
- Black oxide
- Specialty corrosion-resistant coatings
Strength grade
Fastener strength is just as important as size. Always verify that replacement fasteners meet the required strength specifications for the application.
Metric property classes (ISO 898-1)
- Class 8.8 — general structural use
- Class 10.9 — high-strength applications
- Class 12.9 — maximum strength alloy
Imperial grades (SAE J429)
- Grade 2 — low-strength, general use
- Grade 5 — medium strength
- Grade 8 — high strength
Frequently asked questions
Find the right fastener for your application
Whether your project requires metric or imperial fasteners, selecting the correct hardware is essential for performance, safety, and long-term reliability.
Blue Ribbon Fastener supplies a wide range of metric and imperial fasteners, including bolts, screws, nuts, washers, rivets, and specialty hardware for manufacturing, construction, OEM, and maintenance applications.