Need to make sure you’re choosing the right lag bolts? Here’s what most buyers ask us when they’re trying to size lag bolts for their wood, decks, or structural projects:
How do I choose the right lag bolt size? Start with diameter and length. The diameter determines strength, while the length determines how deeply the bolt penetrates and holds. Most projects use diameters between ⅜″ and ½″, with lengths between 4″ and 6″.
How do I measure a lag bolt correctly? Measure the diameter across the shank (not the threads!), and measure the length from the tip of the bolt to the underside of the head. Don’t include the head in the length.
What finish should I use — zinc, galvanized, or stainless steel? It depends. Indoor projects typically use zinc-plated lag bolts, outdoor wood projects usually require hot-dipped galvanized lag bolts, and coastal or high-moisture environments call for stainless steel lag bolts.
If you’d like to go deeper, we’ll walk you through lag bolt sizing step by step, show you how to measure an existing bolt, explain which finishes work best for different environments, and help you choose the right option for wood or concrete applications.
Ready to order? Browse our lag bolt selection — hand-packed and shipped fast from our Pennsylvania warehouse.
Lag bolt sizing is the most important factor in choosing the right fastener. The correct diameter and length ensure the bolt can safely support weight, resist pull-out, and remain secure over time.
Before buying, make sure you understand:
Lag Bolt Diameter: How thick the bolt needs to be to safely support the load
Lag Bolt Length: How deeply the bolt must penetrate to hold securely
Lag Bolt Finish: Whether you need hot-dipped galvanized lag bolts or stainless steel protection based on exposure
If you’re looking for a quick breakdown, check the lag bolt chart below for common lag bolt diameters, length, finishes, and applications.
|
Lag Bolt Diameter |
Typical Length Range |
Common Applications |
Recommended Finishes |
|
¼″ |
1″–3″ |
Light-duty fastening, brackets, trim |
Zinc-plated (indoor) |
|
⅜″ |
2″–5″ |
General construction, light decking |
Zinc-plated (indoor), HDG (outdoor) |
|
½″ |
3″–6″ |
Deck framing, ledger boards, gates |
HDG, 18-8 stainless |
|
⅝″ |
4″–8″ |
Structural wood, heavy outdoor builds |
HDG, 316 stainless |
|
¾″ |
6″–12″+ |
Heavy timber, critical load-bearing |
316 stainless |
Start with diameter based on load and importance of the connection
Choose length based on material thickness and penetration needs
Select a finish based on indoor vs outdoor exposure
Need more info? Here’s a quick guide to lag bolts, including what they are and what they’re used for.
The diameter of a lag bolt refers to the thickness of its shank. Larger diameters provide greater strength and resistance to shear forces, making them ideal for heavy-duty or structural applications.
As a general rule, ⅜″–½″ lag bolts handle most residential and construction projects, while larger diameters are reserved for structural or high-load applications.
If you’re deciding between two sizes, it’s usually safer to size up in diameter rather than rely on added length alone.
Lag bolt length determines how deeply the bolt penetrates the material and how securely it holds under load. A bolt that’s too short may pull out over time, while a bolt that’s too long can bottom out or split the material if not installed correctly.
Lag bolts are measured from the tip of the bolt to the underside of the head. The head is not included in the measurement.
For most wood-to-wood applications, the lag bolt should penetrate at least half the thickness of the second material being fastened into. Longer bolts provide more holding power only when paired with the correct diameter.
Tip: If you’re choosing between two lengths, select the shortest length that still provides adequate penetration. Oversizing length without increasing diameter does not significantly improve strength.
If you already have a lag bolt and want to confirm the size before buying, you only need two measurements. This quick check helps you match an existing bolt or confirm sizing with confidence before ordering.
Measure the diameter across the shank (the smooth part of the bolt). For fully threaded lag bolts (common in shorter lengths), measure across the threads.
Measure the length from the tip of the bolt to the underside of the head. The head is not included.
Most lag bolts use coarse threads, so thread measurement is rarely necessary for standard projects.
Lag bolt finish plays a critical role in how long the fastener lasts and how well it performs in different conditions. The right finish protects against corrosion, moisture, and chemical exposure.
Zinc-Plated Lag Bolts: Zinc-plated lag bolts provide basic corrosion resistance and are best suited for indoor or dry environments. They’re commonly used in interior framing, furniture construction, and light-duty applications.
Hot-Dip Galvanized Lag Bolts: Hot-dip galvanized lag bolts feature a thick zinc coating bonded to the steel. This finish offers moderate corrosion resistance and is ideal for outdoor wood projects such as decks, fences, and landscaping structures.
18-8 Stainless Steel Lag Bolts: 18-8 stainless steel provides strong corrosion resistance and is suitable for outdoor or damp environments where rust prevention is important but conditions are not extreme.
316 Stainless Steel Lag Bolts: 316 stainless steel offers the highest level of corrosion resistance and is the best choice for marine environments, coastal areas, or chemically exposed settings.
Choosing the correct finish prevents premature rusting, staining, and structural degradation over time.
Lag bolt threads determine how the bolt grips the material once installed, but for most buyers, thread type is not a primary decision point. Most lag bolts are designed with coarse threads that work well in common wood and construction applications.
Coarse-thread lag bolts are the most common and are best suited for wood and softer materials. The wider thread spacing provides strong grip and makes installation easier.
Fine-thread lag bolts have more threads per inch and are typically used in harder materials or specialized applications where precise alignment or vibration resistance is required.
For standard wood-to-wood projects, decks, and general construction, coarse threads are almost always the correct choice.
In the U.S., thread spacing is measured as threads per inch (TPI). For lag bolts, TPI typically ranges from 4 to 16, depending on diameter and application. Metric thread pitch measures the distance between threads in millimeters.
In practice, most buyers don’t need to measure or compare TPI when choosing lag bolts. As long as you select the correct size, length, and finish, the thread design will already be appropriate for the intended use.
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 small, family-owned business offering professional-grade lag bolts, hand-packed orders, fast shipping, and a customer-friendly return policy.
Browse lag bolts by size, finish, or application and purchase with confidence, knowing you’re getting fasteners built to perform.
Need help? We’ll help you find the right fastener for your project. Get in touch.
