ISO Metric Thread Chart
Basic dimensions for ISO metric coarse pitch threads (ISO 724), from M1 to M68 — pitch, major, pitch and minor diameter, and recommended tap drill size.
ISO metric screw threads are the most widely used fastener standard in mechanical engineering, defined by ISO 724 (basic dimensions) and ISO 261 (general plan). The table below lists the coarse pitch series — the default choice for most bolts, screws and tapped holes — from M1 up to M68.
For each size you'll find the pitch, the major diameter (the nominal size), the pitch diameter used for thread fit and tolerance calculations, the minor diameter for both external (bolt) and internal (nut) threads, and the recommended tap drill diameter for roughly 75% thread engagement.
Quick reminder: M6 alone means a metric thread with 6 mm major diameter at its coarse pitch (1.00 mm here). Fine-pitch variants are written with an explicit pitch, e.g. M6 × 0.75.
| Size | Pitch (mm) | Major Ø (mm) | Pitch Ø (mm) | Minor Ø — bolt (mm) | Minor Ø — nut (mm) | Tap drill (mm) |
|---|---|---|---|---|---|---|
| M1 | 0.25 | 1.00 | 0.838 | 0.693 | 0.729 | 0.75 |
| M1.1 | 0.25 | 1.10 | 0.938 | 0.793 | 0.829 | 0.85 |
| M1.2 | 0.25 | 1.20 | 1.038 | 0.893 | 0.929 | 0.95 |
| M1.4 | 0.30 | 1.40 | 1.205 | 1.032 | 1.075 | 1.10 |
| M1.6 | 0.35 | 1.60 | 1.373 | 1.171 | 1.221 | 1.25 |
| M1.8 | 0.35 | 1.80 | 1.573 | 1.371 | 1.421 | 1.45 |
| M2 | 0.40 | 2.00 | 1.740 | 1.509 | 1.567 | 1.60 |
| M2.2 | 0.45 | 2.20 | 1.908 | 1.648 | 1.713 | 1.75 |
| M2.5 | 0.45 | 2.50 | 2.208 | 1.948 | 2.013 | 2.05 |
| M3 | 0.50 | 3.00 | 2.675 | 2.387 | 2.459 | 2.50 |
| M3.5 | 0.60 | 3.50 | 3.110 | 2.764 | 2.850 | 2.90 |
| M4 | 0.70 | 4.00 | 3.545 | 3.141 | 3.242 | 3.30 |
| M4.5 | 0.75 | 4.50 | 4.013 | 3.580 | 3.688 | 3.80 |
| M5 | 0.80 | 5.00 | 4.480 | 4.019 | 4.134 | 4.20 |
| M6 | 1.00 | 6.00 | 5.350 | 4.773 | 4.917 | 5.00 |
| M7 | 1.00 | 7.00 | 6.350 | 5.773 | 5.917 | 6.00 |
| M8 | 1.25 | 8.00 | 7.188 | 6.466 | 6.647 | 6.80 |
| M9 | 1.25 | 9.00 | 8.188 | 7.466 | 7.647 | 7.80 |
| M10 | 1.50 | 10.00 | 9.026 | 8.160 | 8.376 | 8.50 |
| M11 | 1.50 | 11.00 | 10.026 | 9.160 | 9.376 | 9.50 |
| M12 | 1.75 | 12.00 | 10.863 | 9.853 | 10.106 | 10.20 |
| M14 | 2.00 | 14.00 | 12.701 | 11.546 | 11.835 | 12.00 |
| M16 | 2.00 | 16.00 | 14.701 | 13.546 | 13.835 | 14.00 |
| M18 | 2.50 | 18.00 | 16.376 | 14.933 | 15.394 | 15.50 |
| M20 | 2.50 | 20.00 | 18.376 | 16.933 | 17.294 | 17.50 |
| M22 | 2.50 | 22.00 | 20.376 | 18.933 | 19.294 | 19.50 |
| M24 | 3.00 | 24.00 | 22.051 | 20.319 | 20.752 | 21.00 |
| M27 | 3.00 | 27.00 | 25.051 | 23.319 | 23.752 | 24.00 |
| M30 | 3.50 | 30.00 | 27.727 | 25.706 | 26.211 | 26.50 |
| M33 | 3.50 | 33.00 | 30.727 | 28.706 | 29.211 | 29.50 |
| M36 | 4.00 | 36.00 | 33.402 | 31.093 | 31.670 | 32.00 |
| M39 | 4.00 | 39.00 | 36.402 | 34.093 | 34.670 | 35.00 |
| M42 | 4.50 | 42.00 | 39.077 | 36.479 | 37.129 | 37.50 |
| M45 | 4.50 | 45.00 | 42.077 | 39.479 | 40.129 | 40.50 |
| M48 | 5.00 | 48.00 | 44.752 | 41.866 | 42.857 | 43.00 |
| M52 | 5.00 | 52.00 | 48.752 | 45.866 | 46.587 | 47.00 |
| M56 | 5.50 | 56.00 | 52.428 | 49.252 | 50.046 | 50.50 |
| M60 | 5.50 | 60.00 | 56.428 | 53.252 | 54.046 | 54.50 |
| M64 | 6.00 | 64.00 | 60.103 | 56.639 | 57.505 | 58.00 |
| M68 | 6.00 | 68.00 | 64.103 | 60.639 | 61.505 | 62.00 |
No thread size matches your search.
Values follow ISO 724 basic dimensions. Data provided for reference — always verify against the current standard for critical or safety-related applications.
Understanding the columns
- Major diameter
- The largest diameter of the thread — at the crest of an external (bolt) thread, or the root of an internal (nut) thread. This is the number in the designation, e.g. the "6" in M6.
- Pitch
- The distance between two adjacent thread crests, measured parallel to the thread axis. Determines how coarse or fine the thread is.
- Pitch diameter
- The diameter at which the thread groove and ridge widths are equal. It's the diameter used to define thread fit and tolerance class, and the most important dimension for gauging.
- Minor diameter
- The smallest diameter of the thread — at the root of an external thread, or the crest of an internal one. Bolt and nut values differ slightly to allow clearance.
- Tap drill
- The recommended drill diameter for the hole before tapping, sized for roughly 75% thread engagement — the standard used in general mechanical design.
Frequently asked questions
What does the "M" mean in a metric thread designation like M6?
The "M" indicates an ISO metric thread as defined by ISO 261. The number that follows (e.g. 6 in M6) is the major diameter in millimetres, measured over the crest of the external thread.
What is the difference between coarse and fine pitch threads?
Coarse pitch is the default, general-purpose series for a given diameter (e.g. M6 = 1.00 mm pitch) and is what "M6" alone refers to. Fine pitch threads use a smaller pitch for the same diameter and are written explicitly, e.g. "M6 × 0.75". Fine threads give a larger stress area and finer adjustment, at the cost of being more sensitive to cross-threading.
How do I choose the right tap drill size?
The tap drill column in this chart gives the diameter that produces approximately 75% thread engagement, the standard used for general mechanical design in most materials. For softer materials you may size up slightly; for harder materials or thin-wall parts you may need a percentage closer to 100%.
What is the difference between major, pitch and minor diameter?
Major diameter is the largest diameter of the thread, measured at the crest of an external thread or the root of an internal thread. Minor diameter is the smallest, at the root of an external thread or the crest of an internal one. Pitch diameter sits between the two, at the point where the thread groove and ridge are equal in width — it is the diameter used to check thread fit and tolerance class.
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