The main content of this article is a comparison of the specifications of bolts, screws, and threaded rods between the imperial and metric systems. Using diameter as a reference benchmark, the sizes(codes) corresponding to the two units are listed side by side, and the mechanical strength grades and markings of bolts, screws, and threaded rods are compared. If one is used to working with metric units but needs to purchase an imperial screw, it would be helpful to have a comparable metric size for reference, since the length units and strength units used in the imperial system are different from those in the metric system.
It should be noted that this is only a comparison of sizes, and even if the sizes are the same, the products cannot be interchangeable - they are not equivalent. For example, a screw that is close in size to an M5 screw cannot be replaced with an M5 screw because the thread shape may be different.
| Diameter/Decimal/Metric Chart for Small Diameters | |||
| Gauge or Diameter | Decimal (in) | mm |  |
| No. 0000 | 0.021 | 0.53 | |
| No. 000 | 0.034 | 0.86 | |
| No. 00 | 0.047 | 1.19 | |
| No. 0 | 0.06 | 1.524 | |
| No. 1 | 0.073 | 1.854 | |
| No. 2 | 0.086 | 2.184 | |
| No. 3 | 0.099 | 2.515 | |
| No. 4 | 0.112 | 2.845 | |
| No. 5 | 0.125 | 3.175 | |
| No. 6 | 0.138 | 3.505 | |
| No. 8 | 0.164 | 4.166 | |
| No. 10 | 0.190 | 4.826 | |
| No. 12 | 0.216 | 5.484 | |
| Fastener Conversion Chart | |||
| Inch Equivalent | Metric Size-Pitch ISO and IFI Rec | ||
| UNC | UNF | ||
| 1 - 64 | 1 - 72 | M2 x 0.4 | |
| 3 - 48 | 3 - 56 | M2.5 x 0.45 | |
| 4 - 40 | 4 - 48 | M3 x .05 | |
| 6 - 32 | 6 - 40 | M3.5 x .06 | |
| 8 - 32 | 8 - 36 | M4 x .07 | |
| 10 - 24 | 10 - 32 | M5 x .08 | |
| 1/4 - 20 | 1/4 - 28 | M6 x 1 | |
| 5/16 -18 | 5/16 - 24 | M8 x 1.25 | |
| 3/8 -16 | 3/8 - 24 | M10 x 1.5 | |
| 7/16 -14 | 7/16 - 20 | M12 x 1.75 | |
| 1/2 - 13 | 1/2 - 20 | M14 x 2 | |
| 5/8 - 11 | 5/8 - 18 | M18 x 2 | |
| 3/4 - 10 | 3/4 - 16 | M20 x 2.5 | |
| 1 - 8 | 1-12 | M24 x 3 | |
| 1-1/4 - 7 | 1-1/8 - 12 | M30 x 3.5 | |
| 1-1/2 - 6 | 1-1/4 - 12 | M36 x 4 | |
Approximate Equivalency Chart Metric/US Roughly Equivalent US Bolt Materials | |||
| Metric Nut Class | SAE J429 Normally Used | Grades | ASTM Grades |
| 4.6 | 4 or 5 | 1 | A306, Grade A |
| 4.8 | 4 or 5 | 2 | . |
| 5.8 | 5 | 2 | . |
| 8.8 | 8 | 5 | A325, A449 |
| 9.8 | 9 | 5+ | A193, B7 and B16 |
| 10.9 | 10 or 12 | 8 | A490, A354, Grade 8D |
| 12.9 | 10 or 12 | . | A540; B21 through B24 |
| Common DIN Numbers for Metric Fasteners | |||
| DIN | Hex Capscrews | ||
| 931 | Coarse thread pitch partially threaded (specify grade) | ||
| 933 | Coarse thread pitch fully threaded (specify grade) | ||
| 930 | Fine thread pitch partially threaded (specify grade) | ||
| 961 | Fine thread pitch fully threaded (specify grade) | ||
| DIN | Nuts | ||
| 934 | Hex nuts (specify pitch and class) | ||
| 985 | Nylon insert locknuts (specify pitch and class) | ||
| 980V | All metal locknuts (specify pitch and class) | ||
| DIN | Washers | ||
| 125 | Flatwashers | ||
| 127 | Lockwashers | ||
| DIN | Socket Produuts | ||
| 912 | Socket head capscrews (normally GR 12.9 and coarse thread) | ||
| 7991 | Flat head socket capscrews (normally GR 12.9 and coarse thread) | ||
| 916 | Socket setscrews (normally GR 12.9 and coarse thread) | ||
| DIN | Machine Screws | ||
| 7985 | Pan head Phillips drive zinc plated | ||
| 965 | Flat head Phillips drive zinc plated | ||
| DIN | Threaded Rod | ||
| 975 | All threaded rod (normally 1 meter lengths, specify grade and pitch) | ||