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Views: 0 Author: Site Editor Publish Time: 2026-08-29 Origin: Site
Bolt size | Thread diameter (d) | Wrench size / across-flats (W) | Head height (k) | Common lengths (mm) |
|---|---|---|---|---|
M3 | 3 mm | 5.5 mm | 2.0 mm | 5–30 |
M4 | 4 mm | 7.0 mm | 2.8 mm | 6–40 |
M5 | 5 mm | 8.0 mm | 3.5 mm | 8–50 |
M6 | 6 mm | 10 mm | 4.0 mm | 10–60 |
M8 | 8 mm | 13 mm | 5.3 mm | 12–80 |
M10 | 10 mm | 16 mm | 6.4 mm | 16–100 |
M12 | 12 mm | 18 mm | 7.5 mm | 20–120 |
M16 | 16 mm | 24 mm | 10.0 mm | 25–200 |
M20 | 20 mm | 30 mm | 12.5 mm | 30–300 |
M24 | 24 mm | 36 mm | 15.0 mm | 40–300 |
Values per ISO 4014 (partial thread) / ISO 4017 (full thread). "Wrench size" = across-flats, the standard hex key or socket size.
How to read this chart: the "M" number is the nominal thread diameter in millimetres — an M8 bolt has an 8 mm thread. To pick a wrench or socket, use the across-flats column (13 mm for M8, 16 mm for M10). To specify length, measure from under the head to the thread end.
Measuring a bolt sounds trivial, but one common mistake causes most ordering errors:
Diameter — measure the thread diameter with a caliper (ignore the threads' peaks, measure the nominal body). M8 = 8.0 mm.
Length — measure from under the head to the end of the threaded shaft. Do not include the head. This is the standard ISO measurement method.
Thread pitch — for metric bolts, count threads per 10 mm or use a pitch gauge. Coarse (default) vs fine pitch matters: M8×1.25 is coarse, M8×1.0 is fine.
Full vs partial thread — ISO 4017 bolts are fully threaded; ISO 4014 bolts have a plain shank. Both share the same head dimensions, so the size chart above applies to both.
Replacing a bolt: match diameter and length exactly, then check grade (strength) — never downgrade.
Thread length rule of thumb: the thread should engage the nut or tapped hole at least 1.0 × the bolt diameter (for steel). M8 bolt → at least 8 mm of thread engagement.
Wrench access: if you can only reach one side of the joint, use a bolt + nut; if both sides are open, a stud or threaded rod may be simpler.
Strength: for structural or load-bearing joints, do not guess — check the grade marking on the head (8.8, 10.9) and match the original specification.
Metric bolts use millimetre threads (M8×1.25); inch bolts use threads per inch (5/16"-18, i.e. 18 TPI). A metric wrench will "feel close" on an inch bolt but will slip or round the corners — check the thread first, then the wrench size. When in doubt, an M8 bolt is close to 5/16", but they are NOT interchangeable.
Q1: Is an M8 bolt 8 mm across the head? No. M8 is the thread diameter (8 mm). The wrench size is 13 mm across-flats.
Q2: How do I measure bolt length? From under the head to the end of the threaded portion. Never include the head height.
Q3: What's the difference between ISO 4014 and ISO 4017? ISO 4017 is fully threaded; ISO 4014 has a plain shank (partial thread). Head dimensions are identical.
Q4: What does "M8×1.25" mean? M8 = 8 mm thread diameter; 1.25 = coarse thread pitch in millimetres. Fine pitch would be M8×1.0.
Q5: Can I use a 13 mm socket on an M8 bolt? Yes — 13 mm is the standard across-flats size for M8 hex bolts.
Q6: What grade hex bolts should I use? Match the original part's grade marking (8.8, 10.9, 12.9). See our bolt grade chart for strength comparison.
Ningbo Weifeng Fastener Co., Ltd. manufactures and exports metric hex bolts in ISO 4014/4017, DIN 931/933 and GB/T 5782/5783, in grades 4.8 to 12.9, with zinc plating, hot-dip galvanizing or stainless steel (304/316). We supply M3–M64 with consistent quality — browse our hex bolt range or send us your drawing and spec for a quote within 24 hours.
Sources: ISO 4014/4017 (hex bolts), ISO 898-1 (mechanical properties), DIN 931/933, GB/T 5782/5783, ANSI/ASME B18.2.1 (inch series).
