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Reference tables · ISO 4014/4032 & ASME B18.2.1/B18.2.2 · Bolt Calculator

Hex Bolt & Nut Dimensions Chart — Wrench Size, Head & Nut Tables

Across flats, head height, nut thickness, thread length, heavy hex, and the SAE↔metric wrench conversion — the physical dimensions that decide which wrench fits and whether the hardware you ordered is actually the hardware the drawing asked for. Metric M5–M30 per ISO 4014/4032, imperial 1/4–1 inch per ASME B18.2.1/B18.2.2.

Metric

Metric Hex Head & Nut Dimensions (ISO 4014 / 4032)

Head and nut dimensions for the common metric hex series. Across flats (s) is the wrench size for both the head and its nut — ISO deliberately keeps them matched. Thread length (b) is the minimum usable thread for bolts up to 125 mm long; longer bolts add 2× pitch per extra 25 mm of grip.

SizeAcross Flats s (mm)Head Height k (mm)Nut Thickness m (mm)Thread Length (min)
M583.55.216
M61045.218
M8135.36.822
M10166.48.426
M12187.510.830
M16241014.838
M203012.51846
M24361521.554
M304618.725.666

Values are ISO 4014 head dimensions (k) with ISO 4032 style-1 nut thickness (m), maximums rounded to table precision. Across-corners runs ≈ 1.15 × AF if you need the diagonal for a socket check.

Imperial

Imperial Hex & Heavy Hex Dimensions (ASME B18.2.1 / B18.2.2)

Standard hex cap screws against heavy hex nuts and heads. Below 1/2 inch the two series share a wrench; from 1/2 inch up, heavy hex runs one wrench size larger — the physical signature of structural-grade hardware.

SizeHex AF (in)Heavy Hex AF (in)Head H (in)Nut Thick (in)
1/4″7/167/16(= hex)11/647/32
1/2″3/47/811/3231/64
3/4″1-1/81-1/433/6447/64
1″1-1/21-5/845/6455/64

Hex AF and head height per ASME B18.2.1 hex cap screws; heavy hex AF per B18.2.2 heavy hex nuts; nut thickness is the standard hex nut (maximum). Heavy hex nuts run thicker still — 63/64″ at 1″ — trading wrench size for thread engagement.

Cross-System

SAE ↔ Metric Size Conversion — and Where the Wrenches Differ

Nominal diameters pair up neatly between systems; across-flats almost never does. This table pairs each metric size with its nearest SAE cousin and prices the difference in wrench reality, so mixed-stock substitution is a decision instead of a surprise.

M6↔ 1/4″M6 AF 10 mm vs 1/4″ AF 9.525 mm — 0.475 mm apart; an SAE 7/16 wrench is loose on an M6 nut.
M8↔ 5/16″13 mm vs 12.7 mm — 0.3 mm apart; a 5/16 wrench fits an M8 nut snugly but the wrench marks corners.
M10↔ 3/8″16 mm vs 15.875 mm — only 0.125 mm apart, the closest pair in the table and the classic mixed-stock trap.
M12↔ 7/16″18 mm vs 17.463 mm — 0.54 mm apart; a 7/16 wrench will start an M12 nut but rounds it under load.
M16↔ 5/8″24 mm vs 23.813 mm — 0.188 mm apart; fits by feel, slips under torque.
M20↔ 3/4″30 mm vs 28.575 mm — 1.4 mm apart; visibly wrong wrench, will not seat.
M24↔ 7/8″36 mm vs 31.75 mm — 4.25 mm apart; different tool class entirely.
M30↔ 1-1/8″46 mm vs 34.93 mm — 11 mm apart; only the nominal thread diameter is 'close'.

⚠️ The M10 / 3/8″ trap

Sixteen millimeters and 15.875 mm are 0.125 mm apart — inside the slop of a worn wrench. Mixing metric and imperial stock in one assembly works at the wrench but fails at the thread: M10×1.5 will not engage 3/8-16 without galling both. When stock gets mixed, sort by thread pitch and AF, never by “it fits the wrench”.

Shop Practice

How to Measure a Nut — and Read the Right Dimension

One measurement decides the wrench, one decides the identity, and confusing them is how parts get re-ordered twice. Here is the sequence a counter person or purchasing agent should run on unidentified hardware:

  1. 1Across flats (AF): caliper two parallel faces. This is the wrench size and the chart’s master dimension. “Spanner size = AF size” — a 16 mm reading means a 16 mm wrench, no conversion.
  2. 2Nut thickness: caliper top face to bearing face. Compare against the m column — a nut noticeably thicker than standard signals a heavy-series or DIN 934 vs ISO 4032 mix.
  3. 3Pitch: thread gauge, or count threads over 10 mm (metric) / over one inch (imperial). Pitch separates the true identity: M10×1.5 and 3/8-16 both wear a 16 mm-class wrench and only the pitch tells them apart.
  4. 4Head marks: radial lines count the SAE grade, stamped characters give metric class or ASTM designation. The marks are the spec — a chart cannot read them for you.

The reverse question — will this wrench turn that nut — has a hard answer: the wrench must match AF within about 0.15 mm to grip without rounding. The famous “close” pairs (16 mm ↔ 5/8″, 13 mm ↔ 1/2″-adjacent) sit right at that limit, which is why they work on light tasks and fail under torque.

Wrenches and hex fasteners arranged on a workbenchAcross flats is the master dimensionPhoto: Unsplash

AF ↔ wrench quick list

  • M6 → 10 mm · M8 → 13 mm · M10 → 16 mm
  • M12 → 18 mm · M16 → 24 mm · M20 → 30 mm
  • 1/4″ → 7/16″ · 1/2″ → 3/4″ · 3/4″ → 1-1/8″
  • 1″ hex → 1-1/2″ · 1″ heavy hex → 1-5/8″
Grades

Heavy Hex vs Standard Hex — and the Grades That Demand It

Heavy hex is not a style — it is the hardware form that structural and high-preload specs require, because the larger AF spreads clamp load over more bearing area and the thicker nut gives full thread engagement at high strength levels. The grades that pair with it:

SAE Grade 2 (no marks)

≈ 74 ksi tensile

Hardware-store general repair. Never in structural or load-rated assemblies — it is the bolt most torque charts warn about.

SAE Grade 5 (3 radial marks)

120 ksi tensile / 92 ksi yield

Automotive and machinery default. Pairs with the standard hex series and the middle torque column of most charts.

SAE Grade 8 (6 marks)

150 ksi tensile / 130 ksi yield

High-load, high-fatigue joints: suspension, implement linkages. Costs more, and it is brittle-prone in hydrogen-rich environments — follow the spec.

ASTM A307 (2 hex head marks? none — grade letter)

60 ksi tensile (Grade A)

The utility anchor of structural steel: low-strength carbon steel for non-structural connections, anchor bolts, and flanged fittings.

ASTM A325 (3 marks: A325)

105–120 ksi tensile by diameter

Structural hex or heavy hex with heavy hex nuts — the 'Grade 5 of building steel'. Head marking reads A325L for low-carbon martensitic.

Class 8.8 / 10.9 metric (no marks — grade number on head)

800 MPa / 1000–1200 MPa tensile

The metric world's Grade 5 / Grade 8 equivalents. The number on the head IS the spec: 8.8 = 800 MPa tensile, 0.8 × that = yield.

Heavy hex is right when

  • The joint is A325/A490 structural or F3125-classified
  • Preload exceeds ~70% of yield on a hardened washer
  • The spec says F1554 anchor rods with heavy hex nuts

Standard hex is right when

  • General machinery, automotive, and fixtures at Grade 5 / 8.8
  • Space around the joint limits wrench swing
  • The drawing shows a hex, not heavy hex, symbol

DIN 934 vs ISO 4032

Older DIN stock runs 19 mm AF at M12 where current ISO uses 18 mm, and 30 mm at M20 where ISO uses 30 (match) but M16 DIN is 24 (match) — the exceptions are exactly where mixed bins bite. When a “standard” nut needs an off-list wrench, check whether it is legacy DIN before assuming it is wrong.

Socket vs open-end reach

Six-point sockets grip by the flats and can take full torque; twelve-point and open-end wrenches grip the corners and round them first. On anything near the table’s torque values — see the torque spec chart — a six-point socket is not a preference, it is what keeps the hexagon a hexagon.

FAQ

Hex Bolt & Nut Dimensions FAQ

Size lookups and the judgment calls around them — answered from ISO and ASME head and nut standards.

How do you calculate the wrench (spanner) size for a bolt?
You do not calculate it from the diameter — you look up the across-flats dimension of the head or nut standard. Spanner size = AF size, full stop: an M10 hex nut needs a 16 mm wrench because ISO 4032 says its across-flats is 16 mm, not because of any formula. The 'bolt head size calculation' people search for is really a lookup of the governing standard (ISO 4014/4032 or ASME B18.2.1/B18.2.2), and the tables above are those lookups.
What are the M8 nut dimensions?
An ISO 4032 M8 hex nut: 13 mm across flats, 6.8 mm thick, 12.9–13 mm across corners, with a 13 mm wrench. The mating M8 hex head (ISO 4014) shares the same 13 mm AF and stands 5.3 mm tall — head and nut are deliberately wrench-compatible.
What are the M10 and M12 nut dimensions?
M10: 16 mm AF, 8.4 mm nut thickness, 16 mm wrench. M12: 18 mm AF, 10.8 mm thick, 18 mm wrench. Note the M12 uses 18 mm AF rather than a '13-series' progression — the 19 mm of older DIN standards and the 18 mm of current ISO are a real procurement trap on mixed stock.
What are the M16 nut dimensions?
M16 ISO 4032: 24 mm across flats, 14.8 mm thick, 24 mm wrench. In heavy-series form (ISO 4033) the nut grows to 16.4 mm thick — same wrench, more thread engagement for higher-preload 10.9 and 12.9 bolts.
What are the 1/4-20 and 5/8 nut dimensions?
A 1/4-20 hex nut (ASME B18.2.2): 7/16 in AF, 7/32 in thick. A 5/8-11 hex nut: 15/16 in AF, 35/64 in thick; the 5/8 heavy hex nut steps up to 1-1/16 in AF and 41/64 in thick. Structural work uses heavy hex almost exclusively from 1/2 in upward.
How do you measure a nut correctly?
Measure across the flats, not corner to corner. Caliper two parallel faces of the hexagon and read that number — that is the AF, and it is the number every wrench and every chart uses. Across-corners (the diagonal) reads 15% larger and matches nothing. If the nut is painted or corroded, clean two flats first; 1 mm of rust halves the accuracy of the answer. Threads and pitch tell you the size class; AF tells you the wrench — measure both and you know everything.
What is the difference between hex nut and heavy hex nut sizes?
Heavy hex nuts are larger across flats and thicker than standard hex at the same thread size — 7/8 vs 3/4 in AF at 1/2, 1-5/8 vs 1-1/2 at 1 inch. The bigger body gives more thread shear area and a bearing surface that spreads structural preload; that is why A325/A490 structural assemblies are always specified heavy hex. Metric mirrors this with ISO 4032 (standard) vs ISO 4033 (heavy) — same AF, thicker nut — plus the separate large-series wrench sizes on some DIN standards.
Grade 5 vs Grade 8 — which hex bolt do I need?
Grade 8 is ~25% stronger in tension and shear than Grade 5 (150 vs 120 ksi tensile) and holds preload better at high temperature, but it is harder and less ductile — it snaps rather than bends when overloaded and is more sensitive to hydrogen embrittlement. Grade 5 is the correct choice where some ductility and price matter; Grade 8 where fatigue and clamp retention matter. Never substitute either into a joint spec'd for the other without an engineer's sign-off — the head marks (3 vs 6 radial lines) exist precisely so substitution gets caught.
A307 vs A325 — when does the structural grade matter?
A307 is the 60 ksi general-purpose bolt for anchor bolts, utility work, and connections where strength is not the design limit; A325 is the 105–120 ksi structural bolt rated for slip-critical and bearing-type steel connections. They share heavy hex geometry on structural jobs, which makes them visually confusable except for the head marking — A325 always carries its designation on the head. Torque values differ by roughly the strength ratio: an A325 3/4-10 takes about 250–290 ft·lb while the same size A307 should stay near 150 ft·lb.
Is there a metric-to-SAE wrench conversion that just works?
Only approximately, and only in the tight pairs: an M10 nut (16 mm AF) can be turned with a 5/8 wrench (15.875 mm) in a pinch, and a 3/8 bolt (15.875 mm AF) accepts a 16 mm wrench. Everything else is 0.3 mm or more apart, which is enough to round hex corners under real torque. The conversion table above exists so the 'close enough' pairs are chosen deliberately, not discovered by damaged fasteners.
How do I estimate the surface area of a hex bolt for coating?
Treat the hex as a circumscribed cylinder for a fast upper bound: shank area π·d·L plus head area ≈ 1.2 × π·AF_equivalent × k. An M10 × 40 bolt works out to about 18 cm² of shank plus 6–7 cm² of head and threads — call it 25 cm² per bolt for galvanizing or plating weight estimates, and multiply by fastener count for the batch. Coating suppliers do the exact math from the thread profile; the cylinder estimate lands within 10%, which is fine for quoting.

Dimensions Are the Static Half — Dynamics Need a Calculator

This hex bolt dimensions chart fixes what the hardware is — head size, wrench, nut thickness, thread length. What the hardware does under your wrench is a different calculation: the free bolt torque calculator on the homepage turns these same diameters and grades into tightening torque and preload, and the bolt calculators hub extends the same treatment to length, shear, weight, and bolt circles. Match the wrench here, set the torque there.

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