Introduction
Precise threading is crucial in manufacturing. To achieve accuracy and optimal function, engineers and designers need to understand screw threads design requirements and standards. While different types of threads may look similar, they may not be compatible, affecting your project. For example, NPT external threads are not compatible with metric internal threads. A metric screw size chart can help in identifying and matching screw components during procurement and assembly.
This article focuses on one of the most widely adopted screw threads standards worldwide – the iso metric screw thread standard. We’ll explore the fundamentals of metric fasteners as well as the metric thread chart.
What is a Metric Thread?
A metric thread is the most common general-purpose thread type. It features a cylindrical shape and is measured in millimeters (mm). The nominal diameter is used alongside the thread pitch and is essential for defining screw and bolt threads. Controlled by the International Organization for Standardization (ISO), the metric thread standard is used to define screw and bolt threads worldwide. This comprehensive system facilitates precise thread design and compatibility.
Metric Thread Size Explained
Metric thread sizes are usually indicated by nominal diameter, pitch, and length. These dimensions are all in millimeters (mm).
Metric threads are either categorized as fine or coarse depending on their pitch. Coarse threads are more common than their fine counterparts and are assumed if the pitch is not given.
Let’s consider a metric thread of size M10-2.0 x 15. Here is a breakdown of these thread dimensions.
M – Metric thread designation
10 – Nominal outer diameter in millimeters
2.0 – Thread pitch in millimeters
15 – Thread length in millimeters
A larger diameter points to a larger bolt or screw. As the thread pitch increases, you get a coarser thread (fewer threads per millimeter).
Metric Thread Dimension Properties
The size of a screw thread is generally measured and specified using the geometric dimensions of its tooth profile. These dimensional parameters include pitch, major diameter, minor diameter, and thread angle. A metric screw size chart can help in visualizing these geometric dimensions of screw threads.
Major Diameter (mm) Explained
The major diameter refers to the largest diameter of a threaded element and is equal to the distance between opposite crests (or tips) in a thread profile. It is measured in millimeters and this crucial dimension dictates all metric bolt sizes. The major diameter can be accurately measured using vernier calipers. The nominal diameter is often used interchangeably with the major diameter in some contexts and is a key component in standardized thread specifications according to ISO standards.
Minor Diameter (mm) Explained
The minor diameter is the smallest diameter of a threaded cylinder or the distance between opposite roots of a thread profile. This critical parameter is specified in millimeters and serves as a measure of the strength and quality of a thread. There are several methods of measuring the minor diameter of a thread. They include;
Optical measurement using profile projectors
Specialized tooling such as micrometer depth gauge
Additionally, the nominal diameter is a key dimension in thread specifications, used alongside the thread pitch and crucial in standardized thread specifications according to ISO standards.
Thread Pitch (mm) Explained
Pitch refers to the distance between two consecutive crests of a screw thread, measured in millimeters (mm). It’s typically obtained by measuring the distance between multiple crests and dividing it by the number of spaces between them. Trying to directly measure the distance between two succeeding peaks using a ruler can be challenging since threads are often tiny. Alternatively, pitch can be measured using specialized equipment such as pitch gauges and vision systems.
Depending on the pitch size, screw threads can be grouped into two categories: coarse threads and fine threads. Fine threads feature a smaller pitch and shallow ridges while coarse threads inherently have a larger pitch and deep ridges. Thread pitch is a crucial dimension that impacts the mechanical properties of a thread as well as its compatibility with mating components. The nominal diameter is used alongside the thread pitch in thread specifications and is a crucial component in standardized thread specifications according to ISO standards.
Root Radius Explained
The root of a screw thread is the bottom surface that connects the two flanks of the thread, whether internal or external. The root radius is simply the radius of this rounded surface.
Additionally, the nominal diameter is a key dimension in thread specifications, used alongside the thread pitch and crucial in standardized thread specifications according to ISO standards.
Thread Angle Explained
Thread angle refers to the angle between the flanks of the thread. This crucial component is a defining factor for the engagement and tightness of the threads.
Additionally, the nominal diameter is a key dimension in thread specifications, used alongside the thread pitch and essential for standardized thread specifications according to ISO standards.
Clearance Hole Diameter (mm) Explained
This is the diameter of a hole that is created through an object and enables a screw or bolt to pass through without the threads engaging with the material. In other words, a clearance hole is slightly larger than the fastener’s diameter, facilitating easy assembly and disassembly.
Clearance hole diameter can be measured using either digital calipers or pin gauges. The nominal diameter is a key dimension in thread specifications, used alongside the thread pitch and crucial in standardized thread specifications according to ISO standards.
Metric Thread Chart: Coarse Pitch (M1-M48)
The metric thread size chart specifies thread dimensions such as pitch, major and minor diameters, tolerances, and tap drill. Here are some of the most commonly used coarse threads up to M68. A metric screw size chart can help in visualizing the dimensions of coarse pitch threads.
Thread size Major diameter Pitch Pitch diameter Minor diameter Minor diameter Tap drill (male thread) (female thread)
| Thread | Major Diameter (d=D) | Pitch (p) | Root Radius(r) | Pitch Diameter (d2=D2) | Minor Diameter Male Thread (d3) | Minor Diameter Female Thread (D1) | Thread Height Male Thread (h3) | Thread Height Female Thread (H1) | Tap Drill Diameter |
| M1 | 1 | 0.25 | 0.036 | 0.838 | 0.693 | 0.729 | 0.153 | 0.135 | 0.75 |
| M1.1 | 1.1 | 0.25 | 0.036 | 0.938 | 0.793 | 0.829 | 0.153 | 0.135 | 0.85 |
| M1.2 | 1.2 | 0.25 | 0.036 | 1.038 | 0.893 | 0.929 | 0.153 | 0.135 | 0.95 |
| M1.4 | 1.4 | 0.3 | 0.043 | 1.205 | 1.032 | 1.075 | 0.184 | 0.162 | 0.162 |
| M1.6 | 1.6 | 0.35 | 0.051 | 1.373 | 1.171 | 1.221 | 0.215 | 0.189 | 1.25 |
| M1.8 | 1.8 | 0.35 | 0.051 | 1.573 | 1.371 | 1.421 | 0.215 | 0.189 | 1.45 |
| M2 | 2 | 0.4 | 0.058 | 1.74 | 1.509 | 1.567 | 0.245 | 0.217 | 1.6 |
| M2.2 | 2.2 | 0.45 | 0.065 | 1.908 | 1.648 | 1.713 | 0.276 | 0.244 | 1.75 |
| M2.5 | 2.5 | 0.45 | 0.065 | 2.208 | 1.948 | 2.013 | 0.276 | 0.276 | 2.05 |
| M3 | 3 | 0.5 | 0.072 | 2.675 | 2.387 | 2.459 | 0.307 | 0.271 | 2.5 |
| M3.5 | 3.5 | 0.6 | 0.087 | 3.11 | 2.764 | 2.85 | 0.368 | 0.325 | 2.9 |
| M4 | 4 | 0.7 | 0.101 | 3.545 | 3.141 | 3.141 | 0.429 | 0.379 | 3.3 |
| M4.5 | 4.5 | 0.75 | 0.108 | 4.013 | 3.58 | 3.688 | 0.46 | 0.406 | 3.8 |
| M5 | 5 | 0.8 | 0.115 | 4.48 | 4.019 | 4.134 | 0.491 | 0.433 | 4.2 |
| M6 | 6 | 1 | 0.144 | 5.35 | 4.773 | 4.917 | 0.613 | 0.541 | 5 |
| M8 | 8 | 1.25 | 0.18 | 7.188 | 6.466 | 6.647 | 0.767 | 0.677 | 6.8 |
| M10 | 10 | 1.5 | 0.217 | 9.026 | 8.16 | 8.376 | 0.92 | 0.812 | 8.5 |
| M12 | 12 | 1.75 | 0.253 | 10.863 | 9.853 | 10.106 | 1.074 | 0.947 | 10.2 |
| M14 | 14 | 2 | 0.289 | 12.701 | 11.564 | 11.835 | 1.227 | 1.083 | 12 |
| M16 | 16 | 2 | 0.289 | 14.701 | 13.546 | 13.835 | 1.227 | 1.083 | 14 |
| M18 | 18 | 2.5 | 0.361 | 16.376 | 14.933 | 15.394 | 1.534 | 1.353 | 15.5 |
| M20 | 20 | 2.5 | 0.361 | 18.376 | 16.933 | 17.294 | 1.534 | 1.353 | 17.5 |
| M22 | 22 | 2.5 | 0.361 | 20.376 | 18.933 | 19.294 | 1.534 | 1.353 | 19.5 |
| M24 | 24 | 3 | 0.433 | 22.051 | 20.319 | 20.319 | 1.84 | 1.624 | 21 |
| M20 | 20 | 2.5 | 0.361 | 18.376 | 16.933 | 17.294 | 1.534 | 1.353 | 17.5 |
| M27 | 27 | 3 | 0.433 | 25.051 | 23.319 | 23.752 | 1.84 | 1.624 | 24 |
| M30 | 30 | 3.5 | 0.505 | 27.727 | 25.706 | 26.211 | 2.147 | 1.894 | 26.5 |
| M33 | 33 | 3.5 | 0.5051 | 30.727 | 28.706 | 29.211 | 2.147 | 1.894 | 2.147 |
| M36 | 36 | 4 | 0.577 | 33.402 | 31.093 | 31.67 | 2.454 | 2.165 | 32 |
| M39 | 39 | 4 | 0.577 | 36.402 | 34.093 | 34.67 | 2.454 | 2.165 | 35 |
| M42 | 42 | 4.5 | 0.65 | 39.077 | 36.479 | 37.129 | 2.76 | 2.436 | 37.5 |
| M45 | 45 | 4.5 | 0.65 | 42.077 | 39.479 | 40.129 | 2.76 | 2.436 | 40.5 |
| M48 | 48 | 5 | 0.722 | 44.752 | 41.866 | 42.857 | 3.067 | 2.706 | 43 |
Advantages of Coarse Threads
Fast installation
One of the main benefits of coarse threads is the quick and simplified installation process. Thanks to the larger thread design, you can insert fasteners into materials without much hassle. Coarse threads are often preferred for applications that require speed and convenience.
Enhanced resistance to vibrations
Due to their larger pitch, coarse threads can absorb vibrations more efficiently. They provide stability and can endure vibrations without loosening.
Resilience
Coarser threads are tougher than fine pitch threads. They can withstand scratches and nicks and still function well.
Disadvantages of Coarse Threads
Limited strength
Compared to fine threads, coarse threads provide reduced tensile strength. Therefore, they’re not as suited to high-strength applications. When picking the ideal thread for your project, you need to take into account the specific load requirements.
Reduced holding power
Another disadvantage of coarse threads is their limited holding power. As the thread pitch increases, the level of grip reduces. This means that coarse threads may not be a good choice in applications where secure connections are crucial.
Metric Thread Chart: Fine Pitch (M1-M24)
Thread size Major diameter Pitch Pitch diameter Minor diameter Minor diameter Tap drill(male thread) (female thread)
| Thread Size | Major Diameter d=D | Pitch (p) | Root Radius(r) | Pitch Diameter (d2=D2) | Minor Diameter Male Thread (d3) | Minor Diameter Female Thread (D1) | Thread Height Male Thread (h3) | Thread Height Female Thread (H1) | Tap Drill Diameter |
| M1.0 x 0.2 | 1 | 0.2 | 0.029 | 0.87 | 0.755 | 0.783 | 0.123 | 0.108 | 0.8 |
| M1.1 x 0.2 | 1.1 | 0.2 | 0.029 | 0.97 | 0.855 | 0.883 | 0.123 | 0.108 | 0.9 |
| M1.2 x 0.2 | 1.2 | 0.2 | 0.029 | 1.07 | 0.955 | 0.983 | 0.123 | 0.108 | 1 |
| M1.4 x 0.2 | 1.4 | 0.2 | 0.029 | 1.27 | 1.155 | 1.183 | 0.123 | 0.108 | 1.2 |
| M1.6 | 1.6 | 0.2 | 0.029 | 1.47 | 1.355 | 1.383 | 0.123 | 0.108 | 1.4 |
| M1.8 | 1.8 | 0.2 | 0.029 | 1.67 | 1.555 | 1.583 | 0.123 | 0.108 | 1.6 |
| M2 x 0.25 | 2 | 0.25 | 0.036 | 1.838 | 1.693 | 1.729 | 0.153 | 0.135 | 1.75 |
| M2.2 | 2.2 | 0.25 | 0.036 | 2.038 | 1.893 | 1.929 | 0.153 | 0.135 | 1.95 |
| M2.5 | 2.5 | 0.35 | 0.051 | 2.273 | 2.071 | 2.121 | 0.215 | 0.189 | 2.1 |
| M3 | 3 | 0.35 | 0.051 | 2.773 | 2.571 | 2.621 | 0.215 | 0.189 | 2.6 |
| M3.5 | 3.5 | 0.35 | 0.051 | 3.273 | 3.071 | 3.121 | 0.215 | 0.189 | 3.1 |
| M4 | 4 | 0.5 | 0.072 | 3.675 | 3.387 | 3.459 | 0.307 | 0.271 | 3.5 |
| M4.5 | 4.5 | 0.5 | 0.072 | 4.175 | 3.887 | 3.959 | 0.307 | 0.271 | 4 |
| M5 x 0.5 | 5 | 0.5 | 0.072 | 4.675 | 4.387 | 4.459 | 0.307 | 0.271 | 4.5 |
| M5.5 x 0.5 | 5.5 | 0.5 | 0.072 | 5.175 | 4.887 | 4.959 | 0.307 | 0.271 | 5 |
| M6 x 0.75 | 6 | 0.75 | 0.108 | 5.513 | 5.08 | 5.188 | 0.46 | 0.406 | 5.2 |
| M8 x 0.75 | 8 | 0.75 | 0.108 | 7.513 | 7.08 | 7.188 | 0.46 | 0.406 | 7.2 |
| M8 x 1.0 | 8 | 1 | 0.144 | 7.35 | 6.773 | 6.917 | 0.613 | 0.541 | 7 |
| M10 x 0.75 | 10 | 0.75 | 0.108 | 9.513 | 9.08 | 9.188 | 0.46 | 0.406 | 9.2 |
| M10 x 1 | 10 | 1 | 0.144 | 9.35 | 8.773 | 8.917 | 0.613 | 0.541 | 9 |
| M10 x 1.25 | 10 | 1.25 | 0.18 | 9.188 | 8.466 | 8.647 | 0.767 | 0.677 | 8.8 |
| M12 x 1 | 12 | 1 | 0.144 | 11.35 | 10.773 | 10.917 | 0.613 | 0.541 | 11 |
| M12 x 1.25 | 12 | 1.25 | 0.18 | 11.188 | 10.466 | 10.647 | 0.767 | 0.677 | 10.8 |
| M12 x 1.5 | 12 | 1.5 | 0.217 | 11.026 | 10.16 | 10.376 | 0.92 | 0.812 | 10.5 |
| M14 x 1 | 14 | 1 | 0.144 | 13.35 | 12.773 | 12.917 | 0.613 | 0.541 | 13 |
| M14 x 1.25 | 14 | 1.25 | 0.18 | 13.188 | 12.466 | 12.647 | 12.647 | 0.677 | 12.8 |
| M14 x 1.5 | 14 | 1.5 | 0.217 | 13.026 | 12.16 | 12.376 | 0.92 | 0.92 | 12.5 |
| M15 x 1 | 15 | 1 | 0.144 | 14.35 | 13.773 | 13.917 | 0.613 | 0.541 | 0.541 |
| M15 x 1.5 | 15 | 1.5 | 0.217 | 14.026 | 13.16 | 13.376 | 0.92 | 0.812 | 13.5 |
| M16 x 1 | 16 | 1 | 0.144 | 15.35 | 14.773 | 14.917 | 0.613 | 0.541 | 15 |
| M16 x 1.5 | 16 | 1.5 | 0.217 | 15.026 | 14.16 | 14.376 | 0.92 | 0.812 | 14.5 |
| M18 x 1 | 18 | 1 | 0.144 | 17.35 | 16.773 | 16.917 | 0.613 | 0.541 | 17 |
| M18 x 1.5 | 18 | 1.5 | 0.217 | 17.026 | 16.16 | 16.376 | 0.92 | 0.812 | 16.5 |
| M18 x 2 | 18 | 2 | 0.289 | 16.701 | 15.546 | 15.835 | 1.227 | 1.083 | 16 |
| M20 x 1.0 | 20 | 1 | 0.144 | 19.35 | 18.773 | 18.917 | 0.613 | 0.541 | 19 |
| M20 x 1.5 | 20 | 1.5 | 0.217 | 19.026 | 18.16 | 18.376 | 0.92 | 0.812 | 18.5 |
| M20 x 2.0 | 20 | 2 | 0.289 | 18.701 | 17.546 | 17.835 | 1.227 | 1.083 | 18 |
| M22 x 1.0 | 22 | 1 | 0.144 | 21.35 | 20.773 | 20.917 | 0.613 | 0.541 | 21 |
| M22 x 1.5 | 22 | 1.5 | 0.217 | 21.026 | 20.16 | 20.376 | 0.92 | 0.812 | 20.5 |
| M22 x 2.0 | 22 | 2 | 0.289 | 20.701 | 19.546 | 19.835 | 1.227 | 1.083 | 20 |
| M24 x 1.0 | 24 | 1 | 0.144 | 23.35 | 22.773 | 22.917 | 0.613 | 0.541 | 23 |
| M24 x 1.5 | 24 | 1.5 | 0.217 | 23.026 | 22.16 | 22.376 | 0.92 | 0.812 | 22.5 |
Advantages of Fine Threads
Enhanced strength
One of the key benefits of fine threads is that they are stronger than coarse pitch threads in terms of both tension and shear. This is because a fine pitch thread has more threads per inch creating a larger surface area for load distribution. The improved stress distribution makes this thread type ideal for high-strength applications.
Increased holding power
Thanks to their smaller thread incline, fine threads have a tighter grip when fastened. Therefore, they create more secure connections that are less prone to loosening under load.
Precise Adjustments
Fine threads are best known for their precision. They allow for finer adjustments and are ideal for sensitive assemblies.
Disadvantages of Fine Threads
Challenging to install
Given their smaller size and tighter tolerances, finer threads are generally trickier to align and install. This is especially true in sections where access is limited. It’s recommended that you employ the right tools and techniques during installation to ensure proper alignment and prevent damage to the screws or bolts.
Poor resistance to vibrations
With fine pitch threads, there is a very small distance between threads. This means they have a reduced capacity to dampen vibrations compared to their coarse counterparts. As such, finer threads are not well-suited to applications where vibrations are an issue.
Long thread engagement
Due to their delicate nature, fine-threaded fasteners require longer thread engagements to prevent damage and fouling.