Hex Head Bolt Dimensions Calculator

Across flats, across corners, head height, washer face, thread length and the plain shank left under it, for ISO 4014 and ISO 4017, for the pre-1982 DIN 931 and 933 across-flats that four sizes still carry, and for ASME B18.2.1 inch hex and heavy hex. The thread length is computed from the standard’s own three-band rule rather than looked up, and the across-corners minimum from a relation checked here against 26 published values: exactly 1.13 × s min, not the geometric 1.1547.

Last updated 24 Sep 2026

Hex bolt dimensions

Size and length → every head and thread dimension

SizeThe metric sizes are ISO 4014 (partially threaded) and ISO 4017 (fully threaded), which share every head dimension. The inch sizes are ASME B18.2.1, a separate family whose across-flats follows a different rule.

Standards family for the across-flatsISO 272:1982 changed the across-flats of M10, M12, M14 and M22 and left every other size alone. A bolt marked DIN 931 may have been made to either figure, so both are shown and the difference is called out.

Nominal bolt length (mm)Thread length is a function of this, not a constant: b = 2d + 6 up to 125 mm, 2d + 12 up to 200 mm and 2d + 25 above. Enter the length in millimetres even for an inch bolt; the inch rule is applied in inches internally.

Grip you need under the head (mm)The unthreaded shank plus washers plus the parts being clamped. A partially threaded bolt is chosen so the plain shank spans the shear plane; this tells you whether it does.

Not a circuit: a hex head in PLAN on the left and the bolt in side view on the right, at two different scales. The plan view is normalised to the nominal diameter — every size is drawn the same width, so what you are looking at is the RATIO s/d rather than the size, and the ISO series' convergence on 1.5 d is visible as the hexagon settling down. At M10, M12, M14 and M22 a SECOND hexagon appears, drawn concentric with the first: that is the pre-1982 DIN 931 across-flats, and the gap between the two outlines is the spanner that does not fit. The inner circle is the washer face dw — the circle the head actually bears on, which is smaller than the flats and much smaller than the corners. The side view is NOT to the same scale as the plan: the bolt is drawn at a fixed on-screen length and the thread is gated on the ratio b/l, because a 26 mm thread on a 300 mm bolt and the same thread on a 45 mm bolt cannot share a scale with a 16 mm head. The two dimensions on it are the plain shank above and the thread below; watch the boundary jump when the length crosses 125 mm and again at 200 mm, which is the thread-length rule stepping.

16.00mmExample

M10 × 60 to ISO 4014, with 30 mm of grip needed

One size and one length in; the head, the thread and the shank out

s — from the standard’s table, and it FORKS at M10, M12, M14, M22 · e min = 1.13 × s min · b = 2d + 6 (l ≤ 125), 2d + 12 (125 < l ≤ 200), 2d + 25 (l > 200) · plain shank = l − b · ASME: F_heavy = 1.5d + 1/8 in, L_T = 2d + 1/4 in (l ≤ 6 in)

d: nominal thread diameter. Everything else is keyed to it, and two of the relations are exact multiples of it
s: width across flats — the spanner size. This is the one quantity on the page that is a pure table entry and not a formula, and it is also the one that changed
e: width across corners, MINIMUM. Exactly 1.13 s min in every ISO and DIN hexagon table checked here, against a geometric 1.1547. The 2.1% is unformed corner
k: head height, nominal. Runs 0.62 to 0.64 d across the metric series
d_w: washer-face diameter, minimum — the circle the head actually bears on. Not s, and not the head across corners: about 0.95 s min from M14 up
b: thread length. A formula with three bands and a floor: below a certain length the bolt cannot be partially threaded at all and you buy ISO 4017 instead
l − b: the plain shank. This is why anyone buys a partially threaded bolt: a shear plane wants to land on shank, not on thread

Worked example

M10 × 60 to ISO 4014, with 30 mm of grip needed
Across flats: ISO 4014 gives 16 mm. DIN 931 gives 17 mm. M10 is one of the four sizes ISO 272:1982 changed, so a 16 mm spanner and a 17 mm spanner are both correct answers to “what fits an M10?” depending on when and to what the bolt was made
Across corners, minimum = 1.13 × s min = 1.13 × 15.73 = 17.77 mm. A perfect hexagon on the same s min would give 18.16 mm, so the standard allows 0.39 mm of unfilled corner
Head height k = 6.4 mm, which is 0.640 d. Washer face dw min = 14.63 mm, which is 0.930 of s min — the head bears on a circle smaller than the flats, which is what decides whether it clears a fillet
Thread length: 60 mm is under 125, so b = 2 × 10 + 6 = 26 mm
Plain shank = 60 − 26 = 34 mm, which covers the 30 mm of grip asked for with 4 mm to spare. Had the bolt been 40 mm long the thread would still be 26 mm and the shank only 14 — and ISO 4014 is not made at 40 mm in M10 for that reason; the shortest is 45
Stretch the same bolt past 125 mm and b jumps to 2 × 10 + 12 = 32 mm; past 200 mm it jumps again to 2 × 10 + 25 = 45 mm. The thread grows with the length, which is not what most people expect

The four sizes ISO 272:1982 changed, and nothing else

This is the whole of the difference between the two families. Eurolink put it plainly: DIN 933 and ISO 4017 “are the exact same” apart from these four sizes. Read the last two columns and the reason shows: ISO pulled M10, M12 and M14 DOWN onto a ratio near 1.5 d and pushed M22 UP onto it, so the series became regular. The cost was that a 17 mm spanner no longer fits an M10 and a 19 mm no longer fits an M12 — and both sizes are still in every toolbox because pre-1982 stock, and parts still ordered to the DIN number, are still out there. The nut standards moved the same four sizes on the same date: see the hex nut dimensions calculator, where the heights moved too. The same designation can mean different dimensions in different standards families — ANSI against ISO, inch against metric, one national standard against another. The family used here is named beside every figure; check which one your part was made to.
Size	ISO 4014/4017 s (mm)	DIN 931/933 s (mm)	Change (mm)	Change (%)	ISO s ÷ d	DIN s ÷ d
M10	16	17	+1	6.3	1.600	1.700
M12	18	19	+1	5.6	1.500	1.583
M14	21	22	+1	4.8	1.500	1.571
M22	34	32	-2	5.9	1.545	1.455

Every head dimension, ISO 4014 and ISO 4017

The e column is computed, not copied, and that is the finding: the standards’ across-corners MINIMUM is exactly 1.13 × s min, rounded to two decimals, at every one of the 26 published values this batch checked — across ISO 4014, ISO 4032, ISO 4033, ISO 4035 and DIN 934, from three different catalogues. It is NOT the geometric 2/√3 = 1.1547, which reproduces none of them. The 2.1% gap is the allowance for corners the forging is not required to fill, and it is the reason a six-point socket that grips on the corners can slip on a bolt that passes inspection. The dw column is tabulated rather than computed — it settles onto 0.95 × s min from M14 up and runs lower below it. The last column matters: product grade A covers threads to M24 at lengths up to 10 d or 150 mm, grade B above, and the two grades have different s minima, so the same designation has two different e minima. Two catalogues printed M22 as e 37.72 and e 37.29 without saying which grade; the 1.13 rule identifies them as grade A and grade B respectively. These dimensions come from a published standard’s table, not from a formula. The standard itself is cited below and the printed values are attributed to the catalogue they were taken from; a different publisher may round differently in the last digit.
Size	s nom (mm)	s min (mm)	e min (mm)	e if the corners were perfect (mm)	k nom (mm)	dw min (mm)	dw ÷ s min	Grade of the printed minimum
M5	8	7.78	8.79	8.98	3.50	7.20	0.925	A
M6	10	9.78	11.05	11.29	4.00	8.88	0.908	A
M8	13	12.73	14.38	14.70	5.30	11.63	0.914	A
M10	16	15.73	17.77	18.16	6.40	14.63	0.930	A
M12	18	17.73	20.03	20.47	7.50	16.63	0.938	A
M14	21	20.67	23.36	23.87	8.80	19.64	0.950	A
M16	24	23.67	26.75	27.33	10.00	22.49	0.950	A
M18	27	26.16	29.56	30.21	11.50	24.85	0.950	B
M20	30	29.67	33.53	34.26	12.50	28.19	0.950	A
M22	34	33.38	37.72	38.54	14.00	31.71	0.950	A
M24	36	35.38	39.98	40.85	15.00	33.61	0.950	A
M27	41	40.00	45.20	46.19	17.00	38.00	0.950	B
M30	46	45.00	50.85	51.96	18.65	42.75	0.950	B
M33	50	49.00	55.37	56.58	21.00	46.55	0.950	B
M36	55	53.80	60.79	62.12	22.50	51.11	0.950	B

Thread length is a formula. Here it is, evaluated

b = 2d + 6, 2d + 12, 2d + 25. Fuller Fasteners print the b column for every size from M1.6 to M24 and the rule reproduces all of it — with exactly ONE exception, M1.6, where the printed 9 / 15 / 28 are 0.2 mm below the formula’s 9.2 / 15.2 / 28.2 because the standard rounded down. The 125 mm and 200 mm boundaries were not taken on trust either: Engineers Edge print the length range each size is made in and how many distinct b values it has, and the count follows from the maximum length in every case — M12 reaches only 120 mm so it has one b; M16 reaches 160 so it has two; M24 reaches 240 so it has three. The last two columns are why the rule has a floor: an M10 is not made shorter than 45 mm as a partially threaded bolt, because 26 mm of that is thread and what is left has to be worth having. Below the floor you buy ISO 4017, fully threaded. These dimensions come from a published standard’s table, not from a formula. The standard itself is cited below and the printed values are attributed to the catalogue they were taken from; a different publisher may round differently in the last digit.
Size	Pitch (mm)	b for l ≤ 125 mm	b for 125 < l ≤ 200	b for l > 200	Shortest ISO 4014 length made	Plain shank at that length
M5	0.80	16	22	35	25	9
M6	1.00	18	24	37	30	12
M8	1.25	22	28	41	40	18
M10	1.50	26	32	45	45	19
M12	1.75	30	36	49	50	20
M14	2.00	34	40	53	—	—
M16	2.00	38	44	57	65	27
M18	2.50	42	48	61	—	—
M20	2.50	46	52	65	80	34
M22	2.50	50	56	69	—	—
M24	3.00	54	60	73	90	36
M27	3.00	60	66	79	—	—
M30	3.50	66	72	85	110	44
M33	3.50	72	78	91	—	—
M36	4.00	78	84	97	140	62

ASME B18.2.1 inch hex and heavy hex — and the rule that generates them

Two exact rules fall out of this table. HEAVY hex across flats is 1.5 d + 1/8 in, at every published size from 3/8 in to 3 in, with no exception whatever. REGULAR hex is 1.5 d at 3/8 in and at every size from 1/2 in up — and the three small sizes are exceptions: 1/4 in is 7/16 in where the rule says 3/8, 5/16 in is 1/2 in where it says 15/32, and 7/16 in is 5/8 in where it says 21/32, which is 1/32 in under. Those are the sizes where a spanner set stops following the pattern. Note also what ASME publishes: across corners as a MAXIMUM, and it is exactly the perfect hexagon, 1.1547 × F basic, to three decimals at every size. ISO publishes across corners as a MINIMUM at 1.13 × s min. Same quantity, opposite end of the tolerance, different number — which is why you cannot read one family’s table with the other family’s habits. The same designation can mean different dimensions in different standards families — ANSI against ISO, inch against metric, one national standard against another. The family used here is named beside every figure; check which one your part was made to.
Size	Hex F basic (in)	F ÷ d	Heavy hex F basic (in)	Heavy F ÷ d	Hex head height H (in)	Hex F (mm)	Heavy hex F (mm)
1/4 in	0.4375	1.7500	—	—	0.1719	11.11	—
5/16 in	0.5000	1.6000	—	—	0.2188	12.70	—
3/8 in	0.5625	1.5000	0.6875	1.8333	0.2500	14.29	17.46
7/16 in	0.6250	1.4286	—	—	0.2969	15.88	—
1/2 in	0.7500	1.5000	0.8750	1.7500	0.3438	19.05	22.22
5/8 in	0.9375	1.5000	1.0625	1.7000	0.4219	23.81	26.99
3/4 in	1.1250	1.5000	1.2500	1.6667	0.5000	28.57	31.75
7/8 in	1.3125	1.5000	1.4375	1.6429	0.5781	33.34	36.51
1 in	1.5000	1.5000	1.6250	1.6250	0.6719	38.10	41.27
1-1/8 in	1.6875	1.5000	1.8125	1.6111	0.7500	42.86	46.04
1-1/4 in	1.8750	1.5000	2.0000	1.6000	0.8438	47.63	50.80
1-1/2 in	2.2500	1.5000	2.3750	1.5833	1.0000	57.15	60.32

What this page does not do, and where it lives instead

The line this project draws: the converters plugin owns the size table and the drill; mechanical owns the feature on the part. A tap drill diameter is a conversion. The depth and diameter of the counterbore that swallows a cap screw head is a machining feature, and that is a mechanical page.
What	Why not here	Where
Tap drill and clearance-hole drill sizes	A drill is a tool size, not a feature on the part	The converters plugin owns the drill and the size tables: tap drill, clearance drill, metric, fractional, number and letter series. This page computes the bearing area under the head over a clearance hole, which is a different question.
Pitch, TPI and the coarse and fine series	Same reason — a thread designation is a conversion	Also converters. The pitch column above is printed only because the thread-length rule needs the nominal diameter beside it.
Tightening torque, preload and proof load	This is a dimensions page; those are strength pages	The bolted-joint cluster. Torque from preload, proof load and stress area, thread engagement length, joint stiffness and separation, fatigue under alternating load, bolt group shear, and how accurate each tightening method actually is.
Socket head, flat head, button head and shoulder screws	Each already has its own page on this site	Those pages exist and are not duplicated here. What IS here that they do not carry is the recess: see the counterbore and countersink calculator.
Hex key and hex socket driver sizes	There is a dedicated page for it already	Not built here, deliberately. The socket size on a set screw is left out of the set screw dimensions calculator for the same reason.

The four sizes that changed, the rule that generates the thread, and the dimension nobody checks

The across-flats of a hex bolt is the one dimension on it that changed, and it changed for four sizes only. ISO 272:1982 took M10 from 17 mm to 16, M12 from 19 to 18, M14 from 22 to 21, and pushed M22 the other way from 32 to 34. Every other size from M1.6 to M64 was left alone. Read the ratio column in the table above and the reason is plain: the ISO series was pulled onto about 1.5 d, and 17, 19, 22 and 32 were the four values that did not sit on that line. The consequence is a toolbox problem that has outlived the standard by four decades. A 17 mm spanner does not fit an ISO 4014 M10 and a 16 mm does not fit a DIN 931 M10, and both bolts are on sale, because the DIN numbers stayed in catalogues long after DIN 931 and DIN 933 were replaced by DIN EN ISO 4014 and 4017. Eurolink, who sell both, put the difference at exactly those four sizes and nothing else.

Across corners is not the geometric figure, and this batch had to work out what it is. A perfect hexagon of width s across the flats is 2/√3 = 1.1547 s across the corners. No standard prints that. What they print is a MINIMUM e, and across 26 published values — ISO 4014 from M1.6 to M36, ISO 4032, ISO 4033, ISO 4035 and DIN 934, from three separate catalogues — that minimum is exactly 1.13 × s min, rounded to two decimals, every single time. The 2.1% shortfall is the corner the forging is not required to fill. It matters in one specific way: a six-point socket that grips on the corners rather than the flats is relying on metal the standard does not guarantee. The rule also let this page recover M14, which one catalogue omits: 1.13 × 20.67 = 23.36, and when a second catalogue’s M14 row turned up it printed 23.36.

And the same rule identified a product-grade fork that neither publisher labelled. ISO 4014 has two product grades — A for threads up to M24 at lengths up to 10 d or 150 mm, B above — and they have different across-flats minima, so the same designation has two different across-corners minima. Two catalogues print M22 as e 37.72 and e 37.29. Divide each by 1.13 and you get s min 33.38 and 33.00: grade A and grade B. Neither page says so. If you are sizing a socket or checking an inspection report, that is a 0.43 mm difference nobody told you about.

Thread length is a formula with three bands and a floor. b = 2d + 6 up to 125 mm, 2d + 12 up to 200, 2d + 25 above — and that reproduces every printed b value from M2 to M24, with one exception worth naming: M1.6, where the standard prints 9, 15 and 28 against the formula’s 9.2, 15.2 and 28.2, having rounded down. The band boundaries were checked without using them: a separate publisher prints the length range each size is made in, and the number of distinct b values it lists follows from the maximum length in every case. M12 tops out at 120 mm so it has one b. M16 reaches 160 so it has two. M24 reaches 240 so it has three. The floor is the other half of the rule. An M10’s thread is 26 mm whatever the bolt’s length, so a 30 mm M10 would be almost all thread — and ISO 4014 M10 is not made below 45 mm. Below the floor you buy ISO 4017, threaded to the head, and the question of shank versus thread in the shear plane stops being yours to choose.

The washer face is the dimension people forget, and it is the one that decides fit. A hex head does not bear on its flats or its corners; it bears on a circle of diameter dw, which is about 0.95 of s min from M14 up and less below. On an M10 that is 14.63 mm against a 16 mm spanner size and a 17.77 mm across-corners. When the question is whether the head clears a fillet, a counterbore wall or an adjacent boss, dw is the number. When the question is whether the member can take the pressure the head puts on it, the bearing area over the clearance hole is computed above — and if it cannot, that is what a washer is for. The washer dimensions calculator gives the areas and the contact pressure, the hex nut dimensions calculator does the other end of the bolt, and the counterbore and countersink calculator does the recess if the head has to go below the surface. For locating rather than clamping, the dowel pin and hole calculator is the right page — a bolt in a clearance hole locates nothing.

Frequently asked questions

Is an M10 bolt 16 mm or 17 mm across the flats?

Both, depending on which standard it was made to. ISO 4014 and ISO 4017 say 16 mm. DIN 931 and DIN 933, before ISO 272:1982 harmonised them, said 17 mm. The change affected exactly four sizes — M10, M12, M14 and M22 — and Wikipedia cites ISO 272 directly for it: the widths “were changed from 17, 19, 22 and 32 mm respectively to the current standard”. Since the DIN numbers are still used commercially for parts that are actually made to the ISO figures, the number on the invoice does not settle it. Measure the flats.

Why did ISO change those four and no others?

Look at the across-flats divided by the nominal diameter, which this page computes in the table and plots in the chart. The ISO series settles on about 1.5 d: M12 at 18 is exactly 1.5, M16 at 24 is exactly 1.5, M20 at 30 is exactly 1.5, M24 at 36 is exactly 1.5. The pre-1982 figures for M10, M12 and M14 sat ABOVE that line and M22’s 32 sat below it. Moving four values made the series regular, which is worth something when you are specifying a spanner set rather than a single bolt. EKINSUN’s wrench chart gives the same reading: “M22 went up because ISO tidied the series to roughly 1.5 × d plus a step, and 32 mm sat under that line.”

How long is the thread on a hex bolt?

b = 2d + 6 mm for bolts up to 125 mm long, 2d + 12 for 125 to 200 mm, and 2d + 25 above 200. So an M10 has 26 mm of thread at any length up to 125 mm, 32 mm from 125 to 200, and 45 mm above. The thread does not grow in proportion to the bolt; it steps. For inch bolts ASME B18.2.1 says the same thing differently: 2d + 1/4 in up to 6 in long and 2d + 1/2 in above. This page evaluates whichever applies and subtracts it from the length to give the plain shank, which is the number you actually need.

What is the shortest partially threaded bolt I can get?

It depends on the size, and the reason is arithmetic. The thread length is fixed by the diameter, so as the bolt gets shorter the plain shank vanishes. An M10 with 26 mm of thread has nothing left below about 35 mm, and in practice ISO 4014 M10 is made from 45 mm up. The table on this page gives the shortest length published for each size and what plain shank it leaves. Below that, the answer is ISO 4017 or DIN 933, threaded to the head. That is not a compromise — a fully threaded bolt is the right part for a short grip — but it does mean any shear plane lands on thread.

What is heavy hex for?

Bearing area and spanner access, not strength. ASME B18.2.1’s heavy hex head is 1.5 d + 1/8 in across the flats — exactly, at every published size from 3/8 in to 3 in, which is one of the two clean rules in the inch series — against 1.5 d for a regular hex. The bolt underneath is the same diameter and the same material, so it is no stronger in tension. What it gives you is a wider face under the head, which spreads the contact pressure on the connected material, and a bigger hex to get a socket on. Structural steelwork uses heavy hex for those reasons.

Is the across-corners dimension the same as the socket size?

No, and this is worth being careful about. ISO publishes e as a MINIMUM — the smallest across-corners a conforming bolt may have — and it is 1.13 × s min, about 2.1% under the perfect hexagon. ASME publishes G as a MAXIMUM and it is exactly the perfect hexagon, 1.1547 × F basic. Same physical quantity, opposite ends of the tolerance band, different numbers. A six-point socket sized on the geometric corner-to-corner figure is relying on metal the ISO standard does not require to be there. Size on the flats.

Does this page cover the clearance hole and the tap drill?

No, deliberately. Drill sizes and thread-designation conversions live in the converters plugin — tap drill and clearance drill in metric, fractional, number and letter series, and pitch to TPI with the coarse and fine series. This page owns the part: the head, the thread length, the shank and the bearing face. Where it needs a clearance hole — for the bearing area under the head — it uses ISO 273’s medium class and says so. The recess that takes a head below the surface is also a feature on the part, and it is on the counterbore and countersink calculator.

Related calculators

References

ISO 4014:2022, Fasteners — Hexagon head bolts — Product grades A and B, and ISO 4017 for the fully threaded form. Cited by number; the standard is copyrighted and was not fetched. Product grade A applies to threads up to M24 and lengths up to 10 d or 150 mm, whichever is shorter, and grade B above — which is why two catalogues can print different across-corners minima for the same designation and both be right.
ISO 272:1982, Fasteners — Hexagon products — Widths across flats. The document that changed four sizes. Wikipedia’s Width across flats article cites it directly: “with ISO 272 1982 the width across flats for M10, M12, M14 and M22 were changed from 17, 19, 22 and 32 mm respectively to the current standard”. Confirmed independently by EKINSUN’s wrench-size chart (M10 17→16, M12 19→18, M14 22→21, M22 32→34) and by the DIN 934 nut tables, which still print 17 and 19.
Fuller Fasteners. ISO 4014 Specifications — Hex Head Bolts. Source for the across-flats, across-corners, head height, washer-face diameter and the three thread-length columns from M1.6 to M24 used here. Its b column is what the 2d + 6 / 2d + 12 / 2d + 25 rule was checked against, size by size.
Torqbolt. ISO 4014 Hex Bolts — Dimensions, Standards, Specifications. Source for M14 (s 21, e 23.36, k 8.8, dw 19.64) and for the grade A figures at M22 (e 37.72, dw 31.71). Its M14 row reproduces, to the last digit, the values this batch had already derived from the 1.13 × s_min and 0.95 × s_min rules before finding it — which is the cross-check that made those rules usable.
BigBoltNut. ISO 4014 Bolt Dimension. Source for M18, M27, M30, M33 and M36, and for the grade B figures at M22 (e 37.29, dw 31.35) that differ from Torqbolt’s grade A ones. Neither publisher says which product grade its column is; the 1.13 rule identifies them.
Engineers Edge. ISO Hex Head Screw per ISO 4014, DIN 931 and ISO 4016. Used only for the nominal LENGTH RANGE each size is made in, and for how many distinct b values it prints per size — which independently pins the 125 mm and 200 mm boundaries of the thread-length rule, because the number of bands a size can reach follows from its maximum length.
Eurolink Fastener Supply Service. Comparing DIN 933 & DIN 931 to ISO 4017 & ISO 4014. States that the two families are otherwise identical and differ on width across flats at M10, M12, M14 and M22 — the same four sizes ISO 272 names.
Würth Industry. Fasteners: Differences between DIN – EN – ISO standards. The single most useful document this batch read. It tabulates DIN 934 against ISO 4032 at the four changed sizes, states that “nuts according to ISO 4032, ISO 4033, ISO 4034 … may not be replaced by nuts according to DIN standards which only have got a reduced loadability according to DIN 267 Part 4 … such as DIN 934”, that “it is absolutely necessary to use at least hardness class 200 HV for high-strength screws and bolts of property class 8.8 and higher”, and that DIN 127, 128, 137, 6797, 6798 and 7980 were “withdrawn without replacement, as there is no functional guarantee in combination with high-strength screws and bolts”.
ASME B18.2.1-2010, Square, Hex, Heavy Hex and Askew Head Bolts and Hex, Heavy Hex, Hex Flange, Lobed Head and Lag Screws (Inch Series). Cited by table. The dimension columns used here were read from a published copy of Table 2 and from pipingpipeline’s transcription of the heavy hex table; the thread-length rule it states — “nominal thread length equals twice the basic thread diameter + 0.25 in. for nominal bolt lengths up to and including 6 in.” — reproduces every printed value.
ISO 273, Fasteners — Clearance holes for bolts and screws. Cited by number; the fine / medium / coarse columns used here are taken from EKINSUN’s What Size Clearance Hole for a Metric Bolt? ISO 273 Chart, which labels them H12, H13 and H14.