Engineering Tools · Hex Bolt Dimension Reference
Bolt Size Chart: Metric Hex Head Bolt Dimensions M1.6–M64 (GB/T 5783 / 5782)
Hex head dimensions of metric hex bolts from M1.6 to M64: width across flats s, width across corners e min, head height k and fillet radius r min, with the thread length b for partially threaded bolts. Values are taken from GB/T 5783-2025 (fully threaded) and GB/T 5782-2025 (partially threaded) for product grade A and grade B, plus the fine pitch options of GB/T 5785-2025. Every row is labelled with the standard it comes from.
M12 hex bolt: width across flats s = 18 mm, which is the socket or spanner size; width across corners e min = 20.03 mm in grade A and 19.85 mm in grade B; head height k = 7.5 mm nominal, 7.79 mm max in grade B; coarse pitch P = 1.75 mm; fillet radius r min = 0.6 mm. Thread length on a partially threaded GB/T 5782 bolt, grade B: b = 30 mm at l ≤ 125 mm, 36 mm at 125 < l ≤ 200 mm and 49 mm at l > 200 mm.
Head figures are the standard's nominal (= max) or minimum values, so a delivered bolt measures at those bounds or inside them. s is what you buy a socket by, e is the diagonal a 12-point socket has to clear, and k is the head height. Small and large sizes follow the same three columns, up to s = 95 mm at M64.
Next: work out your order, or check how we supply this standard
Other sizes
Full PDF
Download the full PDF (M1.6–M64, all sizes)
Printable chart covering every size in this family — click to download the PDF directly.
Metric bolt size chart M1.6–M64 (mm)
The two standards share the same hex head: GB/T 5783 (fully threaded) and GB/T 5782 (partially threaded) give identical s, e, k and r for a size, so only the thread length differs. Switch the thread form to add the b columns, and the product grade to see the A-grade or B-grade tolerance band.
Thread form
Product grade
Partially threaded (GB/T 5782) · GB/T 5782-2025 · Grade B
Grade A and grade B are dimensional tolerance grades, not quality grades: the nominal s and k are the same, only the tolerance band differs (s min, e min, k max). Grade A applies to d ≤ M24 with l ≤ 10d or l ≤ 150 mm; grade B covers every size in this table.
| Size | Coarse pitch P (mm) | Width across flats s (mm, nominal = max) | s min (mm) | Width across corners e min (mm) | Head height k (mm, nominal) | Fillet radius r min (mm) | b at l ≤ 125 mm | b at 125 < l ≤ 200 mm | b at l > 200 mm |
|---|---|---|---|---|---|---|---|---|---|
| M1.6 | 0.35 | 3.2 | 2.9 | 3.28 | 1.1 | 0.1 | 9 | 15 | 28 |
| M2 | 0.4 | 4 | 3.7 | 4.18 | 1.4 | 0.1 | 10 | 16 | 29 |
| M2.5 | 0.45 | 5 | 4.7 | 5.31 | 1.7 | 0.1 | 11 | 17 | 30 |
| M3 | 0.5 | 5.5 | 5.2 | 5.88 | 2 | 0.1 | 12 | 18 | 31 |
| M3.5 | 0.6 | 6 | 5.7 | 6.44 | 2.4 | 0.1 | 13 | 19 | 32 |
| M4 | 0.7 | 7 | 6.64 | 7.5 | 2.8 | 0.2 | 14 | 20 | 33 |
| M5 | 0.8 | 8 | 7.64 | 8.63 | 3.5 | 0.2 | 16 | 22 | 35 |
| M6 | 1 | 10 | 9.64 | 10.89 | 4 | 0.25 | 18 | 24 | 37 |
| M7 | 1 | 11 | 10.57 | 11.94 | 4.8 | 0.25 | 20 | 26 | 39 |
| M8 | 1.25 | 13 | 12.57 | 14.2 | 5.3 | 0.4 | 22 | 28 | 41 |
| M10 | 1.5 | 16 | 15.57 | 17.59 | 6.4 | 0.4 | 26 | 32 | 45 |
| M12 | 1.75 | 18 | 17.57 | 19.85 | 7.5 | 0.6 | 30 | 36 | 49 |
| M14 | 2 | 21 | 20.16 | 22.78 | 8.8 | 0.6 | 34 | 40 | 53 |
| M16 | 2 | 24 | 23.16 | 26.17 | 10 | 0.6 | — | 44 | — |
| M18 | 2.5 | 27 | 26.16 | 29.56 | 11.5 | 0.6 | — | 48 | — |
| M20 | 2.5 | 30 | 29.16 | 32.95 | 12.5 | 0.8 | — | 52 | — |
| M22 | 2.5 | 34 | 33 | 37.29 | 14 | 0.8 | — | 56 | 69 |
| M24 | 3 | 36 | 35 | 39.55 | 15 | 0.8 | — | 60 | 73 |
| M27 | 3 | 41 | 40 | 45.2 | 17 | 1 | 60 | 66 | 79 |
| M30 | 3.5 | 46 | 45 | 50.85 | 18.7 | 1 | 66 | 72 | 85 |
| M33 | 3.5 | 50 | 49 | 55.37 | 21 | 1 | 72 | 78 | 91 |
| M36 | 4 | 55 | 53.8 | 60.79 | 22.5 | 1 | — | 84 | 97 |
| M39 | 4 | 60 | 58.8 | 66.44 | 25 | 1 | — | 90 | 103 |
| M42 | 4.5 | 65 | 63.1 | 71.3 | 26 | 1.2 | — | 96 | 109 |
| M45 | 4.5 | 70 | 68.1 | 76.95 | 28 | 1.2 | — | 102 | 115 |
| M48 | 5 | 75 | 73.1 | 82.6 | 30 | 1.6 | — | 108 | 121 |
| M52 | 5 | 80 | 78.1 | 88.25 | 33 | 1.6 | — | 116 | 129 |
| M56 | 5.5 | 85 | 82.8 | 93.56 | 35 | 2 | — | — | 137 |
| M60 | 5.5 | 90 | 87.8 | 99.21 | 38 | 2 | — | — | 145 |
| M64 | 6 | 95 | 92.8 | 104.86 | 40 | 2 | — | — | 153 |
Partially threaded: the standard gives b as a reference value in three nominal-length bands (b ≈ 2d + 6 at l ≤ 125 mm, ≈ 2d + 12 at 125 < l ≤ 200 mm, ≈ 2d + 25 at l > 200 mm). A dash means that band is not applicable for the size.
References
- GB/T 5783-2025 Hexagon head bolts, fully threaded - width across flats s, s min, width across corners e min, head height k and fillet radius r min, product grades A and B; thread length rule b = l − a
- GB/T 5782-2025 Hexagon head bolts, partially threaded - the same head dimensions, plus the thread length b in three nominal-length bands (l ≤ 125 mm, 125 < l ≤ 200 mm, l > 200 mm)
- GB/T 5785-2025 Hexagon head bolts with fine pitch thread - fine pitch options per nominal diameter and the same hex head (s, e, k unchanged)
- Table values are the nominal (= max) or minimum figures of the standard editions above, checked row by row against the official text. No table values are reproduced from third-party dimension tables, and every row is labelled with the standard it comes from.
Thread length b on partially threaded bolts (GB/T 5782)
A partially threaded hex bolt carries thread over part of the shank, so the threaded length b depends on the nominal length l as well as on the diameter. GB/T 5782 tabulates b as a reference value in three nominal-length bands; the same M-size therefore has up to three different thread lengths.
b ≈ 2d + 6 at l ≤ 125 mm, b ≈ 2d + 12 at 125 < l ≤ 200 mm, b ≈ 2d + 25 at l > 200 mm; the tabulated figure is that value rounded to a whole millimetre. Grade A tables carry the shorter bands only; grade B carries the band that applies to each size.
Thread length b by length band, grade B (mm)
A dash means the standard does not tabulate that band for the size — the size's standard length range does not reach that length band (large sizes start above 125 mm) or the table carries only the bands in use.
Quote the diameter and the length together: b follows l, so only "M12×100" pins the thread length down.
Fine pitch hex bolts (GB/T 5785): fine thread against coarse thread
GB/T 5785-2025 tabulates the same hex head with a fine pitch thread on nominal diameters from 8 mm to 64 mm. The head does not change — s, e, k and r stay exactly as in the coarse table above — so a fine thread must be called out with its pitch: M12×1.5 is not M12, whose coarse pitch is 1.75 mm.
One diameter, one head: every fine pitch of the same d shares the same s, e, k and r, so the socket size does not change with the pitch.
| Nominal diameter d (mm) | Fine pitch P (mm) | Coarse pitch P (mm) |
|---|---|---|
| M8 | 1 | 1.25 |
| M10 | 1.25 / (1) | 1.5 |
| M12 | 1.5 / (1.25) | 1.75 |
| M14 | (1.5) | 2 |
| M16 | 1.5 | 2 |
| M18 | (2) / (1.5) | 2.5 |
| M20 | 2 / (1.5) | 2.5 |
| M22 | (2) / (1.5) | 2.5 |
| M24 | 2 | 3 |
| M27 | (2) | 3 |
| M30 | 2 | 3.5 |
| M33 | (2) | 3.5 |
| M36 | 3 | 4 |
| M39 | (3) | 4 |
| M42 | 3 | 4.5 |
| M45 | (3) | 4.5 |
| M48 | 3 | 5 |
| M52 | (4) | 5 |
| M56 | 4 | 5.5 |
| M60 | (4) | 5.5 |
| M64 | 4 | 6 |
A pitch in brackets is the non-preferred fine pitch of that diameter in GB/T 5785-2025; diameters not listed have no fine thread series in that standard.
Which number do you need: s, e or k?
"Bolt size" means three different numbers depending on the job, and order codes use a fourth. Buying a tool uses s, checking a head that will not go into a socket uses e, and a counterbore or a flush-head check uses k.
s — width across flats (nominal = max)
M12 is s = 18 mm. This is the socket, spanner or ring size, and the number stamped on the tool. It is the same in grade A and grade B and the same in GB/T 5783 and GB/T 5782, because the two standards share the hex head.
e — width across corners (min)
M12 is e min = 20.03 mm in grade A and 19.85 mm in grade B. This is the diagonal between two opposite corners of the head, the dimension that decides whether a 12-point socket or a ring spanner passes over the head. Most published bolt size charts leave this column out; it is also what you can still measure with a caliper once the flats have been rounded off.
k — head height (nominal, with a max)
M12 is k = 7.5 mm nominal and 7.79 mm max in grade B. It sets how deep a counterbore or recess has to be and how far the head stands above the part. When the head must not protrude, k is the number to check, not s.
d — nominal thread diameter
The number in the callout: M12 is a 12 mm thread, not a 12 mm head. The head is always wider, 18 mm across the flats here, which is where ordering by "size" alone goes wrong.
r — fillet radius under the head (min)
r min = 0.6 mm at M12. The washer face has to clear this radius, so it matters when the bolt sits in a corner, against a shoulder or in a slotted hole rather than on a flat face.
Metric to inch bolt sizes (approximate, not interchangeable)
Cross-reference by nominal diameter only. The inch column is a straight millimetre-to-inch conversion of d; the last column is the inch hex bolt whose nominal diameter is closest to it.
| Metric size | Nominal d (mm) | Nominal d (in) | Closest inch hex bolt |
|---|---|---|---|
| M1.6 | 1.6 | 0.063 | 1/4 in (6.35 mm) |
| M2 | 2 | 0.079 | 1/4 in (6.35 mm) |
| M2.5 | 2.5 | 0.098 | 1/4 in (6.35 mm) |
| M3 | 3 | 0.118 | 1/4 in (6.35 mm) |
| M3.5 | 3.5 | 0.138 | 1/4 in (6.35 mm) |
| M4 | 4 | 0.157 | 1/4 in (6.35 mm) |
| M5 | 5 | 0.197 | 1/4 in (6.35 mm) |
| M6 | 6 | 0.236 | 1/4 in (6.35 mm) |
| M7 | 7 | 0.276 | 1/4 in (6.35 mm) |
| M8 | 8 | 0.315 | 5/16 in (7.938 mm) |
| M10 | 10 | 0.394 | 3/8 in (9.525 mm) |
| M12 | 12 | 0.472 | 1/2 in (12.7 mm) |
| M14 | 14 | 0.551 | 1/2 in (12.7 mm) |
| M16 | 16 | 0.630 | 5/8 in (15.875 mm) |
| M18 | 18 | 0.709 | 3/4 in (19.05 mm) |
| M20 | 20 | 0.787 | 3/4 in (19.05 mm) |
| M22 | 22 | 0.866 | 7/8 in (22.225 mm) |
| M24 | 24 | 0.945 | 1 in (25.4 mm) |
| M27 | 27 | 1.063 | 1-1/8 in (28.575 mm) |
| M30 | 30 | 1.181 | 1-1/8 in (28.575 mm) |
| M33 | 33 | 1.299 | 1-1/4 in (31.75 mm) |
| M36 | 36 | 1.417 | 1-3/8 in (34.925 mm) |
| M39 | 39 | 1.535 | 1-1/2 in (38.1 mm) |
| M42 | 42 | 1.654 | 1-3/4 in (44.45 mm) |
| M45 | 45 | 1.772 | 1-3/4 in (44.45 mm) |
| M48 | 48 | 1.890 | 2 in (50.8 mm) |
| M52 | 52 | 2.047 | 2 in (50.8 mm) |
| M56 | 56 | 2.205 | 2-1/4 in (57.15 mm) |
| M60 | 60 | 2.362 | 2-1/4 in (57.15 mm) |
| M64 | 64 | 2.520 | 2-1/2 in (63.5 mm) |
matched by nominal diameter only
Do not substitute. Both series use a 60° thread form, but the diameters and thread counts differ, so a near-diameter inch bolt starts and then jams and damages the thread. Use these rows to read which callout a drawing means, not to interchange parts.
Per-size bolt pages
One page per size from M1.6 to M64: the head row in both standards and both grades, the thread length bands, the fine pitch options and the sizes either side for comparison.
FAQ
Which socket or spanner size does a metric hex bolt take?
The width across flats s, which is the third column of the table: at M12 it is 18 mm and it does not change with the product grade, because grade A and grade B differ only in tolerance and GB/T 5783 and GB/T 5782 share the same hex head. Buy the tool by s, then check e if the socket has to clear the corners.
What is e on a hex bolt, and why is it in this table?
e is the width across corners, the diagonal between two opposite corners of the head. The standard tabulates it as a minimum: M12 is e min = 20.03 mm in grade A and 19.85 mm in grade B. A 12-point socket or a ring spanner passes over the corners, so e is the bound to compare an opening against, and it is also the dimension you can still measure with a caliper when the flats are worn.
Is the M number the head size?
No. The number after M is the nominal thread diameter d: M12 is a 12 mm thread, while its head is 18 mm across the flats and 19.85 mm across the corners. Order by the M number plus the length, and confirm s and k against the drawing, the counterbore or the tool you already have.
What is the thread length b on a partially threaded bolt?
b is the threaded length measured from the end of the bolt. GB/T 5782 gives it as a reference value in three bands: b ≈ 2d + 6 at l ≤ 125 mm, 2d + 12 at 125 < l ≤ 200 mm and 2d + 25 at l > 200 mm, rounded to a whole millimetre — 30 mm, 36 mm and 49 mm at M12 in grade B. A fully threaded bolt has no separate b value, because the thread runs over the whole usable shank.
What is the difference between product grade A and grade B?
Only the tolerance band. Grade A applies to d ≤ M24 with l ≤ 10d or l ≤ 150 mm, so every M27 and larger bolt is grade B. The nominal s, k and the pitch are identical in the two grades, while grade B allows a smaller s min (17.57 mm against 17.73 mm at M12), a smaller e min (19.85 mm against 20.03 mm) and a larger k max. Grade A and grade B are dimensional tolerance grades, not quality grades.
Does a fine pitch bolt take the same socket?
Yes. GB/T 5785-2025 tabulates the fine thread version on the same hex head, so an M12×1.5 bolt has s = 18 mm and k = 7.5 mm exactly as a coarse M12 does, and only the pitch changes: 1.5 mm fine against 1.75 mm coarse. Always write the pitch into the callout for a fine bolt, otherwise it reads as coarse.
Related References
Bolt Weight Chart
Unit weight and order total for hex bolts M6-M36 once the size and length are known, with the same catalogue coefficients the product pages use.
Bolt Torque Chart
Tightening torque and preload per size and property class, on the same size axis as this table.
Bolt Weight Calculator
Total weight and packing quantity for an order of hex bolts, with the size carried over from this page.
Wrench and Socket Size Chart
The opening across flats for each metric size and the tools that fit, which is the s column read the other way round.
Carriage Bolt Size Chart
Round, square-neck and coach bolt head dimensions, for the same head-measurement question on a different bolt.
Hex Head Bolts, Partially Threaded (GB/T 5782)
The partially threaded range we quote to GB/T 5782, with the technical package for that standard.
Hex Head Bolts, Fully Threaded (GB/T 5783 / DIN 933)
Sizes, property classes and finishes we quote to GB/T 5783, which is the same form as DIN 933 and ISO 4017.
Hex Head Bolt Category
Standard numbers and head forms in the catalogue; sizes and lengths made to drawing.
Disclaimer: this table is a selection and quotation reference built from the standard editions named on this page, not a design basis or an acceptance standard.