Bolt size describes the thread, but socket size describes the head. An M8 bolt takes a 13mm socket, and a 1/2" bolt takes a 3/4" socket. Almost every wrong-socket moment in a workshop traces back to that one sentence. The designation stamped in a parts catalogue refers to the shank diameter, because that is what determines the bolt's strength and which hole it passes through. The hexagon on top is sized separately, following the fastener standard rather than the thread, and it is the only dimension a socket cares about. This page maps one to the other in both systems, so you can go from a bolt spec to the tool that fits it without measuring anything. Once you have the head size, the metric to standard socket converter will tell you whether a socket from the other system can stand in for it.
Coarse-pitch metric hex bolts with the head size specified by each standard and the socket that fits.
| Thread | Coarse pitch | Head A/F (ISO) | Head A/F (DIN) | Socket needed |
|---|---|---|---|---|
| M5 | 0.80mm | 8mm | N/A | 8mm socket |
| M6 | 1.00mm | 10mm | N/A | 10mm socket |
| M8 | 1.25mm | 13mm | N/A | 13mm socket |
| M10 | 1.50mm | 16mm | 17mm | 16mm socket |
| M12 | 1.75mm | 18mm | 19mm | 18mm socket |
| M14 | 2.00mm | 21mm | 22mm | 21mm socket |
| M16 | 2.00mm | 24mm | N/A | 24mm socket |
| M18 | 2.50mm | 27mm | N/A | 27mm socket |
| M20 | 2.50mm | 30mm | N/A | 30mm socket |
| M22 | 2.50mm | 34mm | 32mm | 34mm socket |
| M24 | 3.00mm | 36mm | N/A | 36mm socket |
Four rows in that table carry two different head sizes, and they are the ones that cause trouble. ISO 4014/4017 specifies a 16mm head on M10, 18mm on M12, 21mm on M14 and 34mm on M22; the older DIN 931/933 hardware still in circulation uses 17mm, 19mm, 22mm and 32mm for the same threads. Both are correct, they simply belong to different standards, and a vehicle can carry both. When an M10 bolt refuses a 16mm socket, reach for 17mm before assuming the head is damaged. Everywhere the DIN column shows a dash, the two standards agree and there is only one answer.
Three things are encoded in a fastener spec, and only one of them tells you which socket to pick up.
An M8 bolt has an 8mm thread and a 13mm head. A 1/2" bolt has a 1/2" shank and a 3/4" head. The number in the bolt designation always describes the shank diameter, and the socket you need always describes the head across the flats. Sizing a socket from the bolt designation is the single most common reason the tool pulled from the drawer does not fit.
A full metric designation is M10 × 1.5: 10mm diameter, 1.5mm between adjacent thread crests. Coarse pitch is assumed when the second number is omitted. Inch fasteners state threads per inch instead, so 1/2-13 is a half-inch bolt with 13 threads to the inch. Two bolts of identical diameter and different pitch will start to thread together and then jam.
Metric bolts carry a numeric grade such as 8.8 or 10.9 stamped on the head face. SAE bolts use radial lines instead. Three lines for Grade 5, six for Grade 8, none for Grade 2. An unmarked head on a load-bearing joint should be treated as the lowest grade until proven otherwise.
Pitch matters beyond identification: it also sets the hole you drill before cutting a thread, which is why the tap drill size chart is indexed by pitch rather than by diameter alone. If you need to compare pitch and form across standards, metric against UNC, UNF and Whitworth, the thread size comparison puts them side by side.
Inch hex cap screws with head size, both thread series, and the socket required.
| Thread | Head A/F | UNC | UNF | Socket needed |
|---|---|---|---|---|
| 1/4" | 7/16" | 20 TPI | 28 TPI | 7/16" socket |
| 5/16" | 1/2" | 18 TPI | 24 TPI | 1/2" socket |
| 3/8" | 9/16" | 16 TPI | 24 TPI | 9/16" socket |
| 7/16" | 5/8" | 14 TPI | 20 TPI | 5/8" socket |
| 1/2" | 3/4" | 13 TPI | 20 TPI | 3/4" socket |
| 9/16" | 13/16" | 12 TPI | 18 TPI | 13/16" socket |
| 5/8" | 15/16" | 11 TPI | 18 TPI | 15/16" socket |
| 3/4" | 1-1/8" | 10 TPI | 16 TPI | 1-1/8" socket |
| 7/8" | 1-5/16" | 9 TPI | 14 TPI | 1-5/16" socket |
| 1" | 1-1/2" | 8 TPI | 12 TPI | 1-1/2" socket |
The relationship between shank and head is looser here than in the metric table. It climbs from a 1/4" bolt needing a 7/16" head, a ratio of 1.75, to a 1" bolt needing 1-1/2", a ratio of 1.5. There is no formula worth memorising, which is exactly why the table exists. Note also that heavy hex variants of these same threads use a head one size larger than shown; if a 3/8" bolt refuses a 9/16" socket, try 5/8" before concluding anything is wrong.
Grade determines how much torque a fastener can carry, and it is stamped on the head of every bolt that has one.
Metric
The general-purpose automotive grade, used on most chassis and engine hardware.
Metric
Suspension, brake carriers and cylinder head bolts.
Metric
Highest common metric grade, used on socket head cap screws and machine tooling.
SAE
Non-structural hardware; the default unmarked hardware-store bolt.
SAE
The SAE equivalent of metric 8.8, used across general automotive work.
SAE
Roughly equivalent to metric 10.9, used on suspension and driveline.
The two systems map onto each other roughly: metric 8.8 sits close to SAE Grade 5, and metric 10.9 sits close to SAE Grade 8. "Roughly" is doing real work in that sentence. The numbers are near enough for identification and not near enough to justify substituting one for the other on a torqued joint. A metric grade reads as two numbers separated by a dot, where the first is the tensile strength in hundreds of megapascals and the second is the yield as a tenth of that tensile figure, so 8.8 means 800 MPa tensile and 80 per cent yield. Fitting a lower-grade bolt into a joint specified for a higher one is one of the few genuinely dangerous mistakes in fastener work, because the assembly looks completely normal right up until it fails.
The two inch thread series share diameters and differ only in how many threads occupy each inch.
| Size | Major dia (in) | UNC | UNF | UNC pitch (in) |
|---|---|---|---|---|
| #4 | 0.1120 | 40 TPI | 48 TPI | 0.0250 |
| #6 | 0.1380 | 32 TPI | 40 TPI | 0.0313 |
| #8 | 0.1640 | 32 TPI | 36 TPI | 0.0313 |
| #10 | 0.1900 | 24 TPI | 32 TPI | 0.0417 |
| #12 | 0.2160 | 24 TPI | 28 TPI | 0.0417 |
| 1/4 | 0.2500 | 20 TPI | 28 TPI | 0.0500 |
| 5/16 | 0.3125 | 18 TPI | 24 TPI | 0.0556 |
| 3/8 | 0.3750 | 16 TPI | 24 TPI | 0.0625 |
| 7/16 | 0.4375 | 14 TPI | 20 TPI | 0.0714 |
| 1/2 | 0.5000 | 13 TPI | 20 TPI | 0.0769 |
| 9/16 | 0.5625 | 12 TPI | 18 TPI | 0.0833 |
| 5/8 | 0.6250 | 11 TPI | 18 TPI | 0.0909 |
| 3/4 | 0.7500 | 10 TPI | 16 TPI | 0.1000 |
| 7/8 | 0.8750 | 9 TPI | 14 TPI | 0.1111 |
| 1" | 1.0000 | 8 TPI | 12 TPI | 0.1250 |
UNC, Unified National Coarse, has fewer, deeper threads per inch, which makes it faster to run down, more tolerant of dirt and damage, and better suited to softer materials like aluminium and cast iron where deeper engagement resists stripping. UNF, Unified National Fine, packs more threads into the same inch, giving a larger stress area, finer torque control and better resistance to vibration loosening, at the cost of being easier to cross-thread. The two are never interchangeable: a 1/2-13 UNC bolt will start into a 1/2-20 UNF nut and jam solid within two turns, ruining both. Identifying which series you have starts with counting threads against a gauge, and continues on the full thread standard comparison. From there, the socket size calculator closes the loop from head size back to the tool.
The rest of the fastener, thread and drill references on this site, every one of them is one click away from here.