Bolt Size Chart
Bolt sizes are defined by diameter and thread pitch. The chart below covers common imperial bolt sizes from #6 through 1 inch, with their metric equivalents, thread pitch in threads per inch, head dimensions, and grade markings. Understanding bolt grades is essential for choosing the right fastener strength for your application.
| Diameter | Metric Equiv. | Thread Pitch | Head Size | Wrench Size | Grade |
|---|---|---|---|---|---|
| #6-32 | M3.5 | 32 TPI | 5/16" | 5/16" | No mark (Grade 2) |
| #8-32 | M4 | 32 TPI | 11/32" | 11/32" | No mark (Grade 2) |
| #10-24 | M5 | 24 TPI | 3/8" | 3/8" | No mark (Grade 2) |
| #10-32 | M5 | 32 TPI | 3/8" | 3/8" | 3 lines (Grade 5) |
| 1/4"-20 | M6 | 20 TPI | 7/16" | 7/16" | 3 lines (Grade 5) |
| 1/4"-28 | M6 | 28 TPI | 7/16" | 7/16" | 6 lines (Grade 8) |
| 5/16"-18 | M8 | 18 TPI | 1/2" | 1/2" | 3 lines (Grade 5) |
| 5/16"-24 | M8 | 24 TPI | 1/2" | 1/2" | 6 lines (Grade 8) |
| 3/8"-16 | M10 | 16 TPI | 9/16" | 9/16" | 3 lines (Grade 5) |
| 3/8"-24 | M10 | 24 TPI | 9/16" | 9/16" | 6 lines (Grade 8) |
| 7/16"-14 | M12 | 14 TPI | 5/8" | 11/16" | 3 lines (Grade 5) |
| 1/2"-13 | M12 | 13 TPI | 3/4" | 3/4" | 3 lines (Grade 5) |
| 1/2"-20 | M12 | 20 TPI | 3/4" | 3/4" | 6 lines (Grade 8) |
| 5/8"-11 | M16 | 11 TPI | 15/16" | 15/16" | 3 lines (Grade 5) |
| 3/4"-10 | M20 | 10 TPI | 1-1/8" | 1-1/8" | 3 lines (Grade 5) |
| 7/8"-9 | M22 | 9 TPI | 1-5/16" | 1-5/16" | 6 lines (Grade 8) |
| 1"-8 | M24 | 8 TPI | 1-1/2" | 1-1/2" | 6 lines (Grade 8) |
Understanding Bolt Grades
Grade 2 (No Markings)
Grade 2 bolts are the most common low-carbon steel fasteners. They have no head markings and are suitable for general-purpose, non-critical applications. Tensile strength is approximately 74,000 PSI for sizes up to 3/4 inch. Use for light fixtures, furniture assembly, and non-structural connections.
Grade 5 (3 Radial Lines)
Grade 5 bolts are made from medium-carbon steel that has been quenched and tempered. They are identified by three evenly spaced radial lines on the head. Tensile strength is approximately 120,000 PSI. Grade 5 is the standard for automotive and general construction applications where higher strength is needed.
Grade 8 (6 Radial Lines)
Grade 8 bolts are the highest-strength standard grade, made from medium-carbon alloy steel. They are identified by six radial lines on the head. Tensile strength is approximately 150,000 PSI. Use for critical structural connections, suspension components, engine mounts, and heavy equipment where failure would be dangerous.