Metric Bolt Size Chart: M2 to M36 Diameter, Pitch & Clearance Holes

Reviewed by the ScrewBoltSize Editorial Team

How to Read a Metric Bolt Designation

If you've never bought metric bolts before, the naming can look cryptic. A full specification like M8 × 1.25 × 30 mm breaks down into three parts, each separated by a multiplication sign (×):

• M8 — Nominal diameter: 8 mm (measured across the outer thread crests)
• 1.25 — Thread pitch: distance in millimeters between adjacent thread crests
• 30 — Length: 30 mm, measured from under the head for hex/cap screws; full length for flat-head screws

The "coarse" version of each size is the most common and what you'll find in hardware stores without any special request. Fine-thread variants exist for precision applications — they're designated separately (e.g., M8 × 1.0 is fine, while M8 × 1.25 is coarse).

Full Metric Bolt Size Chart (Coarse Thread, ISO)

This table covers standard ISO metric coarse threads from M2 through M36 — the sizes you'll encounter in everyday hardware, machinery, automotive, and construction applications. All measurements are in millimeters.

★ = most common sizes. Clearance hole is medium fit (ISO 286). Tap drill produces ~75% thread engagement — standard for most materials.

How to Measure a Metric Bolt You've Already Got

The quickest way to identify an unknown metric bolt is with digital calipers. Three measurements are enough:

1. Outer diameter (OD): Open the calipers and measure across the thread crests — not the root valleys. This gives you the nominal size. 6.0mm = M6, 8.0mm = M8, etc.
2. Thread pitch: Lay the bolt against a ruler and count how many thread crests span 10mm of shank. Divide 10 by that count to get pitch in mm. Alternatively, use a thread pitch gauge — drop the blade over the threads until one fits flush.
3. Length: Measure from under the head (hex/button/pan/socket cap) or full length (flat/countersunk head) to the tip.

If you don't have calipers, our ScrewBoltSize identifier tool lets you pick head type and drive type, enter diameter, and find the match — it even has an on-screen calibration ruler for rough sizing without tools.

Tap Drill vs Clearance Hole: What's the Difference?

People often confuse these two drill sizes, and using the wrong one ruins the part.

Tap drill creates the hole you cut threads into with a tap. It's undersized relative to the bolt so the tap has material to cut. If you're tapping a hole in aluminum, steel, or plastic, use the tap drill size from the chart above.

Clearance hole is the hole through which the bolt passes freely without threading. It should be slightly larger than the bolt's nominal diameter. The chart lists medium-fit clearance holes — roughly 10% oversized. If you're assembling two plates where the bolt passes through one and threads into the other, the clearance hole goes in the first plate.

Metric vs SAE: Dangerous Near-Matches to Avoid

Some metric and SAE sizes are close enough in diameter to cause confusion — but the thread pitches are always different, making substitution impossible without damaging the mating thread.

The M8 vs 5/16 pairing is the most dangerous: the 0.06mm diameter difference means an M8 bolt can often be forced into a 5/16 hole. It will feel like it's threading — but it's cross-threading and destroying both parts. When in doubt, use a thread pitch gauge before assuming compatibility.

Fine Thread Metric Sizes

Fine thread variants use a smaller pitch for the same nominal diameter. They're used in applications requiring vibration resistance, thin-walled sections, or fine adjustment (e.g., automotive, aerospace, precision machinery). The designation explicitly states the non-standard pitch: M10 × 1.0 vs the coarse M10 × 1.5.

Common fine-thread variants:

• M6 × 0.75 (vs coarse 1.0)
• M8 × 1.0 (vs coarse 1.25)
• M10 × 1.0 or M10 × 1.25 (vs coarse 1.5)
• M12 × 1.25 or M12 × 1.5 (vs coarse 1.75)

If a part specifies a pitch you don't recognize, check whether it's a fine thread before ordering. Fine and coarse threads are not interchangeable even when the nominal diameter matches.

Metric Bolt Property Classes (Grade Equivalents)

Metric bolt strength is specified as a "property class" rather than a grade. The two numbers in the class (e.g., 8.8, 10.9, 12.9) encode the tensile strength and yield ratio:

• 4.8 — Low-strength general purpose (common in hardware store bolts)
• 8.8 — Medium-strength structural (SAE Grade 5 equivalent)
• 10.9 — High-strength (SAE Grade 8 equivalent)
• 12.9 — Ultra-high strength (for demanding mechanical assemblies)

The class is stamped on the bolt head. For load-bearing applications, confirm the class matches your specification — a 4.8 bolt installed in an 8.8 position will fail at lower load. For general household and furniture use, 8.8 is the default to reach for.