Lag screws are heavy-duty construction fasteners with sharp threading that cover only a part of the bolt, allowing you to adjust them as needed.
They’re generally used for wood-to-wood and metal-to-wood fastening.
If you’re planning to use a lag screw for your next project, you should always check the lag screw size chart to make sure you’re using the right screw size.
I’m here to guide you with just that.
In this article, I’ll show you how to accurately and easily read a lag screw size.
I’ve also included a full chart of all the necessary measurements you need for your projects.
How to Read a Lag Screw Size Chart
Compared to bolts and nuts, lag screws have a lower lateral strength.
For this reason, they’re used in three main instances: when only one side of the joint is accessible, when a through-bolt length is excessive, or when the addition of a nut is impossible.
The terms “lag screws” and “lag bolts” are often used interchangeably.
Carriage bolts and lag bolts are also often viewed as the same; even though they’re two completely different materials.
Lag screws/bolts are fastened by rotating the head, allowing the screws to solidly penetrate into the material it’s installed in.
Carriage Bolts, on the other hand, are fastened into the material with a nut.
Lags are usually not used with a nut, so the proper technical term is lag screws rather than lag bolts.
Unless stated otherwise, lag bolts aren’t bolts, but rather screws.
Here’s how to read a lag screw size chart:
Identify the Parts of the Lag Screws
To properly read a lag screw size chart, you first need to familiarize yourself with the basic parts of a lag screw.
They are similar to general screw size chart characteristics and are as follows:
Head:
Situated at the top of the screw. It’s either hex or square-shaped.
Shank:
Threadless part of the bolt, situated between the head and the thread.
Thread:
Helical structure that runs around the cylinder of the screw.
Pitch:
Distance between adjacent threads, usually expressed in millimeters.
Thread Root Diameter:
Lower extreme diameter of the thread, also known as minor diameter.
Flat at Root:
Surface of the thread corresponding to the minor diameter of the screw.
Depth of Thread:
Distance between the crest and the base of the thread.
Measure the Length of the Lag Screw
Once you’ve familiarized yourself with the parts of a lag screw, you’re ready to read and understand the chart.
As you can see in the sizing chart below, lag screws typically range from 1/4-inch to 7/8-inch in diameter.
So, to properly use the chart, you’ll first have to identify the dimensions of your lag screw.
You can find the screw’s dimensions printed in its original box.
If you don’t have the box, manually measure the lag screw with a ruler or measuring tape from under the screw’s head to its threaded tip.
Take a Note of the Head Height, Head Width, and Thread Dimension
The next set of columns corresponds with the given dimensions of your lag screws.
The column next to the lag screw diameter gives you the basic, minimum, and maximum head height in decimals.
You’re likewise given the head width across flats and head width across corners, both of which measure the distance between two opposing surfaces on the head of a screw.
Thread dimensions are also listed to allow you to properly secure the screw in the given surface.
Real-Life Lag Screw Size Application
Let’s say you want to fasten two pieces of lumber with a lag screw.
First, measure the width of the combined wooden pieces together. Then, select a lag bolt that’s about 1/4 inch shorter than the wooden pieces.
Check your chart and slide over the corresponding lag screw size as in the row that lines up.
From there, you’ll be able to identify the head height, head width, and thread dimensions of the lag screw, with their subsequent minimum (min.) and maximum (max.) limits.
This will serve as a guide for when you’re fastening the wooden pieces together.
The thread sizes ensure that the threads don’t strip at the tensile strength limit of your wooden material.
Remember: the pilot hole for the body should match the screw size perfectly. So, for a 3/8-inch hole, use a 3/8-inch screw.
Standard Lag Screw Size Chart
Lag Screw Size Chart |
|||||||||||||
Bolt Diameter |
Head Height |
Head Width Across Flats |
Head Width Across Corners |
Thread Dimension |
|||||||||
F |
D |
Basic |
Max |
Min |
Basic |
Min |
Max |
Min |
Max |
Pitch |
Root Diameter |
Flat at Root |
Depth of Thread |
#10 |
0.19 |
1/8 |
0.14 |
0.11 |
9/32 |
0.271 |
0.281 |
0.309 |
0.323 |
0.091 |
0.120 | 0.039 | 0.035 |
1/4 |
0.25 |
11/64 |
0.188 |
0.15 |
7/16 |
0.425 |
0.438 |
0.484 |
0.505 |
0.100 |
0.173 | 0.043 | 0.039 |
5/16 |
0.3125 |
7/32 |
0.235 |
0.195 |
1/2 |
0.484 |
0.5 |
0.552 |
0.577 |
0.111 |
0.227 | 0.048 | 0.043 |
3/8 |
0.375 |
1/4 |
0.268 |
0.226 |
9/16 |
0.544 |
0.562 |
0.62 |
0.65 |
0.143 | 0.265 | 0.062 | 0.055 |
7/16 |
0.4375 |
19/64 |
0.316 |
0.272 |
5/8 |
0.603 |
0.625 |
0.687 |
0.722 |
0.143 | 0.328 | 0.062 | 0.055 |
1/2 |
0.5 |
11/32 |
0.364 |
0.302 |
3/4 |
0.725 |
0.75 |
0.826 |
0.866 |
0.167 | 0.371 | 0.072 | 0.064 |
5/8 |
0.625 |
27/64 |
0.444 |
0.378 |
15/16 |
0.906 |
0.938 |
1.033 |
1.083 |
0.200 | 0.471 | 0.086 | 0.077 |
3/4 |
0.75 |
1/2 |
0.524 |
0.455 |
1-1/8 |
1.088 |
1.125 |
1.24 |
1.299 |
0.222 | 0.579 | 0.096 | 0.085 |
7/8 |
0.875 |
37/64 |
0.604 |
0.531 |
1-5/16 |
1.269 |
1.312 |
1.447 |
1.516 |
0.0.250 |
0.683 | 0.108 | 0.096 |
1 | 1.0 | 43/64 | 0.700 | 0.591 | 1 1/2 | 1.450 | 1.500 | 1.653 | 1.732 | 0.286 | 0.780 | 0.123 | 0.110 |
1 1/88 | 1.125 | 3/4 | 0.780 | 0.658 | 1 11/16 | 11.631 | 1.688 | 1.859 | 1.949 | 0.308 | 0.887 | 0.133 | 0.119 |
1 1/4 | 1.25 | 27/32 | .0876 | 0.749 | 1 7/8 | 1.812 | 1.875 | 2.066 | 2.165 | 0.308 | 1.012 | 0.133 | 0.119 |
FAQs
How Far Should a Lag Screw Go Into Wood?
As a rule, your lag screw should go in a distance that is at least as thick as whatever it’s securing.
In general, the total length of your lag screw should at least be twice the thickness of whatever the material is that you’re attaching.
For example, if you’re attaching wood that is 2-inches thick, your lag screw should be a minimum of 4-inches long, so it can penetrate both the attaching material and go another 2-inches into whatever you’re securing into.
This is only a starting guideline though as there are many factors that determine the best answer for your particular project (type of wood, grain direction, load, number of other lag screws being used, etc.).
How Do You Screw a Lag Bolt Into Wood?
To install a lag screw, you’ll first have to clamp the wooden pieces together so they stay in place.
Then, use a bit with a slightly small diameter to drill a hole where you want the screw to go.
With a nut driver or ratchet, push the lag screw quickly but safely into the wood. Once the screw is in place, you can safely remove the clamps.
Conclusion
The above lag screw size chart will help you determine the right screw size for your project.
See also, our wood screw chart guide if you’re working with smaller woodworking projects.
Just like with eye-bolt sizes or any other screw-in fastener it’s important to use the ideal size to ensure you’re getting the most secure hold.
For a safe and accurate application, the screw thickness and diameter need to work well with the width of the material you’re using.
The wrong lag screw could not only cost you time and money but also risk your safety.