Metric vs. Imperial Screws
Two sizing systems that do not interchange. Know which one your project uses before buying.
| Feature | Metric (M-series) | Imperial (inch / gauge) |
|---|---|---|
| Designation | M6 x 1.0 (diameter x pitch) | 1/4-20 or #8 (size + TPI) |
| Thread pitch | Millimeters per thread | Threads per inch |
| Common where | Most of the world, autos | US construction, older machinery |
| Interchangeable? | No | No |
| Tools | Metric wrenches and hex keys | Inch wrenches and hex keys |
Verdict
Identify the system before you buy — a metric screw will not thread into an imperial hole and vice versa. Keep separate tool sets to avoid rounding fasteners.