A Guide to Metric and Imperial Fastener Systems

Keywords: Metric fasteners, imperial fasteners, thread standards

The world uses two main fastener measurement systems: Metric (ISO) and Imperial (SAE/UNC/UNF). Metric bolts are sized in millimeters (e.g., M10 x 1.5). Imperial bolts use inches (e.g., 1/2″-13). Mixing them is a common error that damages threads. Knowing which system your equipment or design uses is fundamental. While the global trend is towards metric, many older machines and North American industries still rely on Imperial nuts and bolts. Always have both sets of tools and knowledge.