Complete hex bolt dimensions chart covering M6-M48 metric and 1/4-2 inch imperial sizes. Includes head dimensions, thread length, tolerances per ISO 4014/4017 and ASME B18.2.1.
Why Accurate Bolt Dimensions Matter
Incorrect bolt dimensions are one of the top causes of fastener failure in construction and manufacturing. A bolt that is too short may not develop adequate thread engagement, reducing joint capacity by up to 50%. A bolt head that is too large for the fixture creates interference; too small and the washer face cannot distribute load properly. In procurement, ordering the wrong dimensions wastes 3-8% of project fastener budgets on average through returns and reorders. Understanding hex bolt dimension standards ensures you specify correctly the first time.
Metric Hex Bolt Dimensions (ISO 4014 / ISO 4017)
Metric hex bolts are designated by thread diameter (M) and length in millimeters. ISO 4014 defines partially threaded hex bolts (the standard for structural applications), while ISO 4017 defines fully threaded hex bolts. The across-flats (AF) dimension determines the wrench size needed. Thread pitch can be coarse (standard) or fine — coarse pitch is the default for most applications and provides better resistance to stripping and contamination.
| Size | Thread Pitch (mm) | Head AF (mm) | Head Height (mm) | Wrench Size (mm) |
|---|---|---|---|---|
| M6 | 1.0 | 10.0 | 4.0 | 10 |
| M8 | 1.25 | 13.0 | 5.3 | 13 |
| M10 | 1.5 | 16.0 | 6.4 | 16 |
| M12 | 1.75 | 18.0 | 7.5 | 18 |
| M16 | 2.0 | 24.0 | 10.0 | 24 |
| M20 | 2.5 | 30.0 | 12.5 | 30 |
| M24 | 3.0 | 36.0 | 15.0 | 36 |
| M30 | 3.5 | 46.0 | 18.7 | 46 |
| M36 | 4.0 | 55.0 | 22.5 | 55 |
| M42 | 4.5 | 65.0 | 26.0 | 65 |
| M48 | 5.0 | 75.0 | 30.0 | 75 |
Imperial Hex Bolt Dimensions (ASME B18.2.1)
Imperial (inch) hex bolts follow ASME B18.2.1 in the US market. While metric is the global standard, imperial bolts remain common in North American construction, oil & gas, and legacy infrastructure worldwide — including older installations across Africa and the Middle East. UNC (Unified National Coarse) is the standard thread series, similar to metric coarse pitch.
| Size (in) | TPI (UNC) | Head AF (in) | Head Height (in) | Wrench Size (in) |
|---|---|---|---|---|
| 1/4 | 20 | 7/16 | 11/64 | 7/16 |
| 5/16 | 18 | 1/2 | 7/32 | 1/2 |
| 3/8 | 16 | 9/16 | 1/4 | 9/16 |
| 1/2 | 13 | 3/4 | 11/32 | 3/4 |
| 5/8 | 11 | 15/16 | 27/64 | 15/16 |
| 3/4 | 10 | 1-1/8 | 1/2 | 1-1/8 |
| 7/8 | 9 | 1-5/16 | 37/64 | 1-5/16 |
| 1 | 8 | 1-1/2 | 43/64 | 1-1/2 |
| 1-1/4 | 7 | 1-7/8 | 27/32 | 1-7/8 |
| 1-1/2 | 6 | 2-1/4 | 31/32 | 2-1/4 |
| 2 | 4.5 | 3 | 1-5/16 | 3 |
Thread Length Rules for Partially Threaded Bolts
For ISO 4014 partially threaded hex bolts, thread length depends on bolt diameter. The formula is: bolt length ≤ 125mm → thread length = 2d + 6mm; 125mm < bolt length ≤ 200mm → thread length = 2d + 12mm; bolt length > 200mm → thread length = 2d + 25mm (where d = nominal diameter). The unthreaded shank (grip length) is critical for shear-loaded joints — the shank should pass through both connected parts without threads in the shear plane. For structural joints, specify bolt length so that the grip length matches the total thickness of connected parts.
Dimensional Tolerances You Should Know
ISO 4014/4017 specifies tolerances that affect fit and function. Head across-flats tolerance is typically -0 to +0.5mm for sizes up to M24. Body diameter tolerance follows ISO general tolerances (grade g for bolt bodies). Thread tolerance class 6g is standard for general-purpose bolts — this provides a clearance fit with the 6H nut thread. For high-precision applications, specify 4g6g (tighter). Length tolerance for bolt length is typically ±1.5mm for lengths up to 100mm. When ordering, always specify the standard (ISO or ASME) — mixing systems causes fit problems.
| Dimension | Tolerance (ISO) | Impact of Out-of-Spec |
|---|---|---|
| Head AF | 0 to +0.5mm | Wrench fit, torque accuracy |
| Body diameter | ISO g tolerance | Hole fit, shear capacity |
| Thread class | 6g (standard) | Nut engagement, preload |
| Bolt length | ±1.5mm (≤100mm) | Grip length, thread engagement |
| Head height | 0 to +0.6mm | Wrench clearance, bearing area |
Procurement Tips: Avoiding Dimension Mix-Ups
The most common procurement errors are: confusing bolt length with thread length (always specify total bolt length per the standard); mixing metric and imperial in the same order (clearly separate line items); not specifying thread pitch for metric bolts above M18 where fine pitch alternatives exist; and ordering partially threaded when fully threaded is needed (or vice versa). For African infrastructure projects, metric is the standard, but always confirm the project specification — some oil & gas projects use imperial. Create a fastener schedule table for each project listing: size, grade, length, thread type, coating, and quantity.
Frequently Asked Questions
What does M12×1.75 mean?
M12 is the nominal thread diameter (12mm). 1.75 is the thread pitch in millimeters (distance between adjacent threads). This is the standard coarse pitch for M12 bolts.
How do I measure bolt length correctly?
For hex bolts, measure from under the head to the end of the bolt. Do NOT include the head height. For countersunk bolts, measure total length including the head.
Can I use a metric wrench on an imperial bolt?
Not recommended. Close size matches (e.g., 13mm on 1/2" bolt) risk rounding the head. Always use the correct wrench size per the bolt standard.
What thread engagement depth do I need?
Minimum thread engagement = 1x bolt diameter for steel nuts, 1.5x diameter for aluminum, and 2x diameter for cast iron. Full engagement equals the nut height, which is typically 0.8x diameter.
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