Whether you’re buying screws online or instore, if you don’t know how to read screw callouts, you’ll probably end up getting the wrong size.
Although it might seem hard to understand screw sizes, it’s actually a lot simpler than what you think.
Once you figure out the meaning of each number found in screw callouts, you’ll find that reading callouts and screw size charts is a piece of cake.
You also might be familiar with reading imperial screw callouts.
However, you can find some screw packages that are available in metric sizes only.
So, it’s important to familiarize yourself with both of them.
In this article, you’ll find all the information you need to read metric screw sizes easily. We also included an easytofollow metric screw size chart.
Let’s get started!
Terms You Must Know to Read Metric Screw Size Charts
If you’re used to using the imperial screw sizes, reading screw sizes in mm might be confusing to you.
Metric callouts look different from imperial ones. Thus, metric screw size charts will have different info than what is provided in imperial charts.
So, we’ll be starting by explaining how to read metric screw callouts first. This will help you use our metric screw size chart that is included further in the article.
How to Read Metric Screw Sizes?
A metric callout on a screw package usually looks like this: M6 × 1.00 × 25. So, what does this even mean?
Typically, screw callouts contain three numbers whether for imperial or metric ones. Metric callouts usually contain:
 Screw diameter
 Screw pitch
 Screw length
We’ll be explaining each definition of these to help you read our metric screw size chart.
1. Screw Diameter
Unlike imperial callouts, the metric screw size callouts always start with the letter M followed by a number. This number is the screw diameter.
The screw diameter or major diameter is the outside diameter of the screw. In other terms, it’s the diameter of the screw shank.
Don’t get it confused with the US gauge. Just because they’re both written as the first numbers in callouts, doesn’t mean that they’re the same thing.
It’s really complicated to understand the correlation between metric screw diameter and gauge.
In this system, screw sizes are often referred to by a number followed by the symbol “G” or “ga” (for gauge). For instance, a #6 screw has a larger diameter than a #4 screw.
It’s important to note that screw gauges may vary depending on the specific standards used in different regions or industries.
2. Screw Pitch
The screw pitch is represented by the second number you see in the callout. It represents the distance between each thread on the screw in mm.
You can find that the screw pitch isn’t included in imperial callouts. Instead, you’ll find the number of threads per inch (TPI).
In metric screw sizes, you usually find the screw pitch represented in the second column.
You can also find two pitch columns. One for coarse screws and the other for fine screws.
A coarse screw is a screw with a low number of threads along the screw. While a fine screw has a high number of threads.
So, of course, the pitch number will be different between the two.
In our metric screw size chart, we provide the pitch for both coarse and fine threads.
3. Screw Length
The screw length is the third and final number you find in metric callouts. It’s measured differently depending on the type of the screw head.
For countersinking screws, their length is measured from the top of their heads to their tips.
This is because they’re designed to be installed flat or flush with the surface.
On the other hand, noncountersinking threads are designed so that their heads remain above the surface.
So, the screw length in this case is measured from beneath the head to the tip.
To understand how to choose the right screw length for your project, check out this video:
Example: Reading Metric Machine Screw Sizes
To make sure that you understand how to read metric callouts on screw packages, let’s try to read this metric machine screw callout:
M3 × 1.25 × 30
This callout means:
 The first number after the letter M is the screw diameter which equals 3 mm
 The second number is the screw’s pitch which equals 1.25 mm
 The third and final number is the screw’s length which equals 30 mm
Since we also talked about imperial screw callouts, let’s read this screw thread callout as well:
#6 x 1 x 1/2″
We have here:
 The first number represents the US gauge number, which represents the diameter of the screw shaft. It’s essentially a measurement of the thickness of the screw’s body.
 The second number represents the number of threads per inch which is 1
 The third and final number is the screw’s length which equals ½ inch
Protip, you might find some imperial callouts that contain only two numbers. These two numbers are the gauge number and screw length.
What about the thread per inch number? In this case, it means that the screw has the standard number of threads.
The Only Metric Screw Size Chart You’ll Need
In our metric screw size chart, you’ll find all the information you need to buy the right size of screws for your project. Our chart includes:
 Screw diameter
 Screw pitch for both coarse and fine screws
 Tap drill for both coarse and fine screws
 Finally, clearance drill
Metric Screw Size  Metric Screw Diameter  Screw Pitch (Coarse)  Tap Drill (Coarse)  Screw Pitch (Fine)  Tap Drill (Fine)  Clearance Drill 
M1 
1 
0.25 
0.75 
0.2 
0.8 
1.2 
M1.2 
1.2 
0.25 
0.95 
0.2 
1 
1.4 
M1.4 
1.4 
0.3 
1.1 
0.2 
1.2 
2 
M1.6 
1.6 
0.35 
1.25 
0.2 
1.4 
1.8 
M1.8 
1.8 
0.35 
1.45 
0.2 
1.6 
2 
M2 
2 
0.4 
1.6 
0.25 
1.75 
2.4 
M2.5 
2.5 
0.45 
2 
0.35 
2.1 
2.9 
M3 
3 
0.5 
2.5 
0.35 
2.6 
3.4 
M3.5 
3.5 
0.6 
2.9 
0.35 
3.1 
3.9 
M4 
4 
0.7 
3.3 
0.5 
3.5 
4.5 
M5 
5 
0.8 
4.2 
0.5 
4.5 
5.5 
M6 
6 
1 
5 
0.75 
5.2 
6.6 
M7 
7 
1 
6 
0.75 
6.2 
8 
M8 
8 
1.25 
6.8 
0.75 or 1 
7.2 
9 
M9 
9 
1.25 
7.75 
0.75 or 1 
8.25 or 8 

M10 
10 
1.5 
8.5 
1 or 1.15 
8 or 8.75 
12 
M11 
11 
1.5 
9.5 
1 or 1.25 
10 or 9.75 

M12 
12 
1.75 
10.2 
1.25 or 1.5 
10.75 or 10.5 
14 
M14 
14 
2 
12 
1.5 
12.5 
16 
M16 
16 
2 
14 
1.5 
14.5 
18 
M17 
17 
1 or 1.5 
16 or 15.5 

M18 
18 
2.5 
15.5 
1. 5 or 2 
16.5 or 16 
20 
M19 
19 
1 
18 

M20 
20 
2.5 
17.5 
1.5 
18.5 or 18 
22 
M24 
24 
3 
21 
1.5 or 2 
22.5 or 22 
26 
M25 
25 
1.5 or 2 
23.5 or 23 

M27 
27 
3 
24 
2 
25 
30 
M28 
28 


1.5 
26.5 

M28 
28 
2 
26 

M30 
30 
3.5 
26.5 
2 or 3 
28 or 27 
33 
M32 
32 
1. 5 or 2 
30.5 or 30 

M33 
33 
3.5 
29.5 
2 
31 
36 
M35 
35 
2 or 3 
33 or 32 

M36 
36 
4 
32 
2 or 3 
34 or 33 
39 
M38 
38 
1.5 or 2 
36.5 or 36 

M39 
39 
4 
35 
2 or 3 
37 or 36 
42 
M40 
40 
1.5 or 3 
38.5 or 37 

M42 
42 
4.5 
37.5 
3 or 4 
39 or 38 
45 
M45 
45 
4.5 
40.5 
3 or 4 
42 or 41 
48 
M48 
48 
5 
43 
3 or 4 
45 or 44 
52 
M50 
50 
2 or 3 
48 or 47 

M52 
52 
5 
47 
3 or 4 
49 or 48 
56 
M56 
56 
5.5 
50.5 
2 or 4 
54 or 52 
62 
M60 
60 
5.5 
54.5 
4 
56 
65 
FAQs
Here are some common questions people tend to ask about metric screw size charts:
1. What Does the Letter “M” Mean on Screw Packages?
The letter “M” on screw packages refers to the metric system (ISO). So, when you see it at the beginning of a callout, it means that the measurements are in mm.
2. What Is the Shank of a Screw?
The shank is the part of the screw that lies beneath the screw’s head. It can be threaded all the way to the tip, partiallythreaded, or not threaded at all.
3. What Is the Difference Between Metric Machine Screw Sizes and Wood Screw Sizes?
There isn’t a noticeable difference between machine and wood screw sizes. However, the difference is in the physical characteristics.
You can usually spot these with your eye, but you’ll definitely know when you try to fit one with another (we’ve all been there!).
Machine screws always have an allthreaded shank with finer threads.
Despite being smaller, they’re much stronger than other types of screws.
To Sum Up
Here you have it, our metric screw size chart that includes screw sizes in mm up to M60.
As you can see metric callouts and metric screw size charts are much easier to read and understand than imperial.
You only need to find the values for the screw’s diameter, pitch, and length.
Now, you can go and purchase your screw package with all confidence. Good luck!
dickb
Saturday 16th of March 2024
Since we also talked about imperial screw callouts, let’s read this screw thread callout as well:
#6 x 1 x 1/2?
We have here:
The first number represents the US gauge number which equals 6 inches The second number represents the number of threads per inch which is 1 The third and final number is the screw’s length which equals ½ inch
this is so bogus ....ah....... a 6" diameter screw 1 thread per inch and 1/2 inch long, give me a break. A #6 screw is not six inches across, it's a #6 gauge screw.