Screws are among the most widely used fasteners in the world. From furniture assembly and woodworking to automotive repairs and large-scale construction, screws provide the strength and reliability needed to hold materials together. But with so many different sizes, picking the right screw isn’t always simple.
One of the most important factors to understand is screw diameter—the thickness of the screw shaft. The diameter determines how much load a screw can bear, how well it grips into a material, and whether it fits into a pre-drilled hole. To make this easier, a screw diameter chart serves as a quick reference guide for choosing the correct screw size for any project.
In this complete guide, we will explore what screw diameters mean, how to measure them, the difference between metric and imperial screw systems, and provide a detailed screw diameter chart you can use in your workshop or toolbox.
What Is Screw Diameter?
The diameter of a screw is the width of its shaft (the threaded part). It is usually measured in:
Gauge number (imperial system) – e.g., #4, #6, #8.
Millimeters (metric system) – e.g., M3, M4, M6.
Fractional inches (for larger bolts and screws) – e.g., 1/4″, 3/8″.
A screw’s diameter affects:
Holding power – Thicker screws can handle more load.
Material compatibility – Smaller diameters suit thin or fragile materials.
Pre-drilling needs – Large-diameter screws often require pilot holes.
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Why Use a Screw Diameter Chart?
If you’ve ever tried to match screws without a guide, you know how confusing it can get. A screw diameter chart helps you:
Match screws to drill bit sizes for pilot holes.
Compare imperial and metric equivalents.
Avoid over-sizing or under-sizing fasteners.
Save time by quickly finding the right screw for the job.
Screw Diameter Measurement Systems
1. Imperial Screw Gauge (U.S.)
Uses number gauges (#0 through #14) for small screws.
Uses fractional inch diameters (1/4″, 3/8″, etc.) for larger screws and bolts.
Thread count measured in TPI (threads per inch).
👉 Example: #8 screw has a diameter of about 0.164″.
2. Metric Screw Sizes
Uses “M” followed by diameter in millimeters (M2, M3, M4, M6, etc.).
Common worldwide, especially in automotive and machinery applications.
Thread pitch measured in millimeters between threads.
👉 Example: M6 screw has a 6mm shaft diameter.
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Complete Screw Diameter Chart
Below is a comprehensive screw diameter chart covering both imperial gauges and metric equivalents.
Imperial Screw Diameter Chart (Gauge Sizes)
| Gauge (Imperial) | Shaft Diameter (inches) | Shaft Diameter (mm) | Common Uses |
|---|---|---|---|
| #0 | 0.060″ | 1.5mm | Electronics, precision instruments |
| #2 | 0.086″ | 2.2mm | Small hinges, toys |
| #4 | 0.112″ | 2.8mm | Light woodwork, picture frames |
| #6 | 0.138″ | 3.5mm | Cabinets, furniture |
| #8 | 0.164″ | 4.2mm | General woodworking, drywall |
| #10 | 0.190″ | 4.8mm | Decking, heavy cabinets |
| #12 | 0.216″ | 5.5mm | Door hardware, structural wood |
| #14 | 0.242″ | 6.1mm | Construction, large wood projects |
Fractional Inch Screw Diameters (Bolts & Large Screws)
| Fractional Size | Diameter (inches) | Diameter (mm) | Common Uses |
|---|---|---|---|
| 1/4″ | 0.250″ | 6.35mm | General construction, machinery |
| 5/16″ | 0.313″ | 7.94mm | Automotive, heavy-duty fastening |
| 3/8″ | 0.375″ | 9.53mm | Framing, large wood |
| 7/16″ | 0.438″ | 11.1mm | Structural bolts |
| 1/2″ | 0.500″ | 12.7mm | Industrial, heavy load connections |
Metric Screw Diameter Chart
| Metric Size | Diameter (mm) | Diameter (inches) | Common Uses |
|---|---|---|---|
| M2 | 2mm | 0.079″ | Electronics, fine work |
| M3 | 3mm | 0.118″ | Appliances, small hardware |
| M4 | 4mm | 0.157″ | Furniture, cabinetry |
| M5 | 5mm | 0.197″ | Automotive, bicycles |
| M6 | 6mm | 0.236″ | Deck screws, automotive |
| M8 | 8mm | 0.315″ | Machinery, structural |
| M10 | 10mm | 0.394″ | Industrial, heavy-duty |
| M12 | 12mm | 0.472″ | Construction, load-bearing |
Quick Conversion: Metric to Imperial
M3 ≈ #4 screw
M4 ≈ #8 screw
M5 ≈ #10 screw
M6 ≈ 1/4″ screw
M8 ≈ 5/16″ screw
M10 ≈ 3/8″ screw
⚠️ Always try to use exact matches—close equivalents can strip threads or loosen over time.
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How to Measure Screw Diameter
If you don’t have the packaging, you can measure screw diameter in a few simple steps:
Use a caliper or screw gauge to measure the shaft just under the head (major diameter).
If using imperial, compare to a screw chart (gauge or fractional inches).
If using metric, read directly in millimeters.
For threaded screws, also check the thread pitch (distance between threads).
👉 Example: A screw measured at 4.2mm is equivalent to a #8 screw.
Screw Diameter vs. Material Thickness
Choosing the right diameter also depends on the material you’re working with.
Thin wood or sheet metal → Small diameters (#2–#6, M2–M3).
Furniture and cabinetry → Medium diameters (#8–#10, M4–M5).
Decking and framing → Larger diameters (#12, #14, M6–M8).
Industrial or structural → Heavy diameters (1/2″, M10, M12+).
A general rule: The screw diameter should not exceed 1/3 of the material’s thickness to avoid splitting.
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Types of Screws and Their Typical Diameters
Wood Screws – Commonly #6 to #10 (3–5mm).
Drywall Screws – Typically #6 or #8.
Deck Screws – Range from #8 to #12 (4–6mm).
Machine Screws – Usually M3 to M10 depending on machinery.
Lag Screws (Lag Bolts) – Larger diameters (1/4″ up to 1/2″).
Tips for Selecting the Right Screw Diameter
Always consider the load: Heavier loads require thicker diameters.
Use pilot holes for large diameters to prevent splitting.
Match drill bit size: Use a chart to ensure your pilot hole matches screw diameter.
Avoid overtightening: Larger screws can still break if forced.
Choose coated or stainless steel screws for outdoor projects.
Common Mistakes to Avoid
Using a diameter too small → Weak connections, risk of pullout.
Using a diameter too large → Splitting wood, damaging material.
Mixing metric and imperial → Causes misfits and loose joints.
Ignoring thread type → A thick screw with wrong threads may still fail.
Final Thoughts
The screw diameter chart is an essential tool for anyone who works with screws—whether you’re building furniture, fixing electronics, or working on construction sites. By understanding screw diameters, measurement systems, and conversions, you’ll be able to pick the correct fastener every time.
Remember: the right screw diameter ensures strength, safety, and durability in any project. Keep this guide handy, and you’ll never struggle with screw sizing again.