Using the right hardware is essential for a strong, reliable build—and that includes using correct screw size. Screws that are too small may weaken the joint, and screws that are too large may split or damage the material.
This guide breaks down screw anatomy, size measurements, and conversions so you have the tools to choose the correct fastener for a clean, secure, and long-lasting project.
The Three Screw Measurements
There are three key measurements used in screw sizing: diameter, length, and thread count.
- Diameter typically refers to the total width of the screw shaft and threads.
- Length refers to the length of the screw inserted in the material.
- Thread count refers to the coarseness of the screw threads.
When learning about these measurements, keep in mind that sizing methods vary between the metric system and the imperial system.
In the metric system...
- Diameter is measured in millimeters.
- Thread is measured by pitch—the distance in millimeters between threads.
- Length is measured in millimeters.
In the imperial system...
- Diameter is measured by gauge number.
- Thread is measured by threads per inch.
- Length is measured in inches.
Step 1: Identify the Screw Diameter
The first measurement to understand is screw diameter. While diameter can be measured in several ways, the most commonly referenced measurement is the major diameter—the distance across the widest part of the screw threads.
- In the metric system, screw diameter is measured in millimeters (mm).
- In the imperial system, screw diameter is measured in inches or by gauge number.
- Screws with a diameter less than 1/4 inch are typically sized with a gauge number ranging from #0 to #14.
- Larger gauge numbers indicate wider screws.
- Screws that are 1/4 inch in diameter or larger are typically measured in inches.
- Screws with a diameter less than 1/4 inch are typically sized with a gauge number ranging from #0 to #14.
Use this screw size conversion chart to compare sizes between systems:
| Screw Size Conversion Chart | |||
|---|---|---|---|
| Gauge Number | Major Diameter (in) | Major Diameter (mm) | |
| Decimal | Nearest Fraction | ||
| #0 | 0.060 | ~1/16 | 1.52 |
| #1 | 0.073 | ~5/64 | 1.85 |
| #2 | 0.086 | ~3/32 | 2.18 |
| #3 | 0.099 | ~3/32 | 2.51 |
| #4 | 0.112 | ~7/64 | 2.84 |
| #5 | 0.125 | 1/8 | 3.18 |
| #6 | 0.138 | ~9/64 | 3.51 |
| #7 | 0.151 | ~5/32 | 3.84 |
| #8 | 0.164 | ~11/64 | 4.17 |
| #9 | 0.177 | ~3/16 | 4.50 |
| #10 | 0.190 | ~3/16 | 4.83 |
| #11 | 0.203 | ~13/64 | 5.16 |
| #12 | 0.216 | ~7/32 | 5.49 |
| #13 | 0.229 | ~15/64 | 5.82 |
| #14 | 0.242 | ~1/4 | 6.15 |
| #16 | 0.268 | ~17/64 | 6.81 |
| #18 | 0.294 | ~19/64 | 7.47 |
| #20 | 0.320 | ~5/16 | 8.13 |
| #24 | 0.372 | ~3/8 | 9.45 |
The easiest way to find a screw's major diameter is with calipers. Use the tool to measure the width of the entire screw shaft, including threads.
Step 2: Measure the Screw Length
Length typically refers to the part of the screw that is inserted into the material. This may include the screw head, depending on whether it is a countersinking screw or non-countersinking screw.
Countersinking screws are measured from the top of the head to the tip. These screws are designed so the head sits flush with the material surface when fully installed. Some examples of countersinking screws are flat head, bugle head, and trim head screws.
Non-countersinking screws are measured from the bottom of the head to the tip. These screw heads sit above the material surface when fully installed. Examples include hex head, pan head, button head, round head, and truss head screws.
Step 3: Understand Thread Type
The final measurement to understand is the screw's thread count. Metric screws measure thread by pitch, but imperial screws use threads per inch.
Screws can be coarse thread or fine thread.
- Coarse thread screws are the most common for general-purpose fastening because wider thread spacing makes them easier to install.
- Fine thread screws have more tightly spaced threads, which improves vibration resistance and tensile strength. However, they take longer to install and are typically used in dynamic or high-pressure systems.
Pitch (Metric)
In the metric system, thread is measured by pitch—the distance between two threads on the screw shaft, typically measured in millimeters.
Pitch is equivalent to how far the screw moves in one 360° rotation.
Pitch = Screw Length (mm) ÷ Number of Threads
Threads Per Inch (Imperial)
In the imperial system, thread count is measured by threads per inch (TPI). This refers to the number of threads in a one-inch section of the screw shaft.
Threads Per Inch = Number of Threads ÷ Screw Length (in.)
Putting It All Together
The three most important measurements to know about any screw are:
- Diameter—This typically refers to the outside diameter of the screw threads and is measure in gauge numver or inches/millimeter width.
- Length—Countersinking screws' length includes the head, but non-countersinking screws' length only counts the screw shaft.
- Thread count—This is measured by the number of threads per inch (TPI) or by the distance between threads (pitch).
How to Read Screw Size
Screw sizes are often written as a combination of diameter, thread count, and length. Each number in the name refers to a specific measurement.
For example...
- #8-32 x 1"
- #8—Screw diameter (gauge size)
- 32—Threads per inch
- 1"—Screw length
- M5 x 0.8 x 20
- M5—Screw diameter (mm)
- 0.8—Thread pitch
- 20—Screw length (mm)
Mastering these three measurements will make it much easier to read screw specifications and choose the right fastener for your project. With a clear understanding of diameter, length, and thread count, you can ensure a secure fit, proper installation, and long-lasting results.
If you have additional questions or need help finding the right screw, please contact our team by calling us at (406)752-5253, or you can send us an email.