Metric Thread Chart

A Comprehensive Guide Metric Bolt Sizes

Introduction to Metric Thread Charts

A Metric thread dimensions chart is an essential reference tool for engineers, machinists, and DIY enthusiasts working with fasteners like screws, bolts, and nuts. This standardized chart provides critical dimensions for ISO metric screw threads, ensuring compatibility and precision in manufacturing and assembly.

With globalization driving the adoption of the metric system, understanding how to interpret these charts is vital for industries ranging from automotive to aerospace.

What Are Metric Threads?

Metric bolt dimensions, standardized under ISO 68-1 and ISO 261, are the most widely used thread type globally. Characterized by their 60-degree symmetrical V-shape, these threads are defined by two primary measurements: major diameter (outer thread width) and pitch (distance between adjacent threads). Unlike imperial threads, metric variants use millimeters, simplifying calculations in metric-based industries.

Matric Thread Chart

Simple Thread Designation	Thread Designation	Pitch (MM)	Basic Major Diameter (External Threads)	Basic Minor Diameter (Internal Threads)
M1	M1x0.25	0.25	0.982	0.729
M2	M2x0.4	0.4	1.981	1.567
M3	M3x0.5	0.5	2.980	2.459
M4	M4x0.7	0.7	3.978	3.242
M5	M5x0.8	0.8	4.976	4.134
M6	M6x1	1	5.974	4.917
M7	M7x1	1	6.974	5.917
M8	M8x1.25	1.25	7.972	6.647
M9	M9x1.25	1.25	8.972	7.647
M10	M10x1.5	1.5	9.968	8.376
M12	M12x1.75	1.75	11.970	10.106
M14	M14x2	2	13.960	11.835
M16	M16x2	2	15.960	13.835
M18	M18x2.5	2.5	17.960	15.294
M20	M20x2.5	2.5	19.960	17.294
M22	M22x2.5	2.5	21.960	19.294
M24	M24x3	3	23.950	20.752
M27	M27x3	3	26.950	23.752
M30	M30x3.5	3.5	29.950	26.211
M33	M33x3.5	3.5	32.970	29.211
M36	M36x4	4	35.940	31.670
M39	M39x4	4	38.940	34.670
M42	M42x4.5	4.5	41.940	37.129
M45	M45x4.5	4.5	44.940	40.129
M48	M48x5	5	47.930	42.587
M52	M52x5	5	51.930	46.587
M56	M56x5.5	5.5	55.930	50.046

Matric Thread Chart.PDF

Key Features of ISO Metric Screw Threads

Major Diameter (D): The largest diameter of the thread.
Pitch (P): Measured in millimeters between thread crests.
Tolerance Grades: Indicates thread fit precision (e.g., 6g for external threads).

Components of a Metric Thread Chart

A metric thread chart organizes data into columns and rows, detailing specifications for various thread sizes. Below are the core elements:

1. Major Diameter (D)

The major diameter is the outermost measurement of the thread, critical for determining bolt-hole sizes. For example, an M6 bolt has a major diameter of 6 mm.

2. Pitch (P)

Pitch defines the thread’s coarseness or fineness. A smaller pitch (e.g., M6x1) means tighter threads, while a larger pitch (M6x1.5) offers faster assembly.

3. Tapping Drill Size

This specifies the drill bit diameter required to create a hole before threading. For an M6x1 thread, the tapping drill size is typically 5 mm.

4. Thread Tolerance Grades

Tolerance classes like 6H (internal) or 6g (external) define how tightly threads fit. Tighter tolerances (e.g., 4H) are used in high-precision applications.

How to Read a Metric Thread Chart

Let’s decode a sample entry: M10x1.5-6g.

M10: Major diameter = 10 mm.
1.5: Pitch = 1.5 mm between threads.
6g: Tolerance grade for external threads.

Step-by-Step Interpretation

Identify the thread designation (e.g., M8, M12).
Locate the pitch and tolerance in adjacent columns.
Cross-reference tapping drill sizes for machining.

Applications of Metric Thread Charts

From automotive engines to aerospace components, metric threads ensure global compatibility. Common uses include:

Machinery Manufacturing: Standardizing bolt sizes for assembly lines.
Construction: Structural fasteners in metric-based countries.
Consumer Electronics: Precision threads in devices like smartphones.

Advantages of Using Metric Threads

Global Standardization: Simplifies international trade and repair.
Simplified Calculations: Metric units align with CAD software and engineering designs.
Wide Availability: Charts and tools are accessible across industries.

ISO Standards and Thread Fit Classifications

Adherence to ISO 965-1 ensures consistent thread fits:

Fine vs. Coarse Threads: Fine threads (e.g., M6x0.75) offer higher tensile strength.
Fit Types: Loose (e.g., 7g/7H) vs. tight (4g/4H) for varying applications.

Tips for Selecting the Right Metric Thread Tables

Match the thread to the application’s load and stress requirements.
Use finer pitches for vibration resistance.
Verify tolerance grades to avoid assembly issues.

FAQs

A metric thread chart is a reference table that lists ISO metric screw thread sizes with their nominal (major) diameter, pitch, pitch diameter, minor diameter, and recommended tap drill size, all in millimetres. ISO metric threads use a 60° thread angle and follow ISO 68-1 (basic profile), ISO 261 (general plan), and ISO 965 (tolerances). Engineers, machinists, and procurement teams use the chart to specify fasteners, size tapped holes, and check that mating parts are compatible.

“M” means an ISO metric thread.
“10” is the nominal major diameter in millimetres.
“1.5” is the pitch in millimetres, which is the distance between adjacent thread crests.

When no pitch is given, as in “M10”, coarse pitch is assumed. A full designation can also include the tolerance class, for example M10 x 1.5-6g for an external thread or M10 x 1.5-6H for an internal thread. A left-hand thread adds “LH”, for example M10 x 1.5-LH.

The difference is the pitch. Coarse pitch is the default standard for general fasteners. Fine pitch uses a smaller pitch on the same diameter, for example M10 x 1.5 (coarse) versus M10 x 1.25 or M10 x 1.0 (fine).

Size	Coarse pitch (mm)	Common fine pitch (mm)
M6	1.0	0.75
M8	1.25	1.0
M10	1.5	1.25 / 1.0
M12	1.75	1.5 / 1.25
M16	2.0	1.5
M20	2.5	1.5 / 2.0
M24	3.0	2.0

Coarse threads assemble faster, resist cross-threading, and tolerate dirt and damage. They are used in general machinery and structural work.
Fine threads have a larger stress area and finer adjustment. They resist vibration loosening better and suit thin walls, hydraulics, automotive, and precision assemblies.

Use Tap drill size = Major diameter − Pitch.

M10 x 1.5 → 10 − 1.5 = 8.5 mm drill
M12 x 1.75 → 12 − 1.75 = 10.2 mm drill
M8 x 1.0 (fine) → 8 − 1.0 = 7.0 mm drill

This formula gives roughly 75% thread engagement and works for most cutting taps in steel and stainless steel. Thread-forming (roll) taps need a larger drill, so check the tap manufacturer’s chart for those.

They are ISO 965 tolerance classes that define the fit between mating threads:

6H is the internal thread (nut or tapped hole). It has zero fundamental deviation, meaning no allowance.
6g is the external thread (bolt or screw). It has a small allowance that leaves clearance for plating and easy assembly.

The number gives the tolerance grade, where 6 is the standard medium grade. The letter gives the tolerance position: uppercase letters are for internal threads and lowercase letters are for external threads. 6H/6g is the default general-purpose fit. 5H/4h is a tighter, precision fit, and 7H/8g is a looser fit for coated or rough-service parts.

Feature	ISO Metric	Unified (UNC/UNF)
Thread angle	60°	60°
Diameter unit	Millimetres	Inches
Pitch expressed as	mm between crests	Threads per inch (TPI)
Example	M10 x 1.5	3/8″-16 UNC
Governing standard	ISO 68-1, ISO 261, ISO 965	ASME B1.1

Both systems share a 60° profile, but the two are not interchangeable. Their diameters and pitches differ, so mixing them causes cross-threading and joint failure. Metric threads dominate worldwide, while UNC/UNF threads remain common in US-origin equipment.

Conclusion

Mastering the metric thread Mastering the metric thread chart is crucial for precision in engineering and manufacturing. Professionals can ensure reliable, standardized outcomes by understanding its components—from primary diameter to thread tolerance grades. Always consult the chart to select the optimal thread for your project.