Metric vs Imperial Fasteners

What is the difference?

Metric fasteners use millimeters for diameter and thread pitch (for example, M8 x 1.25). Imperial fasteners use inch-based diameters and threads per inch (for example, 5/16-18 UNC). Mixing systems without conversion leads to cross-threading and joint failure.

Metric vs imperial comparison

Common size equivalents. Use our screw converter for instant lookup.

When to use metric fasteners

• Most modern vehicles and imported machinery
• ISO-standardized manufacturing environments
• Builds where metric tools are already standard

When to use imperial fasteners

• Legacy equipment and North American systems
• UNC/UNF thread standards specified in drawings
• Projects matching existing imperial hardware

Best practices

Always verify diameter and pitch before assembly. Never force a near-match thread. Use a tap drill chart before cutting internal threads.

FAQ

What is the difference between metric and imperial fasteners?

Metric fasteners use millimeters for diameter and thread pitch (e.g. M8 x 1.25). Imperial fasteners use inch-based diameters and threads per inch (e.g. 5/16-18 UNC). They are not interchangeable without conversion.

When should I use metric vs imperial bolts?

Use metric for most modern vehicles and ISO-standard machinery. Use imperial for legacy North American equipment and when drawings specify UNC/UNF.

Can I substitute a metric bolt for an imperial one?

Do not substitute without checking diameter and pitch. Near-match sizes can strip threads or fail. Use a conversion chart or our Metric to Imperial Converter to find correct equivalents.