M18 Bolt Size and Thread Specifications
Quick answer
M18 bolts use 18mm major diameter with 2.5mm coarse pitch and a 15.5mm tap drill.
They serve heavy equipment and structural bolting with high clamp loads.
Use the chart below and tap drill calculator when fine pitch or special taps apply.
M18 serves very high-load joints in heavy machinery and large structural connections. Use torque specs and lubrication class appropriate to the bolt grade. Spanish reference: perno M18. For tap drill size, thread types, and thread identifier workflows, pair this page with tools as you verify threads.
Specifications
| Specification | Value |
|---|---|
| Nominal diameter | 18 mm |
| Standard pitch (coarse) | 2.5 mm |
| Fine pitch | 2.0 mm |
| Tap drill size | 15.5 mm |
| Closest imperial equivalent | 3/4-10 |
Hex head size
| Bolt size | Hex head size |
|---|---|
| M18 | 27 mm |
Common uses
Heavy engineering, large spindles, flanges, and industrial equipment with high clamping requirements. Cross-check loads with the bolt size chart and screw size conversion when you mix metric and inch bins.
Related tools
Related charts
M18 vs M20 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 M20 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.
M18 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 M18 bolt?
The common coarse pitch for M18 is 2.5 mm. Fine pitch is often 2.0 mm where specified.
What drill size should be used for tapping M18 threads?
For M18 x 2.5 coarse thread, use approximately a 15.5 mm tap drill. Use our Tap Drill Calculator for other pitches.
What imperial fastener is closest to M18?
3/4-10 is a practical reference for comparing M18 to inch hardware; verify fit for your application.