1 inch = 25.4mm

1/2 inch = 12.7mm which means a 13mm wrench is just a bit wider than a 1/2 inch wrench. They may not the same, depending on manufacturing toleranced, but close to work on many fasteners.

The fasteners themselves are NOT interchangeable, as the thread pitch is not equal, and will ruin the fastener or the tapped hole.

I investigated a case where an oil filter fell off of a car, ruining the engine. The filter had a SAE thread, the engine wanted a metric thread. The SAE was just slightly larger, allowing the filter to fall off 5 miles after it had been installed. Filters looked exactly the same, except for part number...


Best Regards,
Jim