Engineering Tools · Wrench and Socket Openings
M10 Bolt Wrench Size: 16 mm Wrench and Socket Opening
M10 hex head bolt (GB/T 5782 / ISO 4014 basis): width across flats s=16 mm, so it takes a 16 mm wrench or socket - nominally 5/8″ in imperial sizes. The same size hex nut (ISO 4032 / GB/T 6170) measures s=16 mm. This page puts the bolt, nut, flange nut and carriage bolt width across flats side by side, labels the governing standard row by row, and states the series differences that make shop charts disagree. No weight or unit conversion here - this family is a tool-opening reference only.
A M10 hex head bolt takes a 16 mm wrench or socket: its width across flats is s=16 mm on the GB/T 5782 / ISO 4014 basis, nominally 5/8″ in imperial sizes. On the same basis the M10 hex nut measures 16 mm across flats.
Other Sizes
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M10 Width Across Flats, Side by Side
Four rows, one size. Each row gives the value its governing standard sets and the opening it maps to. The tool opening you need is the width across flats of the part you actually turn - for a hex head bolt that is s, for a hex nut that is s, for a flange nut that is s. The carriage bolt row is different in kind: its square neck is not a hex and no wrench goes on it (it bites into the slot and stops the bolt turning), so tighten on the nut side; it is listed because it is the dimension most often asked about alongside the others for the same nominal size.
| Fastener / feature | Width across flats s (mm) | Wrench / socket opening (nominal) | Standard (row-level source) |
|---|---|---|---|
| Hex head bolt | 16 mm | 16 mm (5/8″) | GB/T 5782 / ISO 4014 |
| Hex nut | 16 mm | 16 mm (5/8″) | GB/T 6170 / ISO 4032 |
| Flange nut | 15 mm | 15 mm (9/16″) | GB/T 6177.1-2016 (type 2) |
| Carriage bolt square neck | 10.36 mm | not a wrench opening - tighten on the nut side | GB/T 12-2013 |
A dash means this chart has no value for that combination: the flange nut data behind this page covers M6, M8, M10, M12, M16 and M20, and the carriage bolt data covers M6-M20 except M18. A missing size is not interpolated or extrapolated - ask us for the drawing-level figure. Widths across flats here are nominal; the real fit also depends on the fastener tolerance and on the tool's own tolerance band.
References
- GB/T 5782-2016 Hexagon head bolts (National Public Service Platform for Standards Information, China; the across-flats series used for the bolt row)
- GB/T 6170-2015 Style 1 hexagon nuts (National Public Service Platform for Standards Information, China; across-flats series identical to ISO 4032, used for the nut row)
- GB/T 6177.1-2016 Hexagon nuts with flange, type 2 (National Public Service Platform for Standards Information, China; used for the flange nut row, which covers six sizes in this chart)
- GB/T 12-2013 Round head square neck bolts (carriage bolts) (National Public Service Platform for Standards Information, China; used for the square neck row)
- ISO 4014 / ISO 4032 - equivalent ISO series for hexagon head bolts and style 1 hexagon nuts, cited for series reference only; no table values are reproduced from them
All Sizes: M6-M36 Wrench and Socket Openings
The same four columns across the whole range, one row per size. The wrench/socket column is the hex bolt width across flats with its nearest imperial fraction in brackets (a nominal label, not a conversion - see the notes under the table). Use it to find the tool for a bolt you already have, or to compare what a nut, a flange nut or a carriage bolt of the same size will need.
| Size | Hex bolt s (mm) | Wrench / socket (nominal) | Hex nut s (mm) | Flange nut s (mm) | Carriage bolt neck s (mm) |
|---|---|---|---|---|---|
| M6 | 10 mm | 10 mm (3/8″) | 10 mm | 10 mm | 6.3 mm |
| M8 | 13 mm | 13 mm (1/2″) | 13 mm | 13 mm | 8.36 mm |
| M10 | 16 mm | 16 mm (5/8″) | 16 mm | 15 mm | 10.36 mm |
| M12 | 18 mm | 18 mm (11/16″) | 18 mm | 18 mm | 12.43 mm |
| M14 | 21 mm | 21 mm (13/16″) | 21 mm | — | 14.43 mm |
| M16 | 24 mm | 24 mm (15/16″) | 24 mm | 24 mm | 16.43 mm |
| M18 | 27 mm | 27 mm (1 1/16″) | 27 mm | — | — |
| M20 | 30 mm | 30 mm (1 3/16″) | 30 mm | 30 mm | 20.52 mm |
| M22 | 32 mm | 32 mm (1 1/4″) | 34 mm | — | — |
| M24 | 36 mm | 36 mm (1 7/16″) | 36 mm | — | — |
| M27 | 41 mm | 41 mm (1 5/8″) | 41 mm | — | — |
| M30 | 46 mm | 46 mm (1 13/16″) | 46 mm | — | — |
| M33 | 50 mm | 50 mm (1 15/16″) | 50 mm | — | — |
| M36 | 55 mm | 55 mm (2 3/16″) | 55 mm | — | — |
A dash means this chart has no value for that combination: the flange nut data behind this page covers M6, M8, M10, M12, M16 and M20, and the carriage bolt data covers M6-M20 except M18. A missing size is not interpolated or extrapolated - ask us for the drawing-level figure. Widths across flats here are nominal; the real fit also depends on the fastener tolerance and on the tool's own tolerance band.
Heavy Hex and Other Series: Why Charts Give Different Numbers
Heavy hex fasteners (the heavy series in the ASME B18.2.1 / B18.2.6 family, and the high-strength structural series) use a larger width across flats than the regular hex series at the same nominal diameter. If you measure a head or nut that has been called "M12" and get a noticeably larger opening than the value in the table above, that is the tell. A wrench that fits the regular hex series will not fit the heavy hex series, and the two are not interchangeable in the same joint. This chart does not tabulate heavy hex: the governing tables are copyrighted and are not reproduced here, and the ISO/GB series this chart is built on has no heavy hex column. Order those items against the drawing or the standard the buyer names.
The second reason shop charts disagree is the size series. Two bases are in circulation for the same nominal diameter: the reduced across-flats series used here (ISO 4014 / GB/T 5782 for bolts, ISO 4032 / GB/T 6170 for nuts - M10 16 mm, M12 18 mm, M14 21 mm) and the withdrawn German DIN 933 / DIN 934 series, which is one step larger at exactly those three sizes (17 / 19 / 22 mm). The widely repeated line "an M12 bolt takes a 19 mm wrench" is the DIN basis; on the ISO/GB basis used throughout this page an M12 bolt takes an 18 mm wrench. Each is correct for its own basis, and what decides is which one your drawing or contract names - so check the basis before you buy a tool or a box of fasteners.
Data Basis and Assumptions Disclosed
Every value on this page is derived at render time from fastenerCalc, the same library that drives the calculators and the weight charts on this site - no second table and no copied figures. The bolt row reads SIZES[].hexHead.s (ISO 4014 / GB/T 5782), the nut row SIZES[].hexNut.s (ISO 4032 / GB/T 6170), the flange nut row FN_DIMS (GB/T 6177.1-2016 type 2) and the carriage bolt row CB_DIMS (GB/T 12-2013); the row-level standards are named in the table itself. No table values are reproduced from any copyrighted standard (for example ASME B18.2.1 / B18.2.6), and no heavy hex column is published. Imperial fractions are nominal nearest-sixteenth labels, not imperial size equivalents.
Boundary: (1) widths across flats are nominal values, actual parts vary within tolerance and the fit also depends on the tool tolerance - this page is not an acceptance basis; (2) imperial fractions are the nearest sixteenth by rounding and are there to help you find a close tool, not as a conversion (13 mm is not 1/2 in); (3) combinations shown as a dash have no value in this chart's data and are deliberately not interpolated; (4) everything here is on the ISO/GB series, which differs from the withdrawn DIN 933 / DIN 934 series at M10, M12 and M14 - confirm the basis on your drawing before ordering tools or fasteners.
FAQ
What size wrench fits an M10 bolt?
An M10 hex head bolt measures 16 mm across flats on the ISO 4014 / GB/T 5782 basis, so it takes a 16 mm wrench or socket (nominally 5/8″ in imperial sizes). If you are not sure of the size, measure it: the width across flats is the distance between two opposite flat faces of the head, and that millimetre reading is the tool size you need. Do not measure corner to corner - that gives the larger diameter across the corners and will send you one or two steps too big. If the part turns out to be heavy hex or an older DIN 933 item, the reading will not match this table; see the series note above.
Charts disagree for M10 - which number is right?
Both can be right, because two series are in circulation for the same nominal diameter. This page uses the reduced across-flats series (ISO 4014 / GB/T 5782 for bolts, ISO 4032 / GB/T 6170 for nuts): M10 16 mm, M12 18 mm, M14 21 mm. The withdrawn German DIN 933 / DIN 934 series is one step larger at exactly those three sizes - M10 17 mm, M12 19 mm, M14 22 mm - and that is where the widely repeated "M12 takes a 19 mm wrench" comes from. For M10 this page gives 16 mm on the ISO/GB basis. Neither number is wrong in itself; the basis named on your drawing or contract decides which one applies.
Do a bolt and its matching nut need the same wrench size?
Usually, and you can check it row by row above. On the bases used here the bolt and the hex nut match at every size in the table except M22, where the bolt is 32 mm across flats while the ISO 4032 / GB/T 6170 nut is 34 mm - so an M22 nut needs one step larger than the bolt itself. Flange nuts are a separate case again: they can be one step smaller than the plain hex nut of the same size (M10: 15 mm against 16 mm on the GB/T 6177.1 type 2 series). Read the row for the part you are actually turning instead of assuming the bolt size carries over.
Will an imperial socket fit a metric bolt?
Close, but treat it as a fallback rather than a substitute. Only a few nominal sizes sit near each other (13 mm against 1/2 in = 12.7 mm; 19 mm against 3/4 in = 19.05 mm), and even a close pair carries visible play. The fractional label in the table is the nearest sixteenth by rounding: it tells you which imperial socket is closest, not that the two are interchangeable. On a tight or already damaged fastener, or with a worn tool, use the metric size by name and keep the imperial one for emergencies at low torque.
Can I use the same wrench on a heavy hex bolt or nut?
No. Heavy hex fasteners (the heavy series in the ASME B18.2.1 / B18.2.6 family, and the high-strength structural series) are larger across flats than the regular hex series at the same nominal diameter, so a wrench that fits a regular hex item will not fit the heavy hex one, and the two are not interchangeable in a joint either. This chart does not publish heavy hex values - the governing tables are copyrighted and are not reproduced here, and the ISO/GB data this chart is built on has no heavy hex column. Order those to the drawing or the standard the buyer names, and confirm the across-flats figure by measurement.
Why is the wrench opening so much bigger than the bolt diameter?
Because the two numbers measure different things. The diameter (10 mm for M10) is the thread size, while the opening (16 mm) is the width across flats of the head the tool grips. The head is considerably larger than the thread across the whole range, and the gap is not a constant factor - so you cannot compute one number from the other, and a M10 bolt is never driven with a 10 mm tool. That mismatch is also why searching for a wrench by thread size returns so many conflicting answers.
Related References
M10 Bolt Torque Chart
How hard to tighten it: torque range and target preload by property class and friction state - the companion to picking the right wrench.
M10 Hex Bolt Weight Chart
Unit weight and kg per 1,000 pcs across the length series at the same size, useful for comparing steel use and freight.
M10 Hex Nut Weight Chart
The matching nut: width across flats, thickness, unit weight and kg per 1,000 pcs on the GB/T 6170 basis.
M10 Hex Key Size Chart
The other end of the same question: for socket head cap screws at this size, which hex key (Allen wrench) fits - same library, same size basis.
Bolt Weight Calculator
For lengths, materials and batch totals outside these tables; same engine and data source as the charts.
Disclaimer: This table is engineering reference data for tool selection, ordering and fit checking only, not a design basis or acceptance standard.