How is an ISO metric thread designated?

By the letter M, the nominal diameter and, for fine pitch, the pitch in millimeters — for example M10 (coarse) or M10×1.25 (fine). The profile has a 60° flank angle per ISO 68-1, and tolerance classes such as 6H (internal) and 6g (external) follow ISO 965.

What is the difference between coarse and fine metric pitch?

Coarse pitch is the default for a given diameter and is more tolerant of damage and faster to assemble. Fine pitch has more threads per length, giving greater tensile strength, finer adjustment and better resistance to loosening under vibration, at the cost of easier cross-threading.

What is the tap drill size for a metric thread?

For a standard 60° metric thread the tap drill is approximately the nominal diameter minus the pitch (drill ≈ D − P). The table lists an approximate tap-drill value per size; always confirm against the required thread-engagement percentage for your material.

Thread Tables

ISO 261 / ISO 262

ISO Metric Thread

The global fastening standard — 60° angle, pitch in mm

ISO metric thread is the world's most widely used fastening system. It specifies pitch in millimeters, has a triangular profile with 60° flank angle, and uses the M designation followed by nominal diameter (e.g. M10) or M-diameter × pitch (e.g. M10×1.25 for fine pitch).

Dimension table

Designation	Series	Pitch (mm)	d₂ pitch (mm)	D₁ int (mm)	Tap drill approx.
M1.6	grueso/coarse	0.35	1.373	1.221	1.25 mm
M2	grueso/coarse	0.40	1.740	1.567	1.60 mm
M2.5	grueso/coarse	0.45	2.208	2.013	2.05 mm
M3	grueso/coarse	0.50	2.675	2.459	2.50 mm
M3.5	grueso/coarse	0.60	3.110	2.851	2.90 mm
M4	grueso/coarse	0.70	3.545	3.242	3.30 mm
M5	grueso/coarse	0.80	4.480	4.134	4.20 mm
M6	grueso/coarse	1.00	5.350	4.917	5.00 mm
M8	grueso/coarse	1.25	7.188	6.647	6.80 mm
M10	grueso/coarse	1.50	9.026	8.376	8.50 mm
M12	grueso/coarse	1.75	10.863	10.106	10.20 mm
M14	grueso/coarse	2.00	12.701	11.835	12.00 mm
M16	grueso/coarse	2.00	14.701	13.835	14.00 mm
M18	grueso/coarse	2.50	16.376	15.294	15.50 mm
M20	grueso/coarse	2.50	18.376	17.294	17.50 mm
M22	grueso/coarse	2.50	20.376	19.294	19.50 mm
M24	grueso/coarse	3.00	22.051	20.753	21.00 mm
M27	grueso/coarse	3.00	25.051	23.753	24.00 mm
M30	grueso/coarse	3.50	27.727	26.211	26.50 mm
M33	grueso/coarse	3.50	30.727	29.211	29.50 mm
M36	grueso/coarse	4.00	33.402	31.670	32.00 mm
M39	grueso/coarse	4.00	36.402	34.670	35.00 mm
M42	grueso/coarse	4.50	39.077	37.129	37.50 mm
M45	grueso/coarse	4.50	42.077	40.129	40.50 mm
M48	grueso/coarse	5.00	44.753	42.588	43.00 mm
M52	grueso/coarse	5.00	48.753	46.588	47.00 mm
M56	grueso/coarse	5.50	52.428	50.046	50.50 mm
M60	grueso/coarse	5.50	56.428	54.046	54.50 mm
M64	grueso/coarse	6.00	60.103	57.505	58.00 mm
M6×0.75	fino/fine	0.75	5.513	5.188	5.25 mm
M8×1.00	fino/fine	1.00	7.350	6.917	7.00 mm
M10×1.25	fino/fine	1.25	9.188	8.647	8.75 mm
M12×1.25	fino/fine	1.25	11.188	10.647	10.75 mm
M12×1.50	fino/fine	1.50	11.026	10.376	10.50 mm
M16×1.50	fino/fine	1.50	15.026	14.376	14.50 mm
M20×1.50	fino/fine	1.50	19.026	18.376	18.50 mm
M24×2.00	fino/fine	2.00	22.701	21.835	22.00 mm

Formulas ISO 68-1: d₂ = d − 0.6495 × P · D₁ = d − 1.0825 × P. Coarse pitch per ISO 261/262. Tap drill = D₁ rounded up to nearest standard drill size.

Key technical characteristics

Flank angle: 60°
Pitch: In millimeters (mm)
Profile: Triangular (ISO 68-1)
Designation: M + diameter (+ × pitch for fine)
Main sizes: M1 – M300 (ISO 261); preferred ISO 262: M3 – M64
Tolerances: ISO 965-1 (classes 6H/6g most common)

Typical applications

Structural fasteners in machinery, automotive and electronics.
Internal threading in aluminum and steel CNC plates.
Workpiece clamping in precision assemblies.
Standard pitch for coarse thread, fine pitch for vibration environments.

Compatibility and interoperability

✗NOT compatible with UNC/UNF thread even when nominal diameter matches — pitch and tolerance systems are different.
✓Interchangeable between manufacturers complying with ISO 261/ISO 965 in the same tolerance class (6H/6g).
✓Fine metric thread (M10×1 vs standard M10×1.5) uses the same profile but different pitch — always verify pitch when ordering tooling.

Selection guide

01For standard applications use coarse pitch — higher tensile load resistance and more alignment-tolerant.
02Use fine pitch for vibration applications (automotive, aerospace) where torque retention is critical.
03For soft materials (aluminum, plastic) consider threaded inserts (Helicoil) with 6H hole tolerance.
04Class 6H/6g is the standard production combination; 4H/4h is more precise (instrumentation).

Back to Thread Tables