- When to use the metric system or the imperial system?It doesn’t have a standard answer, depends on which target market you sale, commonly United States use imperial, most of country use metric. There is still a slight difference in the conversion of metric and imperial systems.
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Do you know what the differences between Imperial (American Standard) fasteners and metric fasteners are?
The biggest difference is metric expressed by thread pitch, imperial expressed by thread count. (Please read below for examples and details.)
Metric and Imperial unit conversion table
Inches (in) x 25.4 = Metric (mm)
| Inches (in) | Metric (mm) | Inches (in) | Metric (mm) |
| 1/64 | 0.3969 | 25/64 | 9.9219 |
| 1/32 | 0.7938 | 13/32 | 10.3188 |
| 1/16 | 1.5875 | 7/16 | 11.1125 |
| 5/64 | 1.9844 | 29/64 | 11.5094 |
| 3/32 | 2.3813 | 15/32 | 11.9063 |
| 7/64 | 2.7781 | 1/2 | 12.7000 |
| 1/8 | 3.1750 | 33/64 | 13.0969 |
| 9/64 | 3.5719 | 17/32 | 13.4938 |
| 5/32 | 3.9688 | 9/16 | 14.2875 |
| 11/64 | 4.3656 | 37/64 | 14.6844 |
| 3/16 | 4.7625 | 19/32 | 15.0813 |
| 13/64 | 5.1594 | 5/8 | 15.8750 |
| 7/32 | 5.5563 | 41/64 | 16.2719 |
| 1/4 | 6.3500 | 21/32 | 16.6688 |
| 17/64 | 6.7469 | 3/4 | 19.0500 |
| 9/32 | 7.1438 | 49/64 | 19.4469 |
| 5/16 | 7.9375 | 25/32 | 19.8438 |
| 21/64 | 8.3344 | 7/8 | 22.2250 |
| 11/32 | 8.7313 | 57/64 | 22.6219 |
| 3/8 | 9.5250 | 1 | 25.4000 |
What is thread pitch and thread count?
Imperial fasteners commonly use thread count, so 20 would represent 20 threads per inch(TPI). Metric fasteners instead specify a thread pitch which is the distance between the threads. Therefore, a 1.5 pitch would have 1.5 millimeters between each thread.
How are the metric and imperial systems marked?
The marking of a metric thread starts with the letter M, followed by the outer diameter of the thread in mm, for example, M8 is a metric thread with an outer diameter of 8 mm.
Imperial is commonly marked with W*N. The pitch of inch screw indicates the number of threads per inch.Most of US fasteners, the size consists of two or three parts. Example: 1/4”-20×3” (Diameter – Thread count x Length)
Such as: 1/8”-40 (outer diameter=1/8”= 3.175mm, 40 threads per inch), and some marked as 1/4”x0.50 are 1/4” screws with outer diameter and 0.5 inches in length.
Standard: the DIN, ISO, JIS or ANSI standard number that applies to the fastener.