ANSI/ASME B18.2.1 Hex Bolts (Inch Series)

ANSI/ASME B18.2.1 square and hex head bolts, inch series 1/4″–2-1/2″ — the collected per-size tables: shank d s, across-flats s (Basic/max/min), across-corners e, head height k, k w, the two-regime thread length b and fillet r.

Last updated: 2026-11

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ANSI/ASME B18.2.1 Hex Bolts (Inch Series) in an industrial setting — Illustrative image redrawn from real product references — not a photograph of stocked items.

Specifications

ANSI B18.2.1 is the inch-series hex bolt standard that North American drawings mean when they say "hex cap screw or hex bolt": the same head geometry family that ISO 4014 metric standardised later, running 1/4″ through 2-1/2″ in the collected tables. The head ladder is pure inch practice — across-flats printed as Basic fractions (7/16 at 1/4″ up to 3-3/4 at 2-1/2″) alongside decimal max/min — and the thread length follows a two-regime rule (b for L≤6 inches and L>6 inches of bolt length), simpler than the metric three-regime ladder because the longer standard lengths collapse into one regime. Two details worth attention when mixing with metric stock: the r fillet row is irregular in the collected source (0.025 through the small sizes, then jumps as printed at 9/16″–3/4″ — see the table footnotes), and above 2″ the b ≤6 regime is not tabulated ("/"), so the longest sizes carry the long-regime thread only. The standard pairs with inch hex nuts (ANSI B18.2.2, covered in this catalog) and SAE J429 property classes. Finishes and classes stay per RFQ.

Standard	ANSI/ASME B18.2.1 — square and hex head bolts, inch series (hex head bolt tables)
Series	1/4″ – 2-1/2″ (seven source column groups of three sizes each; fractions as printed, decimals in max/min)
Tabulated geometry	Per size: d s (max/min), s across flats (Basic fraction/max/min), e across corners (max/min), k head height (Basic/max/min), k w (min), b thread length (L≤6 / L>6 regimes, "/" above 2″ in the L≤6 regime), r fillet (max/min)
Inch head ladder	Across-flats Basic 7/16″ (1/4″) to 3-3/4″ (2-1/2″); head height Basic 5/32″ to 1-17/32″ — wrenching follows the fraction column, tolerances the decimals
Property classes / finishes	Not tabulated by the collected specification — per RFQ (typically SAE J429 grades; state grade and finish explicitly)

ANSI B18.2.1 hex head bolts (inch) - per-size dimensions (1/4 - 3/8)All dimensions in inches. Attribute labels follow the source: d s = shank diameter (max/min); s = width across flats (Basic/max/min, Basic printed as fractions); e = width across corners (max/min); k = head height (Basic/max/min); k w = width across flats of the head bearing (min); b = thread length in two length regimes (L≤6 / L>6, inches of thread); r = fillet radius (max/min). '/' where the source does not tabulate the cell.
Attribute	1/4	5/16	3/8
d s — max	0.25	0.3125	0.375
min	0.245	0.3065	0.369
s — Basic	7/16	1/2	9/16
max	0.438	0.5	0.562
min	0.428	0.489	0.551
e — max	0.505	0.577	0.65
min	0.488	0.557	0.628
k — Basic	5/32	13/64	15/64
max	0.163	0.211	0.243
min	0.15	0.195	0.226
k w — min	0.106	0.14	0.16
b — L≤6	0.75	0.875	1
L＞6	1	1.125	1.25
r — max	0.025	0.025	0.025
min	0.015	0.015	0.015

ANSI B18.2.1 hex head bolts (inch) - per-size dimensions (7/16 - 9/16)All dimensions in inches. Attribute labels follow the source: d s = shank diameter (max/min); s = width across flats (Basic/max/min, Basic printed as fractions); e = width across corners (max/min); k = head height (Basic/max/min); k w = width across flats of the head bearing (min); b = thread length in two length regimes (L≤6 / L>6, inches of thread); r = fillet radius (max/min). '/' where the source does not tabulate the cell.
Attribute	7/16	1/2	9/16
d s — max	0.4375	0.5	0.5625
min	0.4305	0.493	0.5545
s — Basic	5/8	3/4	13/16
max	0.625	0.75	0.812
min	0.612	0.736	0.798
e — max	0.722	0.866	0.938
min	0.698	0.84	0.91
k — Basic	9/32	5/16	23/64
max	0.291	0.323	0.371
min	0.272	0.302	0.348
k w — min	0.195	0.215	0.25
b — L≤6	1.125	1.25	1.375
L＞6	1.375	1.5	1.625
r — max	0.025	0.025	0.45
min	0.015	0.015	0.02

Row 14 (r — max): The source prints r max 0.45 at 9/16 (vs 0.025 across the small series) — reproduced verbatim as printed.

ANSI B18.2.1 hex head bolts (inch) - per-size dimensions (5/8 - 7/8)All dimensions in inches. Attribute labels follow the source: d s = shank diameter (max/min); s = width across flats (Basic/max/min, Basic printed as fractions); e = width across corners (max/min); k = head height (Basic/max/min); k w = width across flats of the head bearing (min); b = thread length in two length regimes (L≤6 / L>6, inches of thread); r = fillet radius (max/min). '/' where the source does not tabulate the cell.
Attribute	5/8	3/4	7/8
d s — max	0.625	0.75	0.875
min	0.617	0.741	0.866
s — Basic	15/16	1-1/8	1-5/16
max	0.938	1.125	1.312
min	0.922	1.1	1.285
e — max	1.083	1.299	1.516
min	1.051	1.254	1.465
k — Basic	25/64	15/32	35/64
max	0.403	0.483	0.563
min	0.378	0.455	0.531
k w — min	0.269	0.324	0.378
b — L≤6	1.5	1.75	2
L＞6	1.75	2	2.25
r — max	0.45	0.45	0.065
min	0.02	0.02	0.04

Row 14 (r — max): The source prints r max 0.45 at 5/8 and 3/4 but 0.065 at 7/8 (min 0.04) — an irregular r ladder in the source, kept verbatim.

ANSI B18.2.1 hex head bolts (inch) - per-size dimensions (1 - 1-1/4)All dimensions in inches. Attribute labels follow the source: d s = shank diameter (max/min); s = width across flats (Basic/max/min, Basic printed as fractions); e = width across corners (max/min); k = head height (Basic/max/min); k w = width across flats of the head bearing (min); b = thread length in two length regimes (L≤6 / L>6, inches of thread); r = fillet radius (max/min). '/' where the source does not tabulate the cell.
Attribute	1	1-1/8	1-1/4
d s — max	1	1.125	1.25
min	0.99	1.114	1.239
s — Basic	1-1/2	1-11/16	1-7/8
max	1.5	1.688	1.875
min	1.469	1.631	1.812
e — max	1.732	1.949	2.165
min	1.675	1.859	2.066
k — Basic	39/64	11/16	25/32
max	0.627	0.718	0.813
min	0.591	0.658	0.749
k w — min	0.416	0.461	0.53
b — L≤6	2.25	2.5	2.75
L＞6	2.5	2.75	3
r — max	0.095	0.095	0.095
min	0.06	0.06	0.06

ANSI B18.2.1 hex head bolts (inch) - per-size dimensions (1-3/8 - 1-3/4)All dimensions in inches. Attribute labels follow the source: d s = shank diameter (max/min); s = width across flats (Basic/max/min, Basic printed as fractions); e = width across corners (max/min); k = head height (Basic/max/min); k w = width across flats of the head bearing (min); b = thread length in two length regimes (L≤6 / L>6, inches of thread); r = fillet radius (max/min). '/' where the source does not tabulate the cell.
Attribute	1-3/8	1-1/2	1-3/4
d s — max	1.375	1.5	1.75
min	1.363	1.488	1.738
s — Basic	2-1/16	2-1/4	2-5/8
max	2.062	2.25	2.625
min	1.994	2.175	2.538
e — max	2.382	2.598	3.031
min	2.273	2.48	2.893
k — Basic	27/32	15/16	1-3/32
max	0.878	0.974	1.134
min	0.81	0.902	1.054
k w — min	0.569	0.64	0.748
b — L≤6	3	3.25	3.75
L＞6	3.25	3.5	4
r — max	0.095	0.095	0.095
min	0.06	0.06	0.06

ANSI B18.2.1 hex head bolts (inch) - per-size dimensions (2 - 2-1/2)All dimensions in inches. Attribute labels follow the source: d s = shank diameter (max/min); s = width across flats (Basic/max/min, Basic printed as fractions); e = width across corners (max/min); k = head height (Basic/max/min); k w = width across flats of the head bearing (min); b = thread length in two length regimes (L≤6 / L>6, inches of thread); r = fillet radius (max/min). '/' where the source does not tabulate the cell.
Attribute	2	2-1/4	2-1/2
d s — max	2	2.25	2.5
min	1.988	2.238	2.488
s — Basic	3	3-3/8	3-3/4
max	3	3.375	3.75
min	2.9	3.262	3.625
e — max	3.464	3.897	4.33
min	3.306	3.719	4.133
k — Basic	1-7/32	1-3/8	1-17/32
max	1.263	1.423	1.583
min	1.175	1.327	1.479
k w — min	0.825	0.933	1.042
b — L≤6	4.25	/	/
L＞6	4.5	5	5.5
r — max	0.095	0.095	0.095
min	0.06	0.06	0.06

Row 12 (b — L≤6): The source prints '/' for b at L≤6 for 2-1/4 and 2-1/2 — the regime is tabulated only for the smallest size in the table; kept verbatim.

ANSI B18.2.1 hex head bolts (inch) - per-size dimensions (2-3/4 - 3)All dimensions in inches. Attribute labels follow the source: d s = shank diameter (max/min); s = width across flats (Basic/max/min, Basic printed as fractions); e = width across corners (max/min); k = head height (Basic/max/min); k w = width across flats of the head bearing (min); b = thread length in two length regimes (L≤6 / L>6, inches of thread); r = fillet radius (max/min). '/' where the source does not tabulate the cell.
Attribute	2-3/4	3
d s — max	2.75	3
min	2.738	2.988
s — Basic	4-1/8	4-1/2
max	4.125	4.5
min	3.988	4.35
e — max	4.763	5.196
min	4.546	4.959
k — Basic	1-11/16	1-7/8
max	1.744	1.935
min	1.632	1.815
k w — min	1.151	1.29
b — L≤6	/	/
L＞6	6	6.5
r — max	0.095	0.095
min	0.06	0.06

Row 12 (b — L≤6): The source prints '/' for b at L≤6 for both 2-3/4 and 3 — the regime is not tabulated at these sizes; kept verbatim.

ANSI/ASME B18.2.1 hex cap screws — tolerance on length (inch)All dimensions in inches. Length tolerance (all minus) by nominal screw size group and nominal screw length band. The source prints "-" where no tolerance applies to that band (short lengths of the large sizes are not in the table) — reproduced verbatim. The per-size head/thread dimension ladder for this page is tabulated separately; this table adds the length tolerance the existing tables do not carry.
Nominal Screw Size	Up to 1" length	Over 1" – 2-1/2"	Over 2-1/2" – 4"	Over 4" – 6"	Longer than 6"
1/4 – 3/8	-.03	-.04	-.06	-.10	-.18
7/16 & 1/2	-.03	-.06	-.08	-.10	-.18
9/16 – 3/4	-.03	-.08	-.10	-.10	-.18
7/8 and 1	-	-.10	-.14	-.16	-.20
1-1/8 – 1-1/2	-	-.12	-.16	-.18	-.22

ANSI/ASME B18.2.1 hex bolts (inch) — body diameter, head and thread-length ladder (1/4 – 4 in)All dimensions in inches. E = body diameter (max/min); F = width across flats (basic/max/min, basic printed as hyphenated fractions); G = width across corners (max/min); H = head height (basic/max/min); R = fillet radius (max/min); LT = thread length, tabulated in two regimes (bolts ≤ 6 in long / bolts > 6 in long). The source splits its ladder into three column groups (1/4 – 1, 1 1/8 – 2, 2 1/4 – 4) with repeated header blocks; groups are merged here and repeated headers are not reproduced as data. The x58 tables on this page carry the ds/s/e/k/kw/b/r attribute ladder from a different collected source and the x76 table adds length tolerance; this chart is the only collected source for E and LT across the full size ladder.
Nominal Size	Basic Dia.	E — Body Dia. (max)	E (min)	F — Across Flats (basic)	F (max)	F (min)	G — Across Corners (max)	G (min)	H — Head Height (basic)	H (max)	H (min)	R — Fillet Radius (max)	R (min)	LT — Thread Length (≤ 6 in)	LT — Thread Length (> 6 in)
1/4	0.2500	0.260	0.237	7/16	0.438	0.425	0.505	0.484	11/64	0.188	0.150	0.03	0.01	0.750	1.000
5/16	0.3125	0.324	0.298	1/2	0.500	0.484	0.577	0.552	7/32	0.235	0.195	0.03	0.01	0.875	1.125
3/8	0.3750	0.388	0.360	9/16	0.562	0.547	0.650	0.628	1/4	0.280	0.238	0.03	0.01	1.000	1.250
7/16	0.4375	0.451	0.423	5/8	0.625	0.609	0.722	0.697	5/16	0.328	0.275	0.03	0.01	1.125	1.375
1/2	0.5000	0.514	0.485	3/4	0.750	0.734	0.866	0.836	11/32	0.370	0.320	0.06	0.02	1.250	1.500
9/16	0.5625	0.577	0.547	13/16	0.812	0.796	0.938	0.904	3/8	0.420	0.365	0.06	0.02	1.375	1.625
5/8	0.6250	0.641	0.610	15/16	0.938	0.922	1.083	1.042	7/16	0.468	0.410	0.06	0.02	1.500	1.750
3/4	0.7500	0.767	0.734	1-1/8	1.125	1.094	1.299	1.249	17/32	0.530	0.467	0.09	0.03	1.750	2.000
7/8	0.8750	0.892	0.859	1-5/16	1.312	1.281	1.516	1.454	19/32	0.592	0.528	0.09	0.03	2.000	2.250
1	1.0000	1.017	0.984	1-1/2	1.500	1.469	1.732	1.661	21/32	0.655	0.590	0.09	0.03	2.250	2.500
1 1/8	1.1250	1.149	1.098	1-11/16	1.688	1.631	1.949	1.859	3/4	0.780	0.658	0.09	0.03	2.500	2.750
1 1/4	1.2500	1.274	1.223	1-7/8	1.875	1.812	2.165	2.063	13/16	0.842	0.720	0.09	0.03	2.750	3.000
1 3/8	1.3750	1.399	1.348	2	2.000	1.937	2.381	2.255	27/32	0.905	0.783	0.09	0.03	3.000	3.250
1 1/2	1.5000	1.524	1.473	2-1/4	2.250	2.187	2.598	2.464	29/32	0.967	0.845	0.12	0.04	3.250	3.500
1 5/8	1.6250	1.649	1.598	2-7/16	2.438	2.375	2.815	2.660	31/32	1.030	0.908	0.12	0.04	3.500	3.750
1 3/4	1.7500	1.774	1.723	2-5/8	2.625	2.562	3.031	2.854	1 1/32	1.092	0.970	0.12	0.04	3.750	4.000
1 7/8	1.8750	1.899	1.848	2-13/16	2.812	2.750	3.248	3.049	1 3/32	1.155	1.033	0.12	0.04	4.000	4.250
2	2.0000	2.024	1.973	3	3.000	2.937	3.465	3.236	1 3/16	1.217	1.095	0.12	0.04	4.250	4.500
2 1/4	2.2500	2.305	2.214	3-3/8	3.375	3.262	3.897	3.719	1 1/2	1.548	1.327	0.19	0.06	4.750	5.000
2 1/2	2.5000	2.555	2.464	3-3/4	3.750	3.625	4.330	4.134	1 5/8	1.717	1.496	0.19	0.06	5.250	5.500
2 3/4	2.7500	2.806	2.715	4-1/8	4.125	4.000	4.764	4.550	1 3/4	1.830	1.609	0.19	0.06	5.750	6.000
3	3.0000	3.056	2.965	4-1/2	4.500	4.375	5.197	4.965	1 7/8	1.905	1.684	0.19	0.06	6.250	6.500
3 1/4	3.2500	3.307	3.216	4-7/8	4.875	4.750	5.630	5.380	2	2.030	1.809	0.19	0.06	6.750	7.000
3 1/2	3.5000	3.557	3.467	5-1/4	5.250	5.125	6.063	5.796	2 1/8	2.155	1.934	0.19	0.06	7.250	7.500
3 3/4	3.7500	3.808	3.718	5-5/8	5.625	5.500	6.496	6.211	2 1/4	2.280	2.059	0.22	0.08	7.750	8.000
4	4.0000	4.058	3.968	6	6.000	5.875	6.929	6.627	2 3/8	2.405	2.184	0.22	0.08	8.250	8.500

Row 1 (1/4): The source prints F basic as hyphenated fractions (1-1/8, 3-3/8, …) and splits the ladder into three groups with repeated header blocks; hyphenation is kept verbatim and duplicate headers are not reproduced as data.

Materials

Material and property class specified per RFQ — inch hardware usually names an SAE J429 grade (2, 5, 8); confirm grade, finish and thread series (UNC/UNF) at quotation

Ordering & documentation

Documents

Shipments are backed by Mill Test Certificate (MTC), Dimensional Report, Salt Spray Test Report on request, with full batch traceability from raw material to finished product. Quality management follows the owner-confirmed compliance list: ISO 9001:2015, RoHS, REACH. Exact document sets are confirmed on the order.

Quotation

The fastener family (ANSI/ASME B18.2.1 Hex Bolts (Inch Series)) plus the standard designation from your drawing — DIN / ISO / JIS / ASTM / ANSI, or a drawing for custom parts.
Size (diameter and length), thread type (metric or inch), property class or grade, and coating or finish.
Quantity per size and your target delivery date.
Destination country, and any required documents (for example a mill test certificate) so the quote includes them.

Sample & supply

Minimum order quantity is 1,000–10,000 pieces depending on size. Stock items run 3–7 days. High-strength and custom parts run 15–25 days. Quantities or variants outside the standard range are confirmed at quotation.

Storage & shipping

Parts are packed for export and shipped factory-direct to buyers in Europe, Asia, the Americas and the Middle East. Store fasteners dry and protected until installation, and keep coated parts in their packaging until use; specific packing and preservation requirements can be stated on the order and confirmed at quotation.

Frequently asked questions

Metric ISO 4014 bolt into an inch-series assembly — close enough?

No — and the head is the giveaway. The collected B18.2.1 tables run in inches with their own across-flats ladder: a 1/2″-13 hex bolt carries a 3/4″ Basic across-flats, while its nearest metric neighbour M12 uses 18 mm (0.709″) — no metric wrench correctly fits an inch head, and the mix propagates to the nuts, the washer ODs and the tapped holes. Thread series makes it worse: UNC/UNF pitches do not engage metric threads beyond a destructive half-turn. Where inch hardware is genuinely needed — US-origin equipment, replacement of existing inch bolt lines, API-pattern flanges — source B18.2.1 bolts with B18.2.2 nuts and J429 grades as a matched set, and quote the fraction size plus grade ("1/2″-13 UNC, J429 Grade 5") rather than a decimal. The collected tables also flag two reading notes: the r fillet row is irregular above 7/16″ (see footnotes) and bolts longer than 2″ tabulate the L>6 thread regime only — check the thread length against your grip before ordering.

What is the MOQ and lead time for this bolt type?

The minimum order quantity is 1,000–10,000 pieces depending on size. Stock items ship in 3–7 days; high-strength and custom parts in 15–25 days. Common sizes in this bolt type are widely stocked; confirm the exact schedule against quantity and size mix at quotation.

Related knowledge

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Get factory-direct pricing from the manufacturer.

What to include in your inquiry

✓ Standard and property class (e.g. DIN 912, ISO 4762, class 8.8) — or a drawing for custom parts.
✓ Sizes, lengths and thread type (metric or inch).
✓ Quantity per size and your target delivery date.
✓ Destination country, plus any coating or finish requirement.
✓ Any documents you need with the shipment (e.g. mill test certificate) so the quote includes them.
✓ Your inquiry goes straight to the factory team by email and gets a reply within 24 hours.

✓ Factory-direct pricing
✓ Standard and custom specifications
✓ Factory-direct export to buyers in Europe, Asia, the Americas and the Middle East
✓ Full fastener family support

ANSI/ASME B18.2.1 Hex Bolts (Inch Series)

Specifications

Materials

Ordering & documentation

Documents

Quotation

Sample & supply

Storage & shipping

Frequently asked questions

Metric ISO 4014 bolt into an inch-series assembly — close enough?

What is the MOQ and lead time for this bolt type?

Related grades & products

Related knowledge

Request a Quote

What to include in your inquiry