Machine screws are used in a wide range of applications. There’s no denying that there are countless types of machine screws used for hundreds of applications.
From construction and manufacturing to industrial and production environments, anything that involves metal parts needs a machine screw to fasten them together.
This begs the question:
Is there a definitive machine screw size chart that can help size the right machine screw for my project?
The simple answer is yes, but only if you know which chart to look at.
Thankfully, I’ve compiled all the data you need in one comprehensive size chart.
I’ve also included a handy guide to help you read and understand any machine screw chart. Let’s get started!
How to Read a Machine Screw Size Chart
The measurements of machine screws follow the Unified Thread Standard (UTS) and the ISO metric screw thread system.
The metric system is used by pretty much the whole world. However, if you live in the United States, this doesn’t apply to you.
American screws follow the standard machine screw sizes of ASME/ANSI, listed under the Unified Thread Standard.
All measurements in the UTS system are in inches.
For the rest of the world, you’ll have to follow the ISO metric system, which is measured by millimeters.
Both systems ultimately convey the same info, but they’re fundamentally different in how they’re structured.
Let’s slowly break it down.
How to Read a UTS Screw Chart (Imperial System)
UTS machine screw charts contain three things:
- Screw gauge
- Threads per inch
- Stress Area
Screw Gauge
The screw gauge stands for the diameter of the outside thread. It’s also called the “major diameter.”
The smallest machine screw gauge is #0, although you could find gauge #00 and #000 for certain projects.
All machine screws with an outside diameter under ¼” are labeled from gauge #0 to gauge #12.
Afterward, the gauge is specified according to the fractional diameter of the screw.
Threads Per Inch (TPI)
Threads per inch (TPI) is the number of threads per one-inch section of the screw.
Fine screws typically have more threads per inch than coarse screws.
Stress Area
The stress area of a machine screw is the weakest point of the bolt. In the UTS system, the stress area in coarse screws is lower than the stress area in fine screws.
Knowing the stress area of a machine screw helps us determine whether it’s suitable for certain applications or not.
For example, if you’re using a screw for clamping, a coarse screw with a high stress area will be a better choice than a fine screw in the same gauge.
How to Read a Metric Screw Chart (ISO)
ISO machine screw charts typically contain four things:
- Major diameter
- Minor diameter
- Pitch diameter
- Stress Area
By now, you should know the major diameter and stress area of a machine screw.
But here’s the tricky part:
In the metric screw chart, the number of threads per inch isn’t calculated.
Instead, the ISO system uses the pitch diameter, which is the distance between the threads on the screw.
Another measurement that you’ll find is the minor diameter. The minor diameter is the smallest point of the thread on a machine screw.
Reading American Machine Screws Callouts
See these and other machine screws offer at The Home Depot,
Let’s say you find a box of machine screws with the label:
#3-48 x .5
Let’s break it down together:
The first number is the screw gauge. Since it’s #3, we can look at the chart and determine the major diameter and threads per inch.
The second number is the TPI. 48 threads per inch indicate that the screws are coarse, not fine.
Finally, the third number indicates the screw length. This means the screws are 0.5 inches long.
Reading International Screws Callouts
If the first letter of the machine screws callout begins with M, this means you’re dealing with international screws.
For example, if you come across a box of machine screws with this label:
M10 x 1.5 x 75
The first number after the letter M indicates the screw’s diameter. The second number is the screw pitch.
The final number is the screw length.
Remember, the measurements here are expressed in millimeters.
You’ll need to convert the numbers to inches before you proceed.
Machine Screw Size Charts
Here’s our in-depth chart using the UTS system.
We’ve also included a metric machine screw size chart for international machine screws.
Imperial Machine Screws Size Chart (UTS)
| Coarse | Fine | |||||
| Size | Diameter | Nearest Fractional | Threads per Inch (TPI) | Stress Area (inches²) | Threads per Inch (TPI) | Stress Area (inches²) |
| #0 | 0.0600 | 1/16″ | N/A | N/A | 80 | 0.00180 |
| #1 | 0.0730 | 5/64″ | 64 | 0.00263 | 72 | 0.00278 |
| #2 | 0.0860 | 3/32″ | 56 | 0.00370 | 64 | 0.00394 |
| #3 | 0.0990 | 7/64″ | 48 | 0.00487 | 56 | 0.00523 |
| #4 | 0.1120 | 7/64″ | 40 | 0.00604 | 48 | 0.00661 |
| #5 | 0.1250 | 1/8″ | 40 | 0.00796 | 44 | 0.00830 |
| #6 | 0.1380 | 9/64″ | 32 | 0.00909 | 40 | 0.01015 |
| #8 | 0.1640 | 5/32″ | 32 | 0.0140 | 36 | 0.01474 |
| #10 | 0.1900 | 3/16″ | 24 | 0.0175 | 32 | 0.0200 |
| #12 | 0.2160 | 7/32″ | 24 | 0.0242 | 28 | 0.0258 |
| 1/4″ | 0.2500 | 1/4” | 20 | 0.0318 | 28 | 0.0364 |
| 5/16″ | 0.3125 | 5/16” | 18 | 0.0524 | 24 | 0.0580 |
| 3/8″ | 0.3750 | 3/8” | 16 | 0.0775 | 24 | 0.0878 |
| 1/2″ | 0.5000 | 1/2” | 13 | 0.1419 | 20 | 0.1599 |
Metric Machine Screws Size Chart (ISO)
| Size | Major Diameter | Minor Diameter | Pitch Diameter | Stress Area (mm²) |
| M1.6 | 1.581 | 1.151 | 1.354 | N/A |
| M2 | 1.981 | 1.490 | 1.721 | N/A |
| M2.5 | 2.480 | 1.928 | 2.188 | N/A |
| M3 | 2.980 | 2.367 | 2.655 | 5.03 |
| M3.5 | 3.479 | 2.743 | 3.089 | 6.78 |
| M4 | 3.978 | 3.119 | 3.523 | 8.78 |
| M5 | 4.976 | 3.995 | 4.456 | 14.2 |
| M6 | 5.974 | 4.747 | 5.324 | 20.1 |
| M8 | 7.972 | 6.438 | 7.160 | 36.6 |
| M10 | 9.968 | 8.128 | 8.994 | 58 |
| M12 | 11.966 | 9.819 | 10.829 | 84.3 |
| M14 | 13.962 | 11.508 | 12.663 | 115 |
| M16 | 15.962 | 13.508 | 12.663 | 157 |
| M20 | 19.958 | 16.891 | 18.334 | 245 |
| M24 | 23.952 | 20.271 | 22.003 | 353 |
| M36 | 35.940 | 31.033 | 33.342 | 817 |
For standard type screws, check out our guide on screw size charts here.
FAQs
Here are some of the most common questions we’ve received.
What’s the Most Distinctive Characteristic of Machine Screws?
If you compare the screw size charts of wood screws and machine screws, you’ll barely notice a difference.
However, the physical characteristics of machine screws are what sets them apart.
Here are the most prominent features:
- They’re smaller but stronger than many screw types
- They generally have finer threads that are better suited with pre-drilled tapped holes – similar to set screws.
- They’re flat-tipped, which is better suited for fastening metal parts together (vehicles, machinery…etc)
What Are the Socket Types of Machine Screws?
Socket types, or drive types, are the driving tools you use to fasten machine screws from a nut or hole. Here are some of the most common machine screw drive types:
- Hex drive sockets
- Cross drives
- Hexalobular recess drives
- Slot sockets
What Are the Different Types of Machine Screws?
There are numerous types of machine screws to choose from. Here are some of the most popular types:
- Phillips head machine screws
- Brass machine screws
- Tamper-resistant machine screws
- Slotted (flathead) machine screws
- Stainless steel machine screws
- Hex head machine screws
- Self-drilling screws are also important in many metal applications
What’s the Difference Between a Machine Screw and a Bolt?
Typically, all machine screws can be used as bolts, but not the other way around.
As a general rule of thumb, a screw with a hex head is considered a bolt, while flatheads are considered a screw.
Wrapping Up
Using my comprehensive machine screw size chart will make your life easier, and here’s why:
It’s easy on the eyes, doesn’t contain any redundant measurements, and comes with the ISO metric screw size standard to help you identify international machine screws.
I’ve also included a handy guide to help you read screw charts like a pro and easily decipher the callouts on your box of screws.