Types Of Lag Bolts | Sizes, TPI, & Thread

Finding the right lag screws, also known as lag bolts, for your project starts with understanding the different characteristics of lag screws that impact their functionality.

Whether you're tackling a DIY project or a professional build, this article will help you select the perfect lag bolt for secure, long-lasting connections. Discover everything you need to know about lag bolts, from their sizes and specifications to types and finishes, in this comprehensive guide.

• Lag Bolt Diameter
• Lag Bolt Length
• Lag Bolt TPI or TP
• Lag Bolt Thread Type: Coarse VS Fine
• Types of Lag Bolts (Lag Bolt Finishes)
• Lag Bolts For Wood
• Lag Bolts For Concrete
• How To Measure Lag Bolt Sizes?
• Browse Lag Bolts

Lag Bolt Diameter

A lag bolt or lag screw has a thick, hexagonal head and a coarse, sharp threaded shaft.

Lag bolt diameter typically refers to the diameter of a lag bolt’s shank or the smooth neck beneath the bolt’s hex head. A lag screw’s diameter typically ranges from ¼ inches to ¾ inches. This measurement impacts the lag bolt’s:

• Tensile strength
• Load-bearing capacity
• Compatibility with standard nuts and washers
• Resistance to shear forces
• Stress distribution

In short, choosing the right diameter helps to ensure that your project remains sturdy and stable.

If you’re not sure which diameter is best for your project, assess the load your bolt will support, the thickness of your materials, industry and legal codes, and the bolt manufacturer’s specifications sheets.

If you’re still unsure, consult a professional. It’s better to be safe than sorry, especially for projects that will store valuables or must safely support heavy weights.

Lag Screw Root and Thread Diameters

In some cases, one may need to measure the diameter of a lag bolt’s thread diameter or root diameter. A lag bolt’s thread diameter measures the diameter of a lag bolt’s threads, rather than its shank, while the bolt’s root diameter measures the diameter at the base of the threads.

Lag Bolt Length

Lag bolt length is the measurement from the tip to the base of the head, not including the head itself. The length of a lag bolt determines its:

• Depth of penetration into materials
• Suitability for binding various material thicknesses
• Stability and resistance to coming loose
• Effectiveness in maintaining structural integrity
• Fit for particular applications

Common lag bolts are sized up to 16 inches in length, however, larger bolts are best suited for industrial or mechanical purposes.

Hobbyist or personal projects, such as building a wood deck or a mattress frame, typically utilize shorter length lag screws. Our stock of stainless steel lag bolts that range from ¼ inch diameter and 1 inch length to ¾ inch diameter and 6 inches in length, so we have exactly what you need to get your project handled.

Lag Bolt TPI or TP

Lag bolt TPI (Threads Per Inch) or thread pitch refers to the spacing between threads on a lag bolt.

A finer thread (higher TPI or smaller TP) generally provides greater holding power because the increased number of threads engages more material, enhancing grip. Coarse threads are better suited for soft materials, while finer threads are ideal for harder materials where more precise alignment is needed.

A lag bolt’s pitch or TPI measurement affects the bolt’s:

• Grip strength in different materials
• Ease of installation
• Ability to withstand pulling forces
• Compatibility with the materials being fastened
• Overall holding power

US TPI VS Metric TP

In the US, thread pitch is expressed in threads per inch (TPI), indicating how many threads exist within one inch of the bolt’s length. Conversely, the metric system measures thread pitch as the distance, in millimeters, between individual threads along the bolt.

• Lag Bolts TPI: TPI for lag bolts commonly ranges from 4 to 16 TPI. For example, a bolt with a TPI of 7 means there are 7 threads per inch.
• Lag Screw Thread Pitch (TP): In the metric system, thread pitch is measured as the distance (in millimeters) between threads. Common pitches for bolts may include 1.25 mm, 1.5 mm, or 2 mm, depending on the bolt's diameter and intended application.

Many projects are best served by lag screws with a pitch of up to 10. Lag screws with a higher pitch offer more threads, but can be harder to drive into harder materials due to increased friction and require more precise alignment.

Lag Bolt Thread Types: Coarse VS Fine Thread

Lag bolt thread types are generally categorized as coarse or fine. For lag screws, coarse threads are the most common as they provide strong grip and are well-suited for penetrating and securing softer materials like wood.

Coarse threads are spaced farther apart than fine threads, providing greater grip and are generally easier to install, especially in softer materials like wood.

Fine threads, on the other hand, have a smaller pitch and more threads per unit length, offering better holding power and resistance to vibrations, but are more suited to harder materials.

A lag bolt’s threading type affects the bolt’s grip, resistance to loosening, use cases, and durability under stress.

Types of Lag Bolts (Lag Bolt Finishes)

Lag bolts are typically made from steel or silicon bronze and finished to enhance their performance and longevity in various environments. The right finish can provide additional protection against corrosion, wear, and other external factors. Here’s a breakdown of how lag bolt finishes impact their performance:

• Zinc-Plated Lag Bolts: Zinc plating involves applying a zinc coating to typically steel bolts through electroplating, providing a layer of protection that’s suitable for dry, non-corrosive environments. It is most effective for screws used in indoor projects where exposure to moisture is minimal.
• Hot-Dipped Galvanized Lag Screws: This process involves dipping steel bolts into molten zinc, creating a thicker zinc layer that is bonded to the steel, offering moderate corrosion resistance. Hot-dipped galvanized bolts are well-suited for general use in outdoor settings where occasional moisture is present but not as challenging as marine or industrial environments.
• 316 Stainless Steel Lag Bolts: Composed largely of chromium, nickel, and molybdenum, 316 stainless steel is renowned for its excellent resistance to corrosion, particularly from chloride and acidic environments. These bolts are ideal for highly corrosive conditions, such as those found in seaside or industrial environments, providing superior durability.
• 18-8 Stainless Steel Lag Screws: This stainless steel alloy consists of approximately 18% chromium and 8% nickel, delivering strong corrosion resistance, especially in highly corrosive environments. They are a reliable choice for projects that require robustness and long-term exposure resilience.

Lag Bolts For Wood

Here are the best types of lag bolts for wood:

• Coarse Thread Lag Bolts: Look for lag bolts with coarse threads, as they provide superior grip in wood, helping to prevent loosening over time and ensuring a secure connection.
• Zinc-Plated Lag Bolts: These are ideal for indoor wood projects or environments with low moisture exposure, offering basic corrosion protection that suits dry conditions.
• Hot-Dipped Galvanized Lag Bolts: If your wood project is outdoors, consider using hot-dipped galvanized lag bolts. They offer moderate corrosion resistance against weather exposure, making them suitable for decks, fences, or garden structures.
• Appropriate Length and Diameter: Select a lag bolt long enough to penetrate deeply into the wood for stability, typically extending halfway into the second piece of wood. Ensure the diameter is suitable for the load and stress the wood structure will endure.

Lag Bolts For Concrete

The best types of lag bolts for concrete include the following characteristics:

• Lag Shields or Anchors: When working with concrete, use lag bolts with lag shield anchors or similar anchors. These provide the necessary grip for threads in a non-pliable material like concrete.
• Coarse Thread Lag Bolts: Choose coarse thread bolts as they work well with concrete anchors, ensuring a secure fit within the sleeve or shield.
• Hot-Dipped Galvanized Lag Bolts: Given the potential exposure to moisture in outdoor or semi-outdoor settings, hot-dipped galvanized lag bolts offer moderate corrosion resistance, prolonging the lifespan of the fastener in concrete.
• 316 Stainless Steel Lag Bolts: For environments with high moisture or chemical exposure, such as marine areas, 316 stainless steel lag bolts provide superior resistance to corrosion, making them ideal for concrete applications that demand durability.
• Correct Size and Length: Ensure the length of the lag bolt and anchor is appropriate for your setup, allowing deep enough penetration to secure the bolt while considering the thickness of the concrete and expected load pressures.

How To Measure Lag Bolt Sizes?

Here’s a step-by-step breakdown on how to measure your lag bolt’s size, including taking measurements for the bolt’s diameter, length, thread pitch, and thread length. You’ll need calipers and/or a tape measure, a lag bolt to measure, and some way to record your measurements for posterity.

1. Gather Your Tools

• Tape Measure or Ruler: For basic measurements of length and diameter.
• Caliper: Ideal for precise measurements of diameter and thread pitch, especially if working with smaller bolts.

2. Measure the Diameter

Identify your lag bolt’s shank. This is the smooth, non-threaded part of the bolt.

Place the caliper or tape measure around the shank or, if fully threaded, around the major diameter of the threads. This gives you the outside diameter, which is the largest diameter of the threads. Record this measurement in inches or millimeters, depending on the bolt type.

3. Determine the Length

Lag bolt length is measured from the tip of the bolt to the underside of the head. Always avoid including the head in your measurement.

To measure with tape, lay the bolt flat and extend the tape measure from the pointed tip to where the shank meets the head's underside. Ensure accuracy by aligning the tape carefully to avoid angled measurements.

To measure with a caliper, open the jaws of the caliper and position one end at the tip of the bolt. Extend the caliper so that the other end touches the underside of the head of the bolt. The caliper will give you a precise measurement of the length, excluding the head, which is the standard way of measuring bolt length.

4. Check the Thread Pitch

For U.S. bolts, you’ll need to determine the lag bolt’s TPI. To do so, measure how many threads occur over a one-inch span. For example, if there are 7 threads in one inch, the TPI is 7.

With metric bolts, use a thread gauge to determine the distance between threads, recorded in millimeters (e.g., a pitch of 1.5 means there is 1.5 mm between each thread).

5. Assess the Thread Length

For partially threaded bolts, use the ruler to measure from the tip of the bolt to where the threads end. This will help you understand how much of the bolt is available for threading into the material.

6. Review and Record

Once you have the measurements, confirm they match your application requirements for both diameter and length, ensuring you have the right size for your project needs.

Browse Lag Bolts From The Fastener Depot

We’ve put together this guide to help make it easier for you to purchase the right lag bolts for your construction project, whether you’re building the deck of your dreams, mounting heavy loads, or serving as the primary support fastener for other projects. We value our customers and want to make the purchase process as seamless as possible.

The Fastener Depot is a family-owned small business committed to providing you with high-quality fasteners and the expert guidance needed to select the perfect fit for your project.

We offer free shipping for orders over $50 and a customer-friendly 30-day return policy, making it easy for you to source the best materials. Count on us for superior products and a commitment to customer satisfaction, ensuring your next project is a success from start to finish.

Let us help you find the right fastener for your project. Get in touch.