Metric vs imperial: know the difference
Mistaking a metric fastener for imperial (or vice versa) is an easy error with expensive consequences. While they may look similar, mismatching threads can damage components, compromise structural integrity, or delay projects.
What's the difference?
Metric fasteners:
- Measured in millimetres (e.g. M6 x 20)
- Thread pitch in mm (e.g. M10 x 1.5)
- Used globally and standard in Australian construction and manufacturing
Imperial fasteners:
- Measured in inches (e.g. 1/4"-20)
- Thread pitch in TPI (Threads Per Inch)
- Common in US-made equipment, mining, defence, older gear
Visual cues and tips
- Metric bolts often have numbers (e.g. M10, PC 8.8)
- Imperial bolts may be marked UNC (coarse), UNF (fine) and G5, G8
- Hex head size differs slightly: a 3/8" bolt won’t fit an M10 spanner exactly
- Use a thread pitch gauge or digital caliper to verify
>> Download our conversion factors PDF
Common conversion pitfalls
- Mismatching M8 with 5/16" (very close in size)
- Assuming UNC bolts will fit metric tapped holes
- Using mixed systems on the same assembly
Tip: Keep metric and imperial stocks clearly separated in bins or kits.
When imperial is still used in Australia
While metric is standard in Australia, imperial fasteners are still common in:
- Imported equipment from the US
- Defence and aerospace sectors
- Mining machinery and legacy infrastructure
Solutions for procurement and maintenance
United Fasteners supports both systems with:
- Metric and imperial stocked lines
- Conversion charts and technical support
- Custom kitting labelled by thread system
Avoid mismatched threads. Get expert support or shop both metric and imperial fasteners online.