Bolt Size Chart: Dimensions, Thread Pitch & Head Sizes
This bolt size chart covers the most common imperial hex bolt dimensions including diameter, threads per inch (TPI), wrench size, and head diameter. Use this reference to identify bolt sizes, find the right wrench, or select the correct bolt for your project. All dimensions are in inches unless noted.
Hex Bolt Size Chart (Imperial — UNC & UNF)
Dimensions per ASME B18.2.1. UNC = Unified National Coarse, UNF = Unified National Fine.
| Bolt Size | Decimal (in) | TPI | Wrench Size | Head Width (in) | Thread Type |
|---|---|---|---|---|---|
| #6-32 | 0.138 | 32 | 5/16 | 0.312 | UNC (Coarse) |
| #8-32 | 0.164 | 32 | 11/32 | 0.344 | UNC (Coarse) |
| #10-24 | 0.190 | 24 | 3/8 | 0.375 | UNC (Coarse) |
| #10-32 | 0.190 | 32 | 3/8 | 0.375 | UNF (Fine) |
| 1/4-20 | 0.250 | 20 | 7/16 | 0.438 | UNC (Coarse) |
| 1/4-28 | 0.250 | 28 | 7/16 | 0.438 | UNF (Fine) |
| 5/16-18 | 0.313 | 18 | 1/2 | 0.500 | UNC (Coarse) |
| 5/16-24 | 0.313 | 24 | 1/2 | 0.500 | UNF (Fine) |
| 3/8-16 | 0.375 | 16 | 9/16 | 0.563 | UNC (Coarse) |
| 3/8-24 | 0.375 | 24 | 9/16 | 0.563 | UNF (Fine) |
| 7/16-14 | 0.438 | 14 | 5/8 | 0.625 | UNC (Coarse) |
| 7/16-20 | 0.438 | 20 | 5/8 | 0.625 | UNF (Fine) |
| 1/2-13 | 0.500 | 13 | 3/4 | 0.750 | UNC (Coarse) |
| 1/2-20 | 0.500 | 20 | 3/4 | 0.750 | UNF (Fine) |
| 5/8-11 | 0.625 | 11 | 15/16 | 0.938 | UNC (Coarse) |
| 5/8-18 | 0.625 | 18 | 15/16 | 0.938 | UNF (Fine) |
| 3/4-10 | 0.750 | 10 | 1-1/8 | 1.125 | UNC (Coarse) |
| 3/4-16 | 0.750 | 16 | 1-1/8 | 1.125 | UNF (Fine) |
How to Read a Bolt Size Designation
Imperial bolt sizes follow the format Diameter-TPI × Length. For example, 3/8-16 × 2" means a bolt with 3/8-inch diameter, 16 threads per inch (UNC coarse), and 2-inch length measured from under the head.
For numbered sizes (#6, #8, #10), the number corresponds to a specific decimal diameter. As bolt size increases past 1/4", fractional dimensions are used (5/16, 3/8, 7/16, 1/2, etc.).
UNC vs UNF: Coarse Thread vs Fine Thread
UNC (Unified National Coarse) is the default thread standard for most fastening applications. Coarse threads are easier to install, more tolerant of minor damage, and less likely to cross-thread. They're the standard choice for construction, general assembly, and most hardware.
UNF (Unified National Fine) has more threads per inch for the same diameter. Fine threads provide slightly higher tensile strength and better resistance to vibration loosening. They're common in automotive, aerospace, and precision machinery applications.
Rule of thumb: If a bolt size doesn't specify UNC or UNF, assume UNC (coarse thread). When ordering, always confirm the thread pitch — a 1/4-20 nut will not fit a 1/4-28 bolt.
Bolt Diameter to Wrench Size Quick Reference
The wrench size for a hex bolt is measured across the flats of the hex head. It is always larger than the bolt diameter. Here is the quick reference:
Bolt
1/4"
Wrench
7/16"
Bolt
5/16"
Wrench
1/2"
Bolt
3/8"
Wrench
9/16"
Bolt
7/16"
Wrench
5/8"
Bolt
1/2"
Wrench
3/4"
Bolt
5/8"
Wrench
15/16"
Bolt
3/4"
Wrench
1-1/8"
Bolt
1"
Wrench
1-1/2"
Frequently Asked Questions
How do I measure bolt size?
Measure bolt diameter across the shank (not the threads) using calipers. Thread pitch is measured by counting threads per inch (TPI) for imperial bolts. The bolt length is measured from under the head to the end of the shank. For hex bolts, measure the wrench size across the flats of the head.
What is the most common bolt size?
The most common bolt sizes in the US are 1/4-20, 5/16-18, 3/8-16, and 1/2-13. These are all UNC (coarse thread) sizes. The 1/4-20 is the most widely used for general-purpose fastening in construction, automotive, and machinery applications.
What is the difference between UNC and UNF bolt threads?
UNC (Unified National Coarse) has fewer threads per inch and is the default standard for most applications. UNF (Unified National Fine) has more threads per inch, providing slightly higher tensile strength and better vibration resistance. For example, a 1/4-20 is UNC (20 TPI) while 1/4-28 is UNF (28 TPI).
What wrench size do I need for a 1/4-20 bolt?
A 1/4-20 hex bolt requires a 7/16-inch wrench or socket. The wrench size corresponds to the distance across the flats of the hex head, not the bolt diameter. Always verify with your specific bolt as some specialty heads may differ.
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