Metric Screw Size Chart: M1–M24, Pitch & Tap Drill

Metric Screw Size Chart Basics: What M, Diameter, and Pitch Mean

M designates the ISO metric screw thread series, and the number after the M is the thread's nominal major diameter in millimeters — the outside diameter of the screw measured across the thread crests. The metric thread system is governed by a family of standards: ISO 261 defines the diameter-and-pitch series, ISO 724 the basic dimensions, and ISO 965 the tolerances. A complete callout adds the pitch — the distance in millimeters from one thread crest to the next:

• M8×1.25 reads as an 8mm major diameter with a 1.25mm pitch: the standard coarse thread for M8.
• M8×1.0 is the same 8mm diameter with a finer 1.0mm pitch, which is a different thread that takes a different nut.
• M8 with no pitch stated means the coarse thread (1.25) is assumed, because coarse is the default in the ISO series. That rule holds for conventional ISO fastener notation; special thread profiles and drawing-specific callouts follow their own specifications.

That is the whole system: diameter, then pitch. Everything else in this chart (tap drills, inch equivalents, fine options) follows from those two numbers.

Quick Reference: The Sizes Most Builds Use

Before the full series, here are the seven sizes that cover most assemblies, with the three numbers people actually look up:

Values are the same reference figures used in the full tables below; for anything outside this range, continue to the complete series.

Metric Screw Size Chart: M1–M24 Coarse Threads

The chart below lists the ISO coarse-pitch series, the sizes most drawings and fastener catalogs use by default. Major diameter equals the nominal size; the inch column converts it at 25.4mm per inch; the tap drill column follows the diameter-minus-pitch convention rounded to standard drills, consistent with our tap drill chart.

The diameter-minus-pitch rule provides an initial estimate, not a fixed value: the tap-drill figures are common reference values for standard metric coarse threads, and the appropriate drill size depends on the required thread engagement, workpiece material, tapping method, and the tap manufacturer's recommendations. Metric coarse-thread sizes continue beyond M24, with nominal diameters and pitches specified by the relevant ISO metric thread standards; bench and machine assembly rarely touches them.

Metric Screw Size Chart: Common Fine-Pitch Options

Alongside the coarse series, ISO 261 defines fine-pitch threads for the same diameters. Fine-pitch options fit specific nominal diameters, and their suitability depends on part design and the mating thread specification. The most common ones in machine assembly:

Fine pitch threads put more threads per millimeter of engagement into the same diameter, which suits thin walls that cannot spare depth and joints that see vibration. The table lists the pitch options only — a fine-pitch tap drill depends on the same diameter-minus-pitch logic but is application-specific, so take it from the tap maker's data for the exact thread you are cutting.

Metric Bolt Sizes in Practice: M3 to M12

The chart covers the full range; these are the sizes most hands actually touch, and where each earns its place:

• M3 — the small-electronics staple: 3D printer parts, RC equipment, enclosure lids. At 0.5mm coarse pitch it takes a 2.5mm tap drill.
• M4 — small mechatronic assemblies and jigs; a step up in strength from M3 while still using compact hex and button heads.
• M5 — the compact European standard, common in aluminum extrusion connections and fixtures where M6 crowds the edge distance.
• M6 — the general assembly workhorse for machine building; our M3–M6 screws guide covers this range's selection and installation in depth.
• M8 — structural connections in machine frames and brackets; the most common bolt size in mid-size mechatronic builds.
• M10 and M12 — heavy joints: motor mounts, linear rail bolts, hydraulic manifolds. M12 starts appearing in fixtures that carry real clamping loads.

Across these sizes, matched nuts and washers matter as much as the screw itself; the thread only performs when the mating hardware meets the same standard. The wrench that fits follows the nut, not the screw, and the common hex-nut widths across flats are:

The dimensions listed are common hex-nut widths across flats. ISO 4032:2023 covers standard Style 1 hexagon nuts from M5 to M39; the M3 and M4 figures follow the same style and should be checked against the applicable smaller-size standards. Older DIN-based hardware and some regional stocks run 1mm wider on M10 and M12, so a wrench that fits one M12 nut may rock on another. When a build mixes fastener sources, sizing wrenches from the actual hardware beats sizing them from the nominal thread.

How to Identify an Unmarked Metric Thread (and Read Its Pitch)

Two measurements sort almost any unmarked male thread:

1. Measure the major diameter with a caliper, across the thread crests at a full thread: a clean 6.0mm reads M6, 8.0mm reads M8.
2. Measure the pitch with a thread pitch gauge, or count threads against a ruler: 1.0mm between crests on the M6, 1.25mm on the M8.

The diameter does most of the sorting in the metric series because the sizes are 0.5mm apart in the small range and spaced widely above M10. The pitch check confirms the callout and catches the fine-versus-coarse distinction the diameter cannot show. For instrument-size threads the same method scales down; the sizes and bands differ for tapered pipe threads, which our NPT thread guide covers separately.

Metric vs. Inch Threads: How to Tell Metric Screw Sizes Apart

Three metric sizes sit dangerously close to common inch sizes on diameter:

Both systems use a 60° thread angle, so the profiles look alike. What separates them is the pitch system: metric pitches run in millimeters (1.0, 1.25, 1.5…), inch pitches in threads per inch (13, 12, 20…), and no callout matches across the two. Although M8 and 5/16-inch fasteners have very similar nominal diameters, their thread pitches belong to different systems: they are not interchangeable, and forcing one into the other can damage the mating threads — the fix is picking one standard for the whole assembly. The inch side runs on a different pitch logic: UNC and UNF series are specified in threads per inch, and their full pitch tables belong to a separate chart.

Choosing Between Coarse and Fine Pitch on a Metric Thread

Coarse is the default for good reasons: it assembles faster, tolerates dirt and slight damage better, and taps with less torque. Fine pitch earns its keep in specific conditions rather than universally:

• Thin walls and short engagement — fine pitch packs more load-bearing threads into a length that a coarse thread cannot fill.
• Vibration-prone joints — fine pitch may offer advantages in certain joint designs, but thread pitch alone does not determine resistance to loosening; proper preload and suitable locking methods are essential.
• Fine adjustment — set screws and optical hardware use fine threads because each turn moves the part less.

The practical rule is to follow the drawing: if it says M8, the coarse 1.25 pitch is assumed; if it says M8×1.0, the fine thread is specified deliberately, and substituting coarse because the nut was at hand strips the joint.

FAQ about Metric Screw Size Chart

About This Metric Screw Size Chart

A metric screw size chart does its job with two numbers per thread: the diameter that names it and the pitch that completes it. The tables above assemble the ISO coarse series M1–M24, the common fine-pitch options, and the size-by-size picture for the everyday M3–M12 range, so a drawing callout can be matched to hardware — and an unmarked bolt can be sorted before it reaches the wrong nut.

Values here are standard reference figures, and the tap manufacturer's chart decides for production tapping. For the threads that pair with these sizes, the tap drill chart covers the drill side, and matched metric screws and nuts are what keep an assembly on one standard from first hole to final torque.