When purchasing screws, you may encounter markings such as “M6×30,” “ST4.2×25,” “8.8,” “A2-70,” or “1/4-20 UNC.” These letters and numbers usually describe the thread type, nominal diameter, length, material, or mechanical properties of the fastener.
However, there is no single screw-designation format that applies to every product. Metric machine screws, inch-thread fasteners, self-tapping screws, and self-drilling screws use different marking systems. Product codes may also vary between standards and manufacturers.
For accurate selection, buyers should not rely on one size number alone. The complete product standard, technical drawing, base material, and operating environment must also be considered.
I. What Information Does a Screw Designation Usually Include?
A complete screw specification may include:
Product type
Thread system and thread form
Nominal diameter
Thread pitch or threads per inch
Screw length
Head style and drive type
Material and property class
Surface treatment
Applicable manufacturing standard
For example:
Hex Socket Screw M6×1.0×30–8.8–Zinc Plated
This describes a hex socket screw with a nominal thread diameter of 6 mm, a pitch of 1.0 mm, a length of 30 mm, property class 8.8, and a zinc-plated surface.
Actual purchase descriptions may be shorter. Any information omitted from the designation should be confirmed through the technical drawing, product data sheet, or applicable standard.
II. How Should “M6×30” Be Interpreted?
“M6×30” is a common metric screw size.
M indicates a metric screw thread.
6 indicates a nominal thread diameter of 6 mm.
30 normally indicates a screw length of 30 mm.
M6 refers to the nominal major diameter of the thread. It does not indicate the diameter of the screw head, and every measured point on the external thread will not necessarily be exactly 6 mm because of thread tolerances and profile geometry.
If the designation is written as M6×1.0×30, the value “1.0” indicates the thread pitch. Thread pitch is the axial distance between corresponding points on two adjacent threads.
When the pitch is omitted, the screw will generally use the coarse pitch specified by the applicable standard. Buyers should still confirm the governing standard to avoid mixing coarse-thread and fine-thread fasteners.
III. What Is the Difference Between Metric and Inch Thread Markings?
Metric fastenersgenerally express the diameter, pitch, and length in millimeters. Inch-thread fasteners may use fractions of an inch, screw gauge numbers, and threads per inch.
Example
Basic Meaning
M6×30
6 mm nominal diameter and 30 mm length
M8×1.0×40
8 mm nominal diameter, 1.0 mm pitch, and 40 mm length
1/4-20 UNC×1″
1/4-inch diameter, 20 threads per inch, Unified National Coarse thread, and 1-inch length
#8-18×1″
No. 8 screw size, 18 threads per inch, and 1-inch length
UNC generally means Unified National Coarse, while UNF means Unified National Fine.
Metric and inch threads should not be substituted merely because their measured diameters appear similar. Their pitch, thread profile, and fit dimensions may differ. Incorrect mixing can prevent assembly, damage the threads, or reduce the reliability of the connection.
IV. How Do You Read a Self-Tapping Screw Size?
A self-tapping screw forms or cuts a mating thread in a prepared hole or suitable base material. A common designation is:
ST4.2×25
In this example:
ST generally indicates a tapping screw thread.
4.2 indicates a nominal thread diameter of approximately 4.2 mm.
25 indicates a screw length of 25 mm.
Self-tapping screws are available with different thread forms, point designs, head styles, and drive recesses. A screw intended for thin steel sheet may differ considerably from one intended for plastic or wood.
Knowing only “ST4.2×25” may therefore be insufficient for accurate product selection. The base material, material thickness, pilot-hole diameter, thread form, and applicable product standard should also be confirmed.
V. How Do You Read a Self-Drilling Screw Size?
Aself-drilling screw has a drill-point tip that can create a hole and form a threaded connection in a suitable material. It is commonly used for light steel framing, metal roofing, cladding, and thin-sheet connections.
For example:
ST5.5×38 Hex Washer Head Self-Drilling Screw
This describes a hex washer head self-drilling screw with a nominal diameter of 5.5 mm and a length of 38 mm.
In addition to diameter and length, the following details should be checked:
Drill-point type
Drilling capacity
Base-material thickness
Total material grip thickness
Head style and drive method
Surface treatment
Service environment
A longer drill point does not automatically make a screw suitable for every material thickness. Drilling capacity should be determined from the manufacturer’s data, the applicable standard, or connection testing.
VI. From Where Is Screw Length Measured?
The method used to measure screw length depends on the head style.
For pan-head, round-head, hex-head, and hex washer head screws, length is generally measured from the bearing surface beneath the head to the tip. The height of the head is normally excluded.
A countersunk screw is designed so that its head sits partly or fully inside the workpiece surface. Its length is therefore generally measured from the top of the head to the tip, including the countersunk head.
Special head styles may use different measuring rules. The relevant product standard or dimensional drawing should be checked before ordering.
VII. What Do Head Style and Drive Type Mean?
The head style affects the bearing area, installation space, finished appearance, and method of load transfer. The drive type determines which tool or bit is required.
Common screw head styles include:
Pan head
Countersunk head
Round head
Hex head
Hex washer head
Socket head cap style
Common drive types include:
Phillips
Slotted
Hex socket
External hex
Hexalobular or star drive
Square drive
Markings such as PH2, PZ2, T25, and H5 generally identify the drive system and tool size.
PH and PZ drives may both look cross-shaped, but their geometries are different. Using the wrong bit can cause slipping, poor torque transfer, or damage to the recess.
The first number multiplied by 100 represents a nominal tensile strength of 800 MPa.
The second number divided by 10 represents a nominal yield-strength-to-tensile-strength ratio of 0.8.
The nominal yield strength can therefore be understood as 800 × 0.8, or 640 MPa.
Property classes must always be interpreted together with the applicable fastener standard, size range, and inspection requirements.
Not every small screw is required to carry a visible property-class marking. Mechanical properties should never be identified solely by color or appearance.
IX. What Does A2-70 Mean on a Stainless Steel Screw?
A2-70 is a common designation for stainless steel fasteners.
A2 identifies a commonly used group of austenitic stainless steels.
70 indicates a tensile strength class with a specified minimum tensile strength of 700 MPa under the conditions and size ranges covered by the relevant standard.
A4 stainless steel is generally selected when greater resistance to certain corrosive environments is required. However, suitability for coastal, chemical, humid, or high-temperature applications must be evaluated according to the specific exposure conditions.
Stainless steel is not completely corrosion-proof in every environment. It also does not automatically have a higher mechanical strength than every carbon steel fastener.
X. How Should Surface Finish Markings Be Interpreted?
Surface treatment can improve corrosion resistance, appearance, or assembly characteristics.
Some finish abbreviations are manufacturer-specific or region-specific and may not have a universal meaning. Buyers should confirm:
Exact coating type
Required coating thickness
Color
Corrosion-test requirements
Effect on thread fit
Suitability for the operating environment
Friction requirements where preload is important
A coating does not replace the properties of the base material. Zinc-plated carbon steel and stainless steel differ in material composition, mechanical behavior, and corrosion-resistance mechanisms.
XI. How Should a Complete Screw Specification Be Confirmed?
When purchasing screws, confirm the specification in the following order:
Identify the materials being joined and the service environment.
Determine whether metric, inch, self-tapping, or self-drilling threads are required.
Confirm the diameter, pitch, and length.
Select the head style and drive type.
Confirm the material and mechanical property class.
Select the appropriate surface treatment.
Verify the applicable standard, drawing, and inspection requirements.
If the screw is used in a structural connection, vibration-prone assembly, or safety-related equipment, it should be selected by qualified personnel according to the applied loads, required preload, joined materials, installation method, and possible failure modes.
A replacement should not be chosen solely because it looks similar to the original screw.
Conclusion
Understanding a screw designation requires identifying the product type, thread diameter, pitch, length, head style, drive type, material, and property class separately.
“M6×30” provides only part of the information required to define a screw. Two screws with the same nominal dimensions may have different thread forms, strength levels, materials, coatings, and intended applications.
Before bulk purchasing or installation, the complete specification should be verified against the relevant product standard, technical drawing, supplier data, and actual operating conditions. This helps prevent thread mismatches, incorrect lengths, insufficient corrosion resistance, and inadequate connection strength.
