Learn the five key parameters, metric and imperial systems, thread pitch, thread angle, and head type classification from scratch
Screws are the most common fasteners in both industrial and everyday applications. Each screw has a defined set of specification parameters that describe its dimensions, shape, and physical properties. Correctly identifying screw specifications is fundamental to procurement, maintenance, design, and quality control.
A screw's specification is typically determined by the following five core parameters:
Outer Diameter — The diameter at the outermost edge of the thread, and the most critical nominal dimension in screw specifications. Metric screws are designated with "M + number," e.g., M6 means an outer diameter of approximately 6mm.
Thread Pitch — The distance between adjacent threads, which determines the screw's tightening characteristics. The same outer diameter may come in coarse and fine pitch variants.
Length — The effective length of the screw, typically measured from below the head to the tip. The measurement reference point varies slightly by head type.
Head Type — The shape of the screw head, such as pan head, countersunk, hex head, etc. This directly affects tool selection and installation method.
Material — Common materials include carbon steel, stainless steel (SUS 304, SUS 316), alloy steel, brass, and aluminum. Material determines the screw's strength grade, corrosion resistance, and suitable environment.
For example, a complete specification reads: "M6 x 1.0 x 20mm Stainless Steel 304 Phillips Pan Head Machine Screw" — this means an outer diameter of 6mm, thread pitch of 1.0mm, length of 20mm, SUS 304 stainless steel material, with a Phillips drive pan head. Learning to interpret these parameters enables you to find exactly the screw you need.
Metric screws are the most widely used screw standard globally, following ISO international standards. Their naming convention is intuitive, starting with the letter "M" followed by the outer diameter value (in mm), hence also called M series screws.
Common metric screw specifications include:
The naming convention is "M + outer diameter x pitch x length." For standard coarse pitch, the pitch is often omitted. For example, "M6 x 20" means a 6mm OD, 20mm long coarse-pitch screw; while "M6 x 0.75 x 20" specifically indicates a fine-pitch version (0.75mm pitch, finer than the standard 1.0mm coarse pitch).
Metric screw thread angle is 60 degrees, conforming to ISO 68-1 standard. The thread profile approximates an equilateral triangle — this 60-degree angle achieves the optimal balance between strength and manufacturing feasibility.
Imperial screws are primarily used in the United States and parts of North America, following the ANSI/ASME Unified Thread Standard. Imperial screw outer diameter is measured in inches, and pitch is expressed as "Threads Per Inch (TPI)" — the opposite concept from metric's "mm/thread."
Imperial threads are divided into two main categories:
UNC (Unified National Coarse) — Coarse Thread
Fewer threads, larger pitch. Advantages include faster assembly, lower tolerance requirements for tapped holes, and less susceptibility to cross-threading damage. Suitable for general assembly and non-precision applications. Example: 1/4"-20 UNC (1/4 inch OD, 20 threads per inch).
UNF (Unified National Fine) — Fine Thread
More threads, smaller pitch. Advantages include stronger clamping force, better vibration resistance, and higher fine-adjustment precision. Suitable for aerospace, precision instruments, and vibration environments. Example: 1/4"-28 UNF (1/4 inch OD, 28 threads per inch).
Imperial screw naming follows the format "OD-TPI Thread Type." Larger sizes use fractional inches (e.g., 1/4", 3/8", 1/2"), while smaller sizes use number sizes (#4, #6, #8, #10). For example, #10-24 UNC means number 10 OD (approximately 4.83mm), 24 threads per inch, coarse thread.
In practice, imperial screws are commonly found in imported equipment maintenance, US-standard piping systems, and aerospace subcontracting. The most reliable way to determine whether a screw is metric or imperial is to use a thread gauge for comparison, or measure the outer diameter with calipers and cross-reference a specification chart.
Thread pitch is one of the most easily overlooked yet most critical parameters in screw specifications. Pitch is defined as the axial distance between adjacent thread crests. For metric screws, pitch is measured in mm; for imperial screws, it's expressed as TPI (Threads Per Inch).
Reading metric pitch is straightforward: use a thread gauge or calipers. For example, if the span across 10 threads measures 10mm, the pitch is 1.0mm. Imperial pitch reading works the opposite way: count the complete thread crests within one inch to get the TPI.
The differences between coarse and fine pitch are critical in practice:
In practice, unless the design drawing or OEM specification explicitly calls for fine pitch, always default to coarse pitch screws. In maintenance scenarios, if uncertain about an existing hole's pitch, comparing with a thread gauge is the safest approach — wrong pitch leads to stripped threads or thread damage, causing irreversible consequences.
Thread angle refers to the included angle between the two flanks of a thread profile. This angle determines the screw's load-bearing characteristics, self-locking performance, and manufacturing method.
In mainstream industrial standards, both metric screws (ISO Metric) and unified imperial screws (Unified Thread) share a 60-degree thread angle. The thread profile approximates an equilateral triangle. This means metric and imperial screws are very similar in thread shape — but they must never be interchanged, as the outer diameter and pitch value systems are completely different.
Notably, not all thread systems use a 60-degree angle. The British BSW/BSF Whitworth Thread uses a 55-degree angle with rounded crests and roots instead of flat surfaces. This thread type still occasionally appears in legacy British pipe fittings and certain specialized equipment. Pipe threads (such as BSP, NPT) also have different angles and profile designs.
For everyday fastener engineering, remember this principle: standard metric and imperial machine screw thread angles are both 60 degrees; for non-standard threads, use a thread gauge or optical measuring instrument to confirm the angle and profile.
The screw head type determines the installation method, required tools, and aesthetic result. Here are the six most common head types in industrial and everyday applications:
Features a low, rounded dome-shaped top with a flat bottom surface. This is the most versatile head type, widely used in electronics, appliances, and general mechanical assembly. Usually paired with Phillips or slotted drives. Since the head protrudes above the mounting surface, it's suitable for applications that don't require a flush finish.
Features a conical head designed to sit flush with the mounting surface after installation. Requires a pre-machined countersink in the workpiece. Standard countersink angles are 82 degrees (US standard) or 90 degrees (metric). Suitable for applications requiring smooth surfaces, such as furniture panels, enclosure covers, and aerospace structural components.
Features a hexagonal head, tightened with a wrench or socket tool. This is the preferred head type for high-torque applications, widely used in steel structures, machine assembly, automotive chassis, and flange connections. Hex bolts are typically used with nuts and washers to form a complete bolted joint.
Features a cylindrical head with a hexagonal recess on top, driven by an Allen key. The smaller diameter and taller profile make it ideal for space-constrained precision machinery. Commonly found in CNC machines, molds, automation equipment, and precision fixtures. Material is typically high-strength alloy steel (Class 12.9).
Features a low, hemispherical dome-shaped head with a smooth, aesthetically pleasing appearance. Also uses hex socket drive, but with a wider and shorter head than SHCS. Suitable for applications requiring both aesthetics and hex socket drive, such as consumer electronics housings and display equipment. Due to the thinner head, maximum torque capacity is lower than SHCS.
Features an extra-wide, flat head — essentially a built-in washer effect. Distributes pressure over a larger contact area, ideal for thin sheet mounting and preventing screws from pulling through soft materials. Common in sheet metal assembly, plastic parts, and lightweight partition construction.
Facing the vast variety of screw specifications, even experienced engineers may not be able to accurately determine every screw's complete specs by sight alone. Screw Genius's AI identification feature is built for exactly this purpose.
Usage is simple: take a photo of a screw, upload it to Screw Genius, and the AI analyzes the image within seconds — automatically identifying the head type, estimating outer diameter and pitch, determining likely specification models, and providing both metric and imperial cross-reference information. For engineering drawings, the AI can also read annotations and parse screw specifications into structured data.
AI identification is particularly useful for:
AI identification results should serve as a professional reference starting point. For critical applications, supplement with thread gauges, calipers, and other physical measurement tools for final confirmation. Screw Genius's goal is to transform screw identification from a time-consuming, expertise-heavy task into an efficient "snap and get results" workflow.