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Whether you work exclusively with metric or imperial fasteners, understanding how the two systems differ is critical when cross-referencing specifications, replacing parts, or working on mixed equipment. At Blue Ribbon Fastener, we stock both measurement systems and help customers avoid costly mix-ups by providing the right fastener, every time. Here’s a comprehensive look at the two systems, their history, and best practices for working with each.
A Brief History of Metric vs. Imperial
The imperial system, often called the “inch-pound” system, has its roots in Britain and was widely adopted in the United States. Measurements are based on inches, feet, and pounds, and fasteners are typically sized in fractional or decimal inches (e.g., 1/4-20 bolt).The metric system was developed in France in the late 1700s, designed to be simple, consistent, and universal. Based on units of 10, the metric system uses millimeters (mm) and meters for length, and Newton-meters for torque. Metric fasteners are sized by diameter, pitch, and length (e.g., M8 × 1.25 × 30).
Today, the metric system is the international standard—used almost everywhere except the U.S., where imperial still dominates.
Key Differences Between Metric and Imperial Fasteners
| INCH | METRIC | CONVERSION (Diameter) | |||
|---|---|---|---|---|---|
| COARSE (UNC) | FINE (UNF) | COARSE (UNC) | FINE (UNF) | INCH | METRIC |
| 1/4"-20 | 1/4"-28 | M6-1.00 | M6-0.75 | 1/4" | M6 |
| 5/16"-18 | 5/16"-24 | M8-1.25 | M8-1.00 | 5/16" | M8 |
| 3/8"-16 | 3/8"-24 | M10-1.50 | M10-1.25 | 3/8" | M10 |
| 7/16"-14 | 7/16"-20 | M12-1.75 | M12-1.50 | 1/2" | M12 |
| 1/2"-13 | 1/2"-20 | M14-2.00 | M14-1.50 | 5/8" | M16 |
| 9/16"-12 | 9/16"-18 | M16-2.00 | M16-1.50 | 3/4" | M20 |
| 5/8"-11 | 5/8"-18 | M20-2.50 | M20-1.50 | 7/8" | M22 |
| 3/4"-10 | 3/4"-10 | M22-2.50 | M22-1.50 | 1" | M24 |
| 7/8"-9 | 7/8"-14 | M24-3.00 | M24-2.00 | 1-1/8" | M27 |
| 1"-8 | 1"-14 | M27-3.00 | M27-2.00 | 1-1/4" | M30 |
| 1-1/8"-7 | 1-1/8"-12 | M30-3.50 | M30-2.00 | 1-3/8" | M33 |
| 1-1/4"-7 | 1-1/4"-12 | M36-4.00 | M36-2.00 | 1-1/2" | M36 |
| 1-3/8"-6 | 1-3/8"-12 | M42-4.50 | M42-2.00 | 1-3/4" | M42 |
| 1-1/2"-6 | 1-1/2"-12 | M48-5.00 | M48-2.00 | 2" | M48 |
Conversion Reference:
Metric vs. Imperial Diameter & Thread Pitch
The conversions table below provides a general reference to illustrate the differences between metric and imperial measurements. For critical applications, always follow your project’s specifications and consult an engineer to confirm suitability.| INCH | METRIC | CONVERSION (Diameter) | |||
|---|---|---|---|---|---|
| COARSE (UNC) | FINE (UNF) | COARSE (UNC) | FINE (UNF) | INCH | METRIC |
| 1/4"-20 | 1/4"-28 | M6-1.00 | M6-0.75 | 1/4" | M6 |
| 5/16"-18 | 5/16"-24 | M8-1.25 | M8-1.00 | 5/16" | M8 |
| 3/8"-16 | 3/8"-24 | M10-1.50 | M10-1.25 | 3/8" | M10 |
| 7/16"-14 | 7/16"-20 | M12-1.75 | M12-1.50 | 1/2" | M12 |
| 1/2"-13 | 1/2"-20 | M14-2.00 | M14-1.50 | 5/8" | M16 |
| 9/16"-12 | 9/16"-18 | M16-2.00 | M16-1.50 | 3/4" | M20 |
| 5/8"-11 | 5/8"-18 | M20-2.50 | M20-1.50 | 7/8" | M22 |
| 3/4"-10 | 3/4"-10 | M22-2.50 | M22-1.50 | 1" | M24 |
| 7/8"-9 | 7/8"-14 | M24-3.00 | M24-2.00 | 1-1/8" | M27 |
| 1"-8 | 1"-14 | M27-3.00 | M27-2.00 | 1-1/4" | M30 |
| 1-1/8"-7 | 1-1/8"-12 | M30-3.50 | M30-2.00 | 1-3/8" | M33 |
| 1-1/4"-7 | 1-1/4"-12 | M36-4.00 | M36-2.00 | 1-1/2" | M36 |
| 1-3/8"-6 | 1-3/8"-12 | M42-4.50 | M42-2.00 | 1-3/4" | M42 |
| 1-1/2"-6 | 1-1/2"-12 | M48-5.00 | M48-2.00 | 2" | M48 |
Industries & Locations Using Imperial & Metric Systems
Metric Fasteners- Best for: Automotive, aerospace, heavy equipment, electronics, and most modern OEM applications.
- Where: Predominantly used across Europe, Asia, South America, Africa, Australia, and Canada, as well as by international suppliers serving U.S. companies.
- Why: Standardized worldwide, easy to source internationally, and simpler for scaling due to decimal increments.
Imperial Fasteners
- Best for: U.S. construction, infrastructure, and industries with older or legacy equipment.
- Where: Most common in the United States and parts of the Caribbean, with limited use in countries that import U.S.-built machinery.
- Why: Many U.S.-made machines and vehicles were originally designed with imperial hardware. Staying consistent avoids mismatched parts and unsafe repairs.
Pro Tips from Blue Ribbon Fastener
- Stick with the original spec – Never swap metric and imperial fasteners in the same assembly.
- Keep tools in both systems – A 13 mm wrench won’t fit a 1/2" bolt.
- Separate and label – Store metric and imperial fasteners in clearly marked bins to avoid mix-ups.
- Measure twice – Use calipers or thread gauges to confirm size and pitch before installation.
The Blue Ribbon Advantage
At Blue Ribbon Fastener, we stock a comprehensive selection of metric and imperial bolts, nuts, screws, washers, and specialty hardware. Whether you’re maintaining legacy U.S.-built machinery or working with international suppliers, our team helps you find the right fastener for your needs.By offering both systems, and the expertise to guide you, we ensure your projects run smoothly, safely, and on schedule.
Final Takeaway
The choice between metric vs. imperial fasteners isn’t about which system is better; it’s about using the right one for your application. Metric is the global manufacturing standard, while imperial continues to play a major role in U.S. industries. Understanding both systems is essential for anyone working in construction, manufacturing, or equipment maintenance.Need help choosing? Contact us today. We’ll make sure you get the perfect fit.