Engineering Tools · Hex Bolt Dimension Reference
M3.5 Bolt Size Chart: Hex Head Dimensions s, e and k
M3.5 hex bolt head dimensions from GB/T 5783-2025 and GB/T 5782-2025: width across flats s = 6 mm, width across corners e min = 6.58 mm in grade A and 6.44 mm in grade B, head height k = 2.4 mm nominal, fillet radius r min = 0.1 mm, coarse pitch P = 0.6 mm, and the thread length bands b of GB/T 5782. All values are nominal or minimum figures of the named standard editions.
M3.5 hex bolt: socket size s = 6 mm; head diagonal e min = 6.58 mm (grade A) / 6.44 mm (grade B); head height k = 2.4 mm nominal, 2.6 mm max in grade B; pitch P = 0.6 mm; fine pitch options —. Head values are the standard's nominal (= max) or minimum figures, not measured guarantees.
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.
M3.5 in both standards and both product grades
One row per thread form and product grade. The head columns repeat because GB/T 5783 and GB/T 5782 share the same head; b is tabulated in GB/T 5782 only, and a dash means the standard does not tabulate that value for the combination.
| Thread form | Product grade | 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) | k max (mm) | b at l ≤ 125 mm | b at 125 < l ≤ 200 mm | b at l > 200 mm |
|---|---|---|---|---|---|---|---|---|---|---|
| Fully threaded (GB/T 5783) | A | 0.6 | 6 | 5.82 | 6.58 | 2.4 | 2.525 | — | — | — |
| Fully threaded (GB/T 5783) | B | 0.6 | 6 | 5.7 | 6.44 | 2.4 | 2.6 | — | — | — |
| Partially threaded (GB/T 5782) | A | 0.6 | 6 | 5.82 | 6.58 | 2.4 | 2.525 | 13 | — | — |
| Partially threaded (GB/T 5782) | B | 0.6 | 6 | 5.7 | 6.44 | 2.4 | 2.6 | 13 | 19 | 32 |
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)
| Size | b at l ≤ 125 mm | b at 125 < l ≤ 200 mm | b at l > 200 mm |
|---|---|---|---|
| M3.5 | 13 | 19 | 32 |
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) |
|---|---|---|
| M3.5 | — | 0.6 |
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)
M3.5 is s = 6 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)
M3.5 is e min = 6.58 mm in grade A and 6.44 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)
M3.5 is k = 2.4 mm nominal and 2.6 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: M3.5 is a 3.5 mm thread, not a 3.5 mm head. The head is always wider, 6 mm across the flats here, which is where ordering by "size" alone goes wrong.
r — fillet radius under the head (min)
r min = 0.1 mm at M3.5. 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 |
|---|---|---|---|
| M3.5 | 3.5 | 0.138 | 1/4 in (6.35 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.
M3.5 beside the next sizes
The sizes either side of M3.5. Measure the thread diameter d with a caliper and compare the head against the rows below before ordering; the nominal diameter steps by 0.5 mm or 1 mm in this range.
Grade B
| Size | Coarse pitch P (mm) | Width across flats s (mm, nominal = max) | Width across corners e min (mm) | Head height k (mm, nominal) |
|---|---|---|---|---|
| M3d = 3 | 0.5 | 5.5 | 5.88 | 2 |
| M3.5d = 3.5 | 0.6 | 6 | 6.44 | 2.4 |
| M4d = 4 | 0.7 | 7 | 7.5 | 2.8 |
FAQ
What are the head dimensions of an M3.5 hex bolt?
M3.5 bolt head, GB/T 5783-2025 and GB/T 5782-2025: width across flats s = 6 mm, which is the socket or spanner size; width across corners e min = 6.58 mm in grade A and 6.44 mm in grade B; head height k = 2.4 mm nominal, 2.6 mm max in grade B; fillet radius r min = 0.1 mm; coarse pitch P = 0.6 mm. The two standards give the same head, so these figures hold for a fully threaded and a partially threaded M3.5 alike.
Which socket fits an M3.5 bolt?
A 6 mm socket or spanner, because the width across flats is what the tool grips. The same 6 mm applies in grade A, grade B, GB/T 5783 and GB/T 5782. If you are not sure of the size, measure across the flats of the head with a caliper and compare with the s column rather than with the thread diameter.
What is the thread length of a partially threaded M3.5 bolt?
It depends on the nominal length l, not on the diameter alone. GB/T 5782 grade B gives b = 13 mm at l ≤ 125 mm, 19 mm at 125 < l ≤ 200 mm and 32 mm at l > 200 mm for M3.5; the grade A tables tabulate only the shorter bands (13 mm and — mm). Always quote the length with the diameter, otherwise the threaded length is undefined.
Is the M number on an M3.5 bolt the head size?
No. M3.5 is the nominal thread diameter d = 3.5 mm, while the head is wider: 6 mm across the flats and 6.44 mm across the corners in grade B. Order by the M number and the length, and check s and k against the drawing or the counterbore.
Does an M3.5 fine pitch bolt take the same socket?
Yes, the head is the same: GB/T 5785-2025 uses the same hex head for the fine thread series, so s = 6 mm and k = 2.4 mm hold for a fine M3.5 too. Only the pitch changes — 0.6 mm coarse against the fine options —. Write the pitch into the callout for a fine bolt: a plain M3.5 will be read as coarse pitch.
Can an M3.5 bolt be replaced by an inch bolt?
No, not as a substitute. The M3.5 thread is d = 3.5 mm with a 0.6 mm pitch, and the nearest inch bolt only matches the diameter to within a few tenths of a millimetre while carrying a different thread count, so it will not run true in the same nut or tapped hole and will damage it. Both are 60° threads, which is why they look close on a drawing; use the metric to inch table to read a callout, and order the thread the joint is made for.
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.