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Lag Screw Size Chart: Measurements and How to Find What You need

Lag Screw Size Chart: Measurements and How to Find What You need

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

Lag Screw

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

Parts of a Lag Screw

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.

Sharing is Caring!

Scott Sater

Sunday 25th of September 2022

Good day, We have a piece of equipment that came with 9mm x 8cm lags. because of a facia and space concern these lags are 1-1.5 inches long. Are 7/16 lags available anywhere in the US? We here in Orlando, have used, It seems, all available options. We are looking for 7/16 x 4-4.5 in. lags...(indoor)

Thank you, Scott Sater

Jerry

Wednesday 7th of September 2022

Brian, I think your root diameters are way too big. These root diameters seem to correspond with much larger screws. You must have read from the wrong part of your original chart when you put this one together.

Sheila Spencer

Wednesday 27th of April 2022

I think you got this backwards. May want to check it.

"How Far Should a Lag Screw Go Into Wood? In general, the total length of your lag screw should at least be double the thickness of the material you’re using.

For example, if your wood is 10-inches thick, your lag screw should be around five inches long or just slightly above that mark."