Hex Bolt Size Chart: Diameter, Length & Thread Reference
3/8-16 × 2" bolt is 3/8" in diameter, 16 threads per inch (UNC coarse), and 2 inches from under head to tip. The matching wrench is 9/16" across the flats.
You're on a job with a mix of SAE and metric fasteners, and someone hands you an unmarked bag of hex bolts. Right tool, right spec, right grade — every time. Here's the complete reference.
Hex Bolt Anatomy
SAE/Imperial Hex Bolt Size Chart (UNC Coarse)
Coarse thread (UNC) is the default for structural, construction, and general mechanical applications. Unless a drawing or spec calls out UNF explicitly, order UNC.
| Size | Dia (in) | UNC TPI | UNF TPI | Wrench AF | Close Hole | Normal Hole |
|---|---|---|---|---|---|---|
| 1/4" | 0.250 | 20 | 28 | 7/16" | 9/32" | 5/16" |
| 5/16" | 0.3125 | 18 | 24 | 1/2" | 11/32" | 3/8" |
| 3/8" | 0.375 | 16 | 24 | 9/16" | 13/32" | 7/16" |
| 7/16" | 0.4375 | 14 | 20 | 5/8" | 15/32" | 1/2" |
| 1/2" | 0.500 | 13 | 20 | 3/4" | 17/32" | 9/16" |
| 9/16" | 0.5625 | 12 | 18 | 13/16" | 19/32" | 5/8" |
| 5/8" | 0.625 | 11 | 18 | 15/16" | 21/32" | 11/16" |
| 3/4" | 0.750 | 10 | 16 | 1-1/8" | 25/32" | 13/16" |
| 7/8" | 0.875 | 9 | 14 | 1-5/16" | 29/32" | 15/16" |
| 1" | 1.000 | 8 | 12 | 1-1/2" | 1-1/32" | 1-1/16" |
AF = across flats (wrench size). Bold rows = most common structural sizes on the job. Close hole for slip-fit; Normal hole for standard through-bolt clearance.
Metric Hex Bolt Size Chart (Standard / Coarse Pitch)
Metric hex bolts use ISO 4014/4016 for partially threaded and ISO 4017/4018 for fully threaded. The designation format is M10 × 1.5 × 40 — diameter, pitch, length in mm.
| Size | Dia (mm) | Coarse Pitch | Fine Pitch | Wrench AF | Close Hole | Normal Hole |
|---|---|---|---|---|---|---|
| M6 | 6.0 | 1.0 | 0.75 | 10 mm | 6.4 mm | 7 mm |
| M8 | 8.0 | 1.25 | 1.0 | 13 mm | 8.4 mm | 9 mm |
| M10 | 10.0 | 1.5 | 1.25 | 17 mm | 10.5 mm | 11 mm |
| M12 | 12.0 | 1.75 | 1.25 | 19 mm | 12.5 mm | 13.5 mm |
| M14 | 14.0 | 2.0 | 1.5 | 22 mm | 14.5 mm | 15.5 mm |
| M16 | 16.0 | 2.0 | 1.5 | 24 mm | 16.5 mm | 17.5 mm |
| M20 | 20.0 | 2.5 | 1.5 | 30 mm | 21 mm | 22 mm |
| M24 | 24.0 | 3.0 | 2.0 | 36 mm | 25 mm | 26 mm |
Fine pitch in gray — use only when spec calls for it. Bold = most common structural metric sizes.
How to Measure Hex Bolt Length
Length is measured from the bearing face — the flat underside of the hex head — to the tip. The head height is not included. A hex bolt listed as 1/2-13 × 3" has exactly 3 inches of shank and threaded section below where the head contacts the material.
Partial vs. full thread: hex bolts under 6 inches are typically threaded for about 2× the bolt diameter plus 1/4", leaving a smooth shank near the head. Fully threaded ("tap bolts" or fully threaded hex) have threads the entire length and are used where thread engagement through the full joint is required.
Determining Grip Length
Grip length is the distance from the bearing face to the first thread — the smooth shank portion that runs through your joint. The standard rule: grip length should approximately equal the material stack thickness so the threads engage only in the nut or tapped hole, not in the clearance hole of the through-member.
Too short a grip and threads bear against the side of the clearance hole — terrible for shear loads. Too long and the nut runs out of thread before the head is seated. Get the grip within 1/16" of the joint stack thickness.
Grade Identification on the Head
Grade is stamped on the head as radial lines (SAE) or a property class number (metric):
| Standard | Marking | Min Tensile (ksi) | Typical Use |
|---|---|---|---|
| SAE Grade 2 | No lines (blank) | 74 | Light hardware, non-structural |
| SAE Grade 5 | 3 lines | 120 | General structural, automotive |
| SAE Grade 8 | 6 lines | 150 | High-strength structural, suspension |
| Metric 8.8 | "8.8" on head | 116 | General structural ≈ Grade 5 |
| Metric 10.9 | "10.9" on head | 145 | High-strength ≈ Grade 8 |
| Metric 12.9 | "12.9" on head | 174 | Max-strength, socket head cap screws |
Why SAE and Metric Hex Bolts Are Not Interchangeable
The diameter numbers are deceptively close. M10 = 10.0 mm and 3/8" = 9.525 mm — less than 0.5 mm apart. But the threads are completely different:
3/8-16 UNC: 16 TPI = 1.588 mm pitchM10 × 1.5: 1.5 mm pitch
Those thread forms will not engage each other. Attempting to run an M10 bolt into a 3/8-16 tapped hole — or vice versa — will cross-thread the hole immediately. The same mismatch applies to M6/1/4-20, M8/5/16, and M12/1/2: close diameters, incompatible threads.
Use our identifier tool to confirm the thread pitch of any unmarked hex bolt before substituting it for another size.
Identify Your Fastener Now
Enter the head type, drive type, and measurements — our tool returns the canonical designation in seconds, including thread pitch.
Open Identifier →Common Length Ranges by Application
| Application | Common Size | Typical Length | Grade |
|---|---|---|---|
| Deck ledger to house framing | 1/2-13 | 3-1/2" to 5" | Grade 5 min |
| Wood beam-to-post connections | 1/2-13 or 5/8-11 | 4" to 8" | Grade 5 |
| Automotive suspension / chassis | M10 or 3/8" | Per service manual | Grade 8 / 10.9 |
| Light machinery / equipment | M8 to M16 | Per drawing | 8.8 typical |
| Fence rail-to-post | 3/8-16 | 2" to 4" | HDG or SS |
Thread Engagement Rules of Thumb
For steel-into-steel tapped holes: minimum thread engagement = 1× bolt diameter. For steel-into-aluminum: 1.5–2× diameter. For steel-into-wood or plastic: through-bolt with nut, not tapped hole.
For structural wood connections (deck bolts, ledger connections), the International Residential Code requires hex bolts with washers under both head and nut. Hot-dip galvanized (HDG) or stainless for PT lumber — zinc-plated bolts react with the copper in ACQ-treated wood and corrode rapidly in outdoor exposure.
Recommended Tool
Combination Wrench Set (SAE + Metric)
One set that covers both standards avoids the job-site bottleneck when you're working with mixed fastener specs. Look for chrome-vanadium construction with 12-point box ends — they grab a rounded hex head better than 6-point in tight spaces.
View on Amazon →Frequently Asked Questions
What wrench size fits a 1/2 inch hex bolt?
A 1/2 inch hex bolt uses a 3/4 inch wrench measured across flats. This is one of the most common bolt-to-wrench pairings on structural jobs. Remember: the wrench size is always larger than the bolt diameter — typically 1.5× for smaller sizes and closer to 1.25× for 3/4" and above.
What is the standard thread for a 3/8 inch hex bolt?
Standard coarse thread is 3/8-16 UNC (16 threads per inch). Fine thread is 3/8-24 UNF. Coarse is the default for construction and general mechanical work. Fine thread is used in automotive and precision applications where vibration resistance or micro-adjustment matters.
How do you measure hex bolt length?
Measure from the underside of the hex head (the bearing face) to the tip of the bolt. The head height is not included in the stated length. A 3/8-16 × 2" bolt has exactly 2 inches of shank below where the head contacts the material.
What clearance hole do I drill for a 1/2 inch hex bolt?
Drill 17/32" (0.531") for a close fit, or 9/16" (0.5625") for a standard clearance fit. Use the close fit when bolt alignment is critical (shear connection); use normal clearance when you need slight adjustment during assembly.
Can you substitute metric for SAE hex bolts?
No. Even when diameters appear identical (M10 = 10.0 mm vs 3/8" = 9.53 mm), the thread pitches are incompatible. Forcing an M10 bolt into a 3/8-16 tapped hole will cross-thread the hole immediately. Always confirm the designation before substituting.
When should I use a hex cap screw vs. a hex bolt?
In practice the terms are often used interchangeably on the job site. Technically, a hex bolt (ASME B18.2.1 Grade A or B) is used with a nut and has looser tolerances on the bearing surface. A hex cap screw (ASME B18.2.1 Grade A) has tighter tolerances and is designed to thread into a tapped hole or flanged joint without a nut. For structural through-bolting with a nut, either works.