Machine screws are one of the most widely used fasteners in mechanical assemblies, electronics, furniture, and industrial applications. Unlike wood screws, which grip wood fibers, machine screws are designed to fasten metal parts or fit into pre-tapped holes. Choosing the correct screw size is crucial to ensure secure connections, prevent stripped threads, and maintain the structural integrity of your projects. A machine screw size chart is an invaluable reference for both professionals and DIY enthusiasts.
In this guide, we’ll explore machine screw types, how to read their sizes, provide a detailed chart, and share tips for selecting the right screw for your projects.
What Are Machine Screws?
A machine screw is a threaded fastener with a uniform diameter designed to be screwed into a tapped hole or used with a nut. They are commonly made from steel, stainless steel, brass, or aluminum, and come in various head types:
Flat head – Countersunk for a flush finish
Pan head – Rounded top, often used in electronics
Round head – Decorative or for light-duty fastening
Hex head – Used with wrenches for heavy-duty applications
Machine screws are widely used in:
Electronics and electrical equipment
Automotive and machinery components
Metal fabrication and structural assemblies
Furniture and cabinetry
How Machine Screw Sizes Are Measured
Machine screws are typically measured using:
Diameter
Small screws: identified by gauge numbers (#2, #4, #6, etc.)
Large screws: listed in fractional inches (1/4″, 3/8″)
Threads Per Inch (TPI)
TPI is the number of threads per inch along the screw shaft
Coarse threads are easier to assemble; fine threads offer stronger holding power
Screw Length
Measured from the flat portion under the head to the tip
Countersunk screws include the head in the total length
Head and Drive Type
Drive types include Phillips, slotted, Torx, and square drives
Head style affects the finish, appearance, and tool needed for installation
Common Machine Screw Sizes
Here’s a reference chart for standard machine screws, including gauge, diameter, TPI, and applications:
| Screw Size | Major Diameter (inches) | Coarse Threads (TPI) | Fine Threads (TPI) | Typical Applications |
|---|---|---|---|---|
| #2 | 0.086″ | 56 | 64 | Electronics, small parts |
| #4 | 0.112″ | 40 | 48 | Light machinery, instruments |
| #6 | 0.138″ | 32 | 40 | Cabinet assembly, small machines |
| #8 | 0.164″ | 32 | 36 | General fastening, furniture |
| #10 | 0.190″ | 24 | 32 | Furniture, machinery parts |
| 1/4″ | 0.250″ | 20 | 28 | Automotive, structural metalwork |
| 5/16″ | 0.312″ | 18 | 24 | Heavy-duty equipment, construction |
| 3/8″ | 0.375″ | 16 | 24 | Industrial machinery, structural assemblies |
| 1/2″ | 0.500″ | 13 | 20 | Large-scale construction, machinery |
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How to Read Machine Screw Sizes
A screw labeled #10-24 x 1″ means:
#10 → Screw gauge
24 → Threads per inch (TPI, coarse thread)
1″ → Screw length
For fractional screws like 1/4″-20 x 2″:
1/4″ → Major diameter
20 TPI → Threads per inch
2″ → Screw length
Tips for Choosing the Right Machine Screw
Match the screw to the material
Metals → Fine-thread screws for a stronger grip
Plastics → Coarse-thread screws to prevent stripping
Pre-drill or tap holes if needed
Ensures accurate alignment
Reduces the risk of stripped threads
Use proper driver bits
Phillips, slotted, Torx, or square drive
Prevents stripped heads and allows smooth assembly
Check load requirements
Larger diameter and longer screws provide higher strength
Heavy-duty applications require thicker screws with coarse threads
Applications of Common Machine Screw Sizes
#2 – #6 screws → Small electronics, precision instruments, light-duty assemblies
#8 – #10 screws → Furniture, cabinets, light machinery, DIY projects
1/4″ – 3/8″ screws → Automotive, structural assemblies, medium-duty machinery
1/2″ and above → Heavy-duty machinery, industrial equipment, structural projects
Advantages of Using a Machine Screw Size Chart
Saves time: Quickly find the correct screw without guesswork
Prevents errors: Reduces the risk of stripped threads or loose connections
Ensures strength: Proper sizing provides maximum holding power
Universal reference: Useful for DIY, industrial, and professional projects
Common Mistakes to Avoid
Using the wrong TPI – Fine threads in soft metals may strip easily
Incorrect screw length – Too short screws reduce holding power; too long may damage materials
Skipping pilot holes – Essential for hard metals and delicate plastics
Wrong driver bit – Can strip the screw head and ruin the assembly
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Final Thoughts
A machine screw size chart is a must-have reference for anyone working with mechanical assemblies, furniture, electronics, or industrial machinery. By understanding screw gauge, diameter, threads per inch, and proper length, you ensure strong, precise, and durable connections.
Keep this chart handy in your workshop or toolbox. Whether you’re a professional engineer, machinist, or DIY enthusiast, using the correct machine screw will save time, reduce mistakes, and improve the quality of your work.