Lag bolts are heavy-duty wood screws with a hex head, enabling higher wrench installation tension. Lag screws are helpful when just one side of the joint is accessible. Using a nut isn’t suitable, or the through-bolt length is excessive.
They come in a variety of sizes and lengths. Conventional applications include wood-to-wood and metal-to-wood fastening, attaching objects to wood, and affixing items to concrete and masonry when combined with lag expansion shields. Lag bolts, hex lag bolts, and coach screws are other names.
Lag screws are thread-forming screws because mating threads are automatically produced during installation, eliminating the need for a through-hole and nut. Because of the range of uses, you can find hex lag screws dimensions vary to cover all tasks, so picking the right ones can be challenging. In our guide, you can use our chart to find the correct lag screws dimensions to meet your needs.
By the end, you’ll see how to measure the nominal screw length and pilot hole sizes to complete your project with our lag screw size chart. (Read Carriage Bolt Sizes Chart)
What Sizes Do Lag Bolts Come In?
Lag screws are fully threaded and come with coarse threads that are widely separated:
- 10 threads per inch for a 1/4″ diameter lag screw.
- 3 1/4 threads per inch for a 1 1/4″ lag screw.
Right-hand threads are standard; longer lengths have a minimum threaded length of one-half the screw length + 1/2″, or 6″, whichever is shorter.
You will see the gimlet point, which is a threaded cone point and an angle of 45 to 50 degrees, so they can start threading into wood:
Lag screws are typically 1/4″ to 3/4″ in diameter, with lengths ranging from 1″ to 16″.
The screw’s length is measured from under the head to the threaded tip.
The screw size determines the size of the head. Because lag screws are externally wrenched, enough room must be around the head for wrench clearance.
Steel and stainless steel are the most frequent materials for lag screws, with zinc coating and hot-dip galvanizing as finishes.
Zinc is the most often used and least expensive metal with moderate corrosion resistance.
Fully threaded lag screws that have been hot-dip galvanized have a thick zinc coating that protects them against corrosion in severe conditions.
If the screws will be used with pressure-treated wood, hot-dip galvanized, and stainless steel is usually recommended. (Read Allen Bolt And Allen Key Size Chart)
Lag Screw Dimensions and Specifications
| Nominal Size | Threads per Inch | Head Width | Head Height |
||
|---|---|---|---|---|---|
| Across Flats | Across Corners | ||||
| Fraction | Decimal | Basic | Avg | Basic | |
| 1/4 | 0.2500 | 10 | 7/16 | 1/2 | 11/64 |
| 5/16 | 0.3125 | 9 | 1/2 | 9/16 | 7/32 |
| 3/8 | 0.3750 | 7 | 9/16 | 41/64 | 1/4 |
| 7/16† | 0.4375 | 7 | 5/8 | 45/64 | 19/64 |
| 1/2 | 0.5000 | 6 | 3/4 | 27/32 | 11/32 |
| 5/8 | 0.6250 | 5 | 15/16 | 1 1/16 | 27/64 |
| 3/4 | 0.7500 | 4 1/2 | 1 1/8 | 1 17/64 | 1/2 |
The pilot hole for lag screws, like the one for wood screws, should be stepped.
The lag screw pilot hole for the unthreaded shank piece adjacent to the head should have the same size as the screw (for example, a 3/8″ hole for a 3/8″ screw).
The screw size and density of the wood define the size of the pilot hole for the threaded section.
A screw may break if the pilot holes are too small. If the hole is excessively large, the resistance to withdrawal may be impaired.
If you use a fully threaded lag screw, be sure to put a wide diameter washer behind the head to distribute clamping force over a broader area and prevent wood compression. (Read Socket Size Chart)
How Is The Diameter Of A Lag Bolt Measured?
Standard Thread Length
The minimum thread length is 1/2 the nominal screw length + 1/2 inch or 6 inches, whichever is less. For excessively lengthy lag screws, greater thread length plus a bit should should be considered. You can often find most follow the ASME b18.2.1 specifications
Note that hex lag screws bigger than 3/4′′ in diameter and all square lag screws are rarely available as stock items in the marketplace.
| Basic Product Diameter | Threads Per Inch | Body or Shoulder Diameter (E) | Width Across Flats (F) | Width Across Corners (G) | Head Height (H) | ||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Max | Min | Basic | Max | Min | Max | Min | Basic | Max | Min | ||
| #10 | 11 | .199 | .178 | 9/32 | .281 | .271 | .323 | .309 | 1/8 | .140 | .110 |
| 1/4 | 10 | .260 | .237 | 7/16 | .438 | .425 | .505 | .484 | 11/64 | .188 | .150 |
| 5/16 | 9 | .324 | .298 | 1/2 | .500 | .484 | .577 | .552 | 7/32 | .235 | .195 |
| 3/8 | 7 | .388 | .360 | 9/16 | .562 | .544 | .650 | .620 | 1/4 | .268 | .226 |
| 7/16 | 7 | .452 | .421 | 5/8 | .625 | .603 | .722 | .687 | 19/64 | .316 | .272 |
| 1/2 | 6 | .515 | .482 | 3/4 | .750 | .725 | .866 | .826 | 11/32 | .364 | .302 |
| 5/8 | 5 | .642 | .605 | 15/16 | .938 | .906 | 1.083 | 1.033 | 27/64 | .444 | .378 |
| 3/4 | 4-1/2 | .768 | .729 | 1-1/8 | 1.125 | 1.088 | 1.299 | 1.240 | 1/2 | .524 | .455 |
| 7/8 | 4 | .895 | .852 | 1-5/16 | 1.312 | 1.269 | 1.516 | 1.447 | 37/64 | .604 | .531 |
What Size Head Is A 3/8 lag Bolt?
Lag Screw Dimensions. Threads per inch, head width across flats and corners, and head height in inches are all given in fractional and decimal inches.
What Diameter Is A #15 Lag Screw?
Lag screws are a hybrid between a wood screw and a bolt. A lag screw pilot hole should be smaller in diameter than the actual screw, similar to wood screw pilot holes.
This generates a tight fit, allowing the lag screw’s teeth to dig into the pilot hole’s walls. As a result, lag screw pilot hole sizes are noticeably smaller than bolt pilot holes.(Learn How To Screw Into Concrete)
It makes no difference what the drill bit’s coating or substance is. The drill bits will not break because the screw holes are so huge.
As a result, HSS drill bits and other low-cost choices are suitable for drilling lag screw pilot holes. However, larger bits require more time to dissipate heat.
Clear the sawdust out of the flute of the drill bit between each stoke to speed things up. This will ensure that the heat isn’t trapped against the metal.
| Pilot Hole Sizes for Softwood | Lag Screw Diameter | Pilot Hole Size for Hardwood |
|---|---|---|
| 3/32″ | 1/4″ | 3/16″ |
| 9/64″ | 5/16″ | 7/32″ |
| 11/64″ | 3/8″ | 1/4″ |
| 1/4″ | 1/2″ | 11/32″ |
| 5/16″ | 5/8″ | 1/2″ |
| 3/8″ | 3/4″ | 9/16″ |
Lag Screws in Softwoods
Always try a smaller bit when drilling softwoods. The lesser-density wood fibers allow for better screw compression. This results in a more secure fastening and tighter fit.
Exceptions include aromatic cedar and other hard softwoods. If you aren’t sure, we offer a chart with all the hardness ratings for North American softwoods.
Though you don’t have to countersink your lag screw, it is a good recommendation.
Lag Screws in Hardwoods
The hardest part about hardwoods is driving the lag screw in.
The 1/4′′ hex shank socket will likely ground out when the lag screw advances deeper into the pilot hole. In this case, insert little washers into the socket.
When the socket wraps around the lag screw head, it becomes shallower. Remember that nut setter bits are safer and faster.
How do you use a lag screw?
When your wood is lined up, clamp them together and drill a lag screw pilot hole through the materials where you want the lag screw gimlet point to grip the wood.
Regular screws cannot be tightened in such small pilot hole sizes, yet because lag screws have hex heads, you can use a larger bolt diameter.
How do I know what size lag bolt to get?
Calculate the combined width of the pieces. Then, select a lag bolt that’s 1/4 inch shorter than the aggregate width of the two parts.
In most cases, lag bolts with a diameter of 5/16 inches will suffice. Use a 3/8-inch diameter or bigger lag bolt for larger or heavier-duty joints.
Do you need washers with lag screws?
The installation of a lag bolt does not necessitate the use of nuts. However, to enhance the surface area in contact with the wood, a washer is used on both screws and bolts. This stops the hex head from ripping through the wood and losing its grip.
