When you are working on DIY projects, woodworking, construction, or even simple household repairs, screws are among the most common fasteners you’ll use. But if you’ve ever stood in the hardware aisle staring at boxes of screws labeled with numbers, fractions, or millimeters, you know how confusing it can be. That’s where a screw size chart becomes invaluable.
This complete guide will help you understand screw sizes, the different systems used (metric vs. imperial), how to read a screw size chart, and how to select the right screw for your project.
What Are Screw Sizes?
Screw sizes describe the diameter, length, and thread count of a screw. These measurements are essential because they determine how well a screw holds materials together and whether it fits into a pre-drilled hole.
The three main aspects of a screw’s size are:
Diameter (gauge or millimeters): The thickness of the screw shaft.
Length: The distance from the head to the tip of the screw.
Thread count/pitch: How many threads are on the screw per inch (imperial) or the distance between threads in millimeters (metric).
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Why Do You Need a Screw Size Chart?
A screw size chart works as a reference table that matches screw dimensions with their common uses. Without it, you might pick the wrong screw, leading to problems like:
Stripped threads
Weak joints
Splitting wood
Poor load-bearing strength
By using a chart, you can quickly confirm which screw works best for drywall, decking, cabinetry, metal, or machinery.
Screw Size Measurement Systems
There are two main systems for screw sizes: Imperial (U.S.) and Metric (international).
1. Imperial Screw Sizes (Gauge and Inches)
Diameter is expressed as a number gauge (#2, #4, #6, #8, #10, etc.) or as a fractional inch (e.g., 1/4″).
Length is measured in inches.
Thread count is measured in TPI (threads per inch).
👉 Example: A screw labeled #8 × 2″ × 18 means:
#8 gauge (0.164″ diameter)
2 inches long
18 threads per inch
2. Metric Screw Sizes
Diameter is expressed in millimeters.
Length is also in millimeters.
Thread pitch is the distance between threads (in mm).
👉 Example: A screw labeled M5 × 40 × 0.8 means:
5mm diameter
40mm long
0.8mm thread pitch
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Screw Size Chart (Imperial & Metric)
Here’s a reference screw size chart that covers both imperial gauges and metric equivalents.
Imperial Screw Gauge Chart
| Gauge | Diameter (inches) | Common Uses |
|---|---|---|
| #2 | 0.086″ | Electronics, small hinges |
| #4 | 0.112″ | Light woodwork, picture frames |
| #6 | 0.138″ | Cabinets, small furniture |
| #8 | 0.164″ | General wood screws, drywall |
| #10 | 0.190″ | Decking, medium construction |
| #12 | 0.216″ | Door hardware, heavy wood |
| #14 | 0.242″ | Structural applications |
Metric Screw Size Chart
| Metric Size | Diameter (mm) | Common Uses |
|---|---|---|
| M2 | 2mm | Electronics, precision devices |
| M3 | 3mm | Small furniture, appliances |
| M4 | 4mm | Cabinets, light hardware |
| M5 | 5mm | General woodworking, metal fastening |
| M6 | 6mm | Deck screws, automotive |
| M8 | 8mm | Heavy-duty wood, machinery |
| M10 | 10mm | Structural, industrial use |
Screw Length Guide
Screw lengths vary depending on application:
1/2″ to 1″ (12–25mm): For thin materials, hinges, brackets.
1 1/4″ to 2″ (30–50mm): For woodworking and drywall.
2 1/2″ to 3″ (65–75mm): Decking, framing.
3 1/2″ and above (90mm+): Heavy-duty construction.
How to Read a Screw Size Chart
Let’s break it down with an example:
A #8 × 1 1/2″ wood screw → #8 gauge (0.164″ diameter), 1.5 inches long.
A M6 × 40 machine screw → 6mm diameter, 40mm length.
By matching these numbers on the chart, you can immediately identify the screw’s diameter, length, and the type of material it’s best suited for.
Types of Screws and Their Applications
Wood Screws
Sharp points, coarse threads
Best for fastening wood to wood
Drywall Screws
Bugle heads, fine or coarse threads
Ideal for securing drywall to studs
Deck Screws
Corrosion-resistant coating
Designed for outdoor use
Machine Screws
Uniform threads, used with nuts or tapped holes
Common in metal and machinery
Sheet Metal Screws
Pointed or self-tapping ends
Used for fastening metal sheets
Lag Screws (Lag Bolts)
Heavy-duty, hex head
Used in structural wood connections
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Tips for Choosing the Right Screw
Match screw to material: Wood screws for wood, machine screws for metal, drywall screws for gypsum boards.
Use correct length: At least half of the screw length should penetrate into the base material.
Check thread type: Coarse threads grip soft materials better; fine threads suit metal and hard materials.
Choose proper head type: Flat, round, or hex heads depending on appearance and torque needs.
Consider corrosion resistance: Use stainless steel or coated screws for outdoor projects.
Common Mistakes to Avoid
Using the wrong diameter – Too small and it won’t hold; too big and it splits the material.
Over-tightening – Can strip threads or break the screw.
Mixing metric and imperial – Always stick to the right measurement system.
Ignoring pilot holes – For hardwood and large screws, pre-drilling prevents splitting.
Final Thoughts
A screw size chart is one of the most useful tools you can keep in your workshop or toolbox. By understanding gauge sizes, metric equivalents, lengths, and thread types, you’ll save time and avoid costly mistakes.
Whether you are hanging shelves, building furniture, repairing electronics, or tackling heavy construction projects, the right screw size ensures a safe, strong, and lasting connection.
Keep a screw size chart handy, and you’ll never second-guess which screw to use again.