AmesWeb
Hex Bolt Sizes Chart (ASME B18.2.1)
Hex bolt sizes and dimensions per ASME B18.2.1 for inch series bolts and screws.
Width across flats, width across corners, head height, fillet radius, and thread length are included.
For related fastener dimensions, see our
hex socket screws,
heavy hex nuts,
and
hex flange nuts.
Typical hex bolt geometry showing E, F, G, H, R and LT.
Filters all hex bolt tables by nominal size (first column). Leave empty to show all rows.
Hex Bolt Dimensions – Small to Medium Sizes
For bolt thread details, check our
UN thread calculator
and
UNC/UNF thread pitch chart.
Hex Bolt Dimensions – Large Sizes
For nut compatibility with these sizes, see
ASME heavy hex nut dimensions
and
hex coupling nuts.
Hex Bolt Dimensions – Extra Large Sizes
Symbols:
- E – Body diameter
- F – Width across flats
- G – Width across corners
- H – Head height
- R – Fillet radius
- LT – Thread length (≤ 6 in and > 6 in)
Frequently Asked Questions About Hex Bolts (ASME B18.2.1)
ASME B18.2.1 is the dimensional standard for inch-series hex bolts, heavy hex bolts, and hex cap screws.
It specifies width across flats (F), width across corners (G), head height (H), fillet radius (R),
and thread length (LT). These dimensions ensure interchangeability between manufacturers.
Hex bolts are typically used with a nut and allow slight clearance in the hole, while hex cap screws
are designed for tighter tolerances and can be used in tapped holes. Both follow ASME B18.2.1,
but hex cap screws usually have full body diameter tolerance control.
Thread length (LT) is the portion of the bolt that contains threads.
ASME B18.2.1 specifies two values: one for bolt length ≤ 6 inches and one for bolt length > 6 inches.
The width across flats (F) is the distance measured between opposing wrenching flats on the bolt head.
It determines which wrench or socket size will fit the bolt and ensures proper tightening torque.
Standard hex nuts, heavy hex nuts, and flange nuts are compatible depending on application.
For critical bolted joints, heavy hex nuts are commonly paired with larger hex bolts. See our
heavy hex nut size chart.
More Fastener Size Charts
References
- ASME B18.2.1-2010, Square, Hex, Heavy Hex Bolts and Screws
- Machinery’s Handbook, 30th Edition, Industrial Press