Hex & Heavy Hex Bolt Dimensions (ASME B18.2.1 – Inch Series)

This calculator provides hex bolt and heavy hex bolt dimensions according to the ASME B18.2.1 inch-series standard. It calculates head dimensions, width across flats and corners, body diameter, fillet radius, and nominal thread length.

What is the difference between hex and heavy hex bolts?

Heavy hex bolts use a larger head (greater bearing area) compared with standard hex bolts. This can be beneficial in structural connections and higher-load applications where a larger bearing surface is preferred.

Important notes (ASME B18.2.1)

Hex bolt nominal diameter range (standard): 1/4 in to 4 in.
Heavy hex bolt nominal diameter range (typical): 3/8 in to 3 in.
Threads may be UNC/UNF/UN-8 (or UNR forms when rolled) with tolerance class 2A unless specified otherwise.
Common material references include ASTM A307 (application-dependent grade). Always follow project and manufacturer requirements.

For thread pitch and minimum pitch diameter, use the Unified inch screw thread calculator.

Input parameters

Nominal diameter (inch)

Bolt length range (L)

Bolt type Hex bolts
Heavy hex bolts

Results – Hex / Heavy Hex Bolt Dimensions

Parameter	Symbol	Value	Unit
Product name	–	---	–
Nominal diameter	d	---	in
Full size body diameter	E_max	---
Full size body diameter	E_min	---
Width across flats	F_basic	---
Width across flats	F_max	---
Width across flats	F_min	---
Width across corners	G_max	---
Width across corners	G_min	---
Head height	H_basic	---
Head height	H_max	---
Head height	H_min	---
Fillet radius	R_max	---
Fillet radius	R_min	---
Thread length (nominal)	L_T	---

Thread length depends on nominal diameter and bolt length range (L ≤ 6 in or L > 6 in) per ASME B18.2.1.

Definitions and engineering notes

Hex cap screw vs hex bolt

Hex cap screws typically have tighter tolerances on body diameter and head height.
Hex cap screws often have a washer-faced bearing surface and tighter runout requirements.
Hex bolts are commonly used in construction and are generally intended for use with a nut.

Unified inch thread terms

Unified threads are used for general-purpose fastening and are defined by standards such as ANSI/ASME B1.2 (gages) and related documents. Common series include UNC (coarse) and UNF (fine).

Supplements

References

ASME B18.2.1-2010, Square and Hex Bolts and Screws (Inch Series)
Machinery’s Handbook, Industrial Press (30th edition)