ISO Metric Screw Threads – Complete Guide

The ISO metric screw thread is the most commonly used type of threads in general purpose fastening.

It uses a simple naming system, a clear geometry, and a shared set of standards that make parts fit together predictably.

ISO metric threads are similar to the Unified Thread Standard (UTS) threads mostly used in North America (UNC and UNF), as both feature a 60° V profile. The key difference is that UTS is inch-based while ISO is metric-based, resulting in different measuring systems and non interchangeable parts.

So, different measuring systems.

ACME threads, with their 29° trapezoidal profile, are also a standardized category found in North America, but primarily used for power transmission in North America, not general purpose fastening.

ISO standards are widely used all around the world, and if you are not working in North America, there is a high chance that you will encounter ISO screws.

Don’t get me wrong, they are also increasingly adopted even in North America for industries like automotive and electronics, it’s just less common than anywhere else.

What is the ISO Standard ? What is it used for ?

When we talk about screws, bolts, and fasteners, one question quickly comes up: how do we make sure they all fit together, no matter where they are made?

The answer is: Standardization.

ISO threads are metric based standards that define the exact shape, angles and size you might work with on a screw or a bolt.

By following these rules, manufacturers everywhere can produce parts that match. A bolt made in Germany, for example, will screw perfectly into a nut made in Japan, because both follow the same standard.

Of course they both need to meet ISO requirements, such as those outlined in ISO 261 and ISO 724 for dimensions and ISO 68-1 for the basic profile.

By using ISO threads, industries across the world avoid unnecessary rework, reduce errors, and make supply chains far easier to manage. Whether in automotive, aerospace, or everyday manufacturing, the same rules apply, and that consistency saves both time and resources.

Today, the metric thread series holds the largest worldwide share for general purpose threads. This dominance is not a coincidence.

Basically, anywhere but in North America (where the Unified Thread Standard or UTS is more common), you will encounter ISO metric threads for general purpose fastening.

This dominance reflects an early international agreement to adopt a common metric system, followed by decades of successful use.

Basic Profile And Geometry

The shape of an ISO thread is based on a simple, symmetric V profile.

This “V” has an included angle of 60 degrees, which means both sides of the thread slope at the same angle.

To describe a thread, three main dimensions are important:

• Major diameter: the outermost point of the thread, at the crest.
• Minor diameter: the innermost point, at the root.
• Pitch: the distance from one crest to the next, measured along the axis of the screw. Thread Pitch is covered by ISO 724.

A useful way to imagine this is to think of a key fitting into a lock.

The ridges on the key (like the threads on a bolt) match the grooves inside the lock (like the threads in a nut).

Because both are cut to the same angle and shape, they slide together smoothly. The tiny gap between them, called the allowance or clearance, is carefully controlled by tolerance classes as specified in ISO 965, ensuring parts turn easily without jamming or excessive wobbling.

Heights, truncations, and practical numbers

In theory, the V shape of a thread has a certain height that depends on the pitch.

This geometric height (H) is exactly (√3/2) times the pitch, which approximates to 0.866 times the pitch.

The very top (exactly 1/8 of the height) and the very bottom (exactly 1/4 of the height) are truncated, creating flat crests and roots that make the thread stronger, less fragile, and easier to manufacture.

ISO 68-1 covers these requirements, including options for rounded roots on external threads with a minimum radius of 0.125 times the pitch.

This creates flat crests and roots, which makes the thread stronger, less fragile, and easier to manufacture.

After these adjustments, the effective thread depth is exactly 5/8 of H, which approximates to 0.541 times the pitch.

This number is important because it appears in both strength calculations and machining formulas.

Using this rule, the tap drill size is approximately the major diameter minus the pitch.

for example, an M10 screw with a coarse pitch of 1.5 mm gives a tap drill of about 8.5 mm. For precise work, confirm the exact size in ISO 965 tables for the required tolerance class.

External and Internal Thread Boundaries

When defining threads, it is important to understand the limits of size for bolts (external threads) and nuts (internal threads).

• For a bolt (external/male thread):
  1. The major diameter is the maximum limit. The crests of the thread must not go beyond this size.
  2. The minor diameter is also a maximum for the root of the thread. For external threads, the 2023 update to ISO 68-1 specifies a minimum root radius of 0.125 times the pitch to improve fatigue resistance.
• For a nut (internal/female thread):
  1. The logic is reversed. Here, the major diameter and the minor diameter are minimum limits.
  2. The thread form must at least reach these values, but it can be cut slightly deeper or rounded beyond them.

This explains two common observations:

First, when you measure across the threads of a bolt, the result is very close to the nominal major diameter (for example, an M10 bolt measures close to 10 mm).

Then, the clear bore of a nut reflects its minimum inner diameter, which is close to the internal minor diameter.

Pitch Diameter and Common Approximations

The pitch diameter is a key concept in thread geometry.

It is the imaginary cylinder where the flanks of the bolt and nut would meet with equal clearance. In other words, it lies halfway through the height of the engaged thread profile.

For the ISO metric thread form, some useful approximations can be made for a basic profile:

Pitch diameter (d₂):

where (D) is the major diameter and (P) is the pitch.

Minor diameter of an external thread (d₃):

These formulas give quick estimates without needing to look up full tables. The exact limits, however, depend on the tolerance class defined in ISO 965, which specifies the allowed clearances and allowances for different applications, such as 6g for external threads in general use.

Even though these are approximations, they are very practical. They are often used for quick strength checks, fit evaluations, or sanity checks during design and machining, when a full set of reference data is not immediately available.

Designation and How to Read It

The designation of an ISO metric thread always begins with the letter M, which indicates that it is a metric thread.

This is followed by the nominal diameter in millimetres.

If the pitch is not the standard coarse pitch, it is written after the diameter. A separator can be either a dash (–) or a multiplication sign (×). both are commonly used, so be aware that means the same thing.

For example: M8×1.25 or M8–1.25 both describe an 8 mm thread with a 1.25 mm pitch.

When the coarse pitch is used, the pitch value is usually omitted.

In that case, M8 by itself means an 8 mm thread with the standard coarse pitch for that size. (which is pitch = 1.25 mm)

If the length of the screw or bolt is specified, it follows after another separator. Once again it can be both symbols.

For example: M8×1.25×30 indicates an 8 mm diameter, 1.25 mm pitch, and 30 mm length.

In many catalogues, the pitch is left out when it is coarse, so you might see M8×30. In that case, the context makes it clear that the coarse pitch is implied.

Tolerance classes and fit

In ISO metric threads, tolerances are used to control how tightly or loosely a bolt and nut will fit together. It is defined in ISO 965 (parts 1 to 5), which sets the principles, limits of size, and special cases.

These tolerances are written after the thread designation when needed, and they consist of a number and a letter:

The number indicates the tolerance grade, which defines how wide the tolerance band is (a smaller number means a tighter tolerance).

The letter indicates the tolerance position, also called the fundamental deviation.

• External threads (bolts) use lowercase letters such as g or h.
• Internal threads (nuts) use uppercase letters such as G or H.

A very common combination can be : External : 6g – Internal 6H.

This pairing is widely used because it provides a reliable fit for most applications.

But many other combinations exist.

Some are designed for tighter or looser fits, some account for special coatings, and others are tailored to specific industries.

One important case is hot-dip galvanizing. Since the coating adds thickness to the threads, the standards include special tolerance classes for bolts that will be galvanized, along with matching nuts that are sized to fit after the coating is applied.

In these cases, the letter positions reflect whether the measurement is taken before or after coating, so it is essential to check the correct part of the standard.

Preferred Sizes and Pitch Series

The set of preferred metric thread sizes is not random, it follows a structured plan.

The full list of combinations is given in ISO 261, while ISO 262 defines a shorter selection of the most commonly used sizes for screws, bolts, and nuts.

The values are based on the Renard series, a system that spaces numbers in a rounded geometric progression.

This approach ensures that the steps between sizes feel natural and practical and avoiding unnecessary overlaps.

For each nominal diameter, the coarse pitch is the default choice. Coarse threads are easier to start, less likely to be damaged, and suitable for most general applications.

However,

Many diameters also have one or two fine pitch options, and in some cases even extra fine (or “superfine”) pitches. These finer threads are used in more specialized situations.

Why choose a fine pitch when coarse threads are simpler and more robust?

Three main reasons explain their use:

• Stronger core: For the same nominal diameter, a fine thread leaves a larger core area in the bolt, which can improve strength.
• Better vibration resistance: Fine threads are slightly less prone to loosening under vibration.
• Thin-wall applications: In parts with limited thickness, a coarse thread might break through the wall, while a fine thread allows more controlled engagement.

Concrete Examples by Size

Looking at specific thread sizes helps to show how the system works.

• M6: The coarse pitch is 1.0 mm, with a fine option of 0.75 mm.
• M8: The coarse pitch is 1.25 mm, with fine options of 1.0 mm and 0.75 mm.
• M10: The coarse pitch is 1.5 mm, with fine options of 1.25 mm and 1.0 mm.
• M20: The coarse pitch increases to 2.5 mm, with fine options around 2.0 mm or 1.5 mm, depending on the series.

This pattern shows that as the nominal diameter increases, the coarse pitch also increases. Larger screws need deeper threads to maintain strength and engagement.

At the other end of the scale, very small diameters use pitches below one millimeter.

For example:

• M2: The coarse pitch is 0.4 mm, with a fine option of 0.25 mm.

In every case, the goal is the same: to balance strength, engagement depth, and ease of manufacturing.

Coarse threads are the default for general use, while fine and extra-fine pitches are chosen when specific design needs (such as vibration resistance or thin walls) make them more suitable.

Here is the complete table:

Standards That Define The System

The ISO metric thread system is defined by a set of international standards that cover the profile, the size series, and the tolerance rules.

Thread profile: The basic geometry of the thread the 60° V-shape, the truncations at crest and root, and the fundamental proportions. All of those are defined in ISO 68-1 (ISO general purpose screw threads – Basic profile).

Diameters and pitches: The full range of preferred combinations is listed in ISO 261 (General purpose metric screw threads – General plan).

A shorter, practical selection of the most commonly used sizes for screws, bolts, and nuts is given in ISO 262 (Selected sizes for screws, bolts and nuts). Together, these two standards define what is normally available in stock.

Tolerances and fits: These are covered in ISO 965 (General purpose metric screw threads – Tolerances).

• Part 1: Principles and basic data
• Part 2: Limits of sizes for general-purpose external and internal threads
• Part 3: Deviations for constructional screw threads
• Part 4: Limits of sizes for hot dip galvanized external threads
• Part 5: Limits of sizes for internal threads to accommodate hot dip galvanized external threads

These documents ensure that threads made in different countries and industries will fit together reliably.

National and regional standards often adopt or mirror these ISO rules:

• British Standards (BS) provide metric thread details aligned with ISO.
• American standards (ANSI/ASME) include a metric profile document and a “limits and fits” guide that closely follow the ISO approach.
• German standards (DIN) have a long history of detailed thread tables, many of which are still used in shop references.
• Japanese standards (JIS) are also based on ISO metric threads but there are some practical differences in head dimensions and pitch selections. These are not fundamental changes, but adaptations for local practice.

Together, these standards form a consistent global system, ensuring that a bolt made in one country will fit a nut made in another.

Related Thread Families and Comparisons

The ISO metric thread is the most widely used system today, but it is not the only one. Other thread families exist, each with its own history and purpose.

Unified threads (UNC/UNF): In the inch-based world, the Unified Thread Standard is common. It uses the same 60° profile angle as ISO metric threads, but the diameters and pitches are defined in inches. For example, a ¼‑20 UNC thread (¼ inch diameter, 20 threads per inch) does not match an M6×1.0 metric thread. The dimensions are close, but not interchangeable — mixing the two systems is not possible.

Whitworth threads (BSW/BSF): An older British system, Whitworth threads use a 55° angle with rounded crests and roots. They are less common today but still appear in legacy equipment.

Pipe threads (BSP, NPT, etc.): Pipe threads are designed for sealing and come in both parallel and tapered forms.

British Standard Pipe (BSP) and National Pipe Thread (NPT) are two widely used systems.

There is also the family of API threads, defined by the American Petroleum Institute. These are specialized thread forms used in the oil and gas industry, particularly for casing, tubing, and drill pipe.

Those standards are not interchangeable with metric threads, since their geometry and purpose are different.

Trapezoidal and square threads:These forms are used for power transmission and motion screws, such as in vises, jacks, or lead screws in machines. Their geometry is optimized for strength and efficiency in moving loads.

Buttress threads:Designed for high loads in one direction, buttress threads are used in applications like presses or heavy-duty clamping systems.

In short, different thread families are like different tools in the same toolbox. The choice is not about style, but about function: sealing, strength, vibration resistance, or motion control.

Legacy And Obsolete Pre-ISO Sizes

Before the adoption of a unified international system, many countries had their own series of metric thread sizes.

Some of these used diameters and pitches that are no longer part of the preferred ISO set.

• Small odd sizes such as M2.3 or M2.6 can still be found in older machines and equipment.
• A few mid range sizes like M5.5 also appear in legacy designs.

These sizes are still documented for service and repair work, but they are not recommended for new designs.

Modern practice almost always uses the current preferred ISO sizes defined in ISO 261 and ISO 262.Finding one of these obsolete sizes in a legacy product is not a cause for concern.

It simply means that replacement fasteners and taps must match that specific size, or that the hole may be carefully reworked to a nearby modern size if appropriate.

Choosing Coarse, Fine, Or Superfine

When selecting a thread pitch, the default choice is coarse pitch.

Coarse threads are used in most general assemblies because they are:

• Easier to start during assembly
• More tolerant of dirt or minor damage
• Cheaper and more widely available in stock

However, there are situations where fine pitch threads are preferred:

• Thin wall sections: A fine pitch allows more controlled engagement without breaking through the wall.
• Slightly higher core strength: For the same nominal diameter, a fine thread leaves a larger uncut core in the bolt.
• Better vibration resistance: Fine threads are less likely to loosen in assemblies exposed to vibration.

At the extreme, superfine threads are used in specialized applications.

These appear in areas such as suspension components or aerospace hardware, where very small pitches are part of the design intent to achieve precise adjustment, high strength, or compact geometry.

Conclusion

The ISO metric thread system may look complex at first glance, but it is built on a clear and logical foundation.

The basic V‑profile defines the geometry, while the preferred diameters and pitches ensure consistency across industries.

Coarse threads serve as the reliable default, while fine and superfine options provide solutions for thin walls, vibration resistance, or specialized applications.

The system is held together by a family of standards: ISO 68 for the profile, ISO 261 and 262 for size series, and ISO 965 for tolerances and fits. These documents guarantee that a bolt made in one country will fit a nut made in another, and that designers, machinists, and mechanics can all work with the same expectations.

We also saw how the system connects to practical details: spanner sizes, tap drill rules, and tolerance classes that define how tight or loose a fit will be.

Even legacy and obsolete sizes have their place, reminding us of the long history behind today’s standards. And when compared with other thread families: Unified, Whitworth, pipe, trapezoidal, or buttress, etc. the ISO metric thread stands out as the global common language for fastening.

The choice of thread (coarse, fine, or superfine) is not about “better” or “worse,” but about matching the design to the job.

The strength of the ISO system is that it gives engineers, manufacturers, and technicians a shared toolbox: predictable, reliable, and versatile. That is why, decades after its adoption, the ISO metric thread remains the backbone of modern fastening.

Questions And Answers

What does the “M” in a thread callout indicate?
The letter M signals that the thread is an ISO metric thread. The number that follows is the nominal outer diameter in millimeters. For example, M6 means a thread with a nominal major diameter of 6 mm, typically pairing with a clearance hole of about 6.6-7 mm depending on the fit class.

When should fine pitch be used instead of coarse?
Fine pitch threads are chosen in thin wall sections, when a larger core area is desired for the same nominal size, or when vibration resistance benefits from a smaller pitch. Coarse pitch remains the general-purpose default, as it is easier to assemble, more tolerant of dirt, and widely available.

What do 6g and 6H mean on a drawing?
These are tolerance classes. The number indicates the grade (tolerance width), and the letter indicates the position (fundamental deviation).

• Lowercase letters (e.g., g, h) apply to external threads.
• Uppercase letters (e.g., G, H) apply to internal threads.A common general-purpose pairing is 6g external with 6H internal.

How do ISO metric threads compare to Unified inch threads?
Both use a 60° V-profile, but they differ in units and in the diameter–pitch combinations. For example, a ¼‑20 UNC thread does not mate with an M6×1.0 thread, even though the diameters are close. Mixing the two families is not functional.

Are spanner sizes fixed for each metric diameter?
Standard hex nuts and hex head bolts follow widely used across-flats sizes, such as 10 mm for M6 and 13 mm for M8. However, some product families (e.g., flange bolts or compact series) use smaller heads to save space and weight. Always check the relevant product standard or data sheet.

Why is the thread angle set at 60 degrees?
The 60° angle is a compromise that balances strength, manufacturability, and interchangeability. It is easy to grind and measure, provides good flank contact, and aligns with other major thread families (like Unified), which simplifies tooling.

What changes when parts are hot-dip galvanized?
Galvanizing adds a thick protective coating to the crests and flanks of the thread. To account for this, ISO 965 includes special tolerance provisions:

• External threads are cut undersize before coating, so that after galvanizing they fall within the correct limits.
• Matching internal threads are specified with allowances to accept the coated fastener.