M8 Bolt Size and Thread Specifications
Quick answer
M8 bolts use 8mm major diameter with 1.25mm coarse pitch and a 6.8mm tap drill.
They are standard for medium-duty machinery and structural brackets.
Use the chart below for hex and drill data, then metric to imperial conversion for stocked UNC parts.
Bolt sizing and thread specifications follow widely recognized mechanical standards used in engineering and manufacturing. Standard bolt sizing references, industrial fastener charts, and mechanical thread standards ensure compatibility, safety, and correct tool selection across applications.
M8 is a widely used metric bolt size in machinery, automotive, and structural applications. An 8 mm nominal diameter with 1.25 mm coarse pitch provides strong threads and is easy to source. Spanish reference: perno M8. For tap drill size, thread types, and thread identifier workflows, pair this page with tools as you verify threads.
Specifications
| Specification | Value |
|---|---|
| Nominal diameter | 8 mm |
| Standard pitch | 1.25 mm |
| Fine pitch | 1.0 mm |
| Tap drill size | 6.8 mm |
| Closest imperial equivalent | 5/16-18 |
Hex head size
| Bolt size | Hex head size |
|---|---|
| M8 | 13 mm |
Common uses
M8 bolts are common in machinery mounts, automotive assemblies, structural connections, and general hardware. They are a standard choice for medium-duty fastening. Cross-check loads with the bolt size chart and screw size conversion when you mix metric and inch bins.
Related tools
Related charts
M8 vs M10 bolt size
Shank diameter and pitch set tap drill, hex key, and strength for each ISO size. Compare values in the tables above with the M10 bolt size page when you resize hardware.
Use the bolt size chart when you compare hex and shank columns across sizes.
Metric vs UNC thread size
Metric threads list diameter in millimeters and pitch in millimeters per thread; UNC lists nominal inch diameter and threads per inch. They are not interchangeable—taps, dies, and gauges differ.
Read metric vs unc thread context when series labels are unclear.
Use screw size conversion when you must match stocked parts.
M8 specifications and tools
How to choose the right bolt size
Choosing the correct bolt size depends on load, material, and thread type. Larger diameters increase strength, while thread pitch affects holding force.
Use the bolt size chart to compare shank and hex dimensions before you standardize hardware.
Use tap drill size guidance when you plan tapped holes.
Use the thread identifier when stamped sizes are missing.
FAQ
What is the standard pitch for an M8 bolt?
The standard pitch for M8 is 1.25 mm. Fine pitch is 1.0 mm.
What drill size should be used for tapping M8 threads?
For M8 x 1.25 coarse thread, use a 6.8 mm tap drill.
What imperial bolt is closest to M8?
5/16-18 UNC is the closest imperial equivalent to M8.