Lag Bolt vs Lag Screw: Is There Actually a Difference?

If you have ever stood in an aisle wondering whether you need a lag bolt or a lag screw, the answer is simpler than the question suggests: they are the same fastener, and you can buy either one.

The Short Answer

One product, two names. Walk into any supplier and ask for either, and you will be handed the same thing: a large-diameter fastener with a hex head, a smooth shank under the head, and coarse tapered threads running to a pointed tip. It is driven with a wrench or a socket, not a screwdriver, and it threads directly into wood.

Catalogues, code documents and manufacturers all use both terms, sometimes on the same page. Nothing about the part changes with the label.

Why Two Names Exist

The confusion comes from a genuine distinction in fastener terminology that most people never learn:

• A bolt passes through both members and is tightened by a nut on the far side. The threads engage the nut, not the material.
• A screw threads directly into the material it is fastening, cutting or forming its own engagement. No nut involved.

By that rule a lag is unambiguously a screw. It has wood screw threads, it cuts its own path into the receiving member, and it never takes a nut. The reason people call it a bolt is that it looks like one — the hex head, the heavy diameter and the wrench-driven installation all read as bolt rather than screw, because ordinary wood screws are small and driven with a screwdriver.

Once the name caught on it stuck, and it is now so widespread that correcting it serves little purpose outside a spec document.

What the Standards Say

The dimensional standard for these fasteners in imperial sizes is ASME B18.2.1 — the same standard that covers hex bolts and screws generally. It refers to them as lag screws throughout, specifying head shape, thread dimensions and point geometry. Metric equivalents fall under DIN 571.

Material is usually specified separately: most commodity lag screws are supplied to ASTM A307 Grade A, a low-carbon steel with a 60,000 psi minimum tensile strength.

Anatomy of a Lag Screw

Four parts, and each one explains something about how the fastener is meant to be installed:

• The head — hex, driven with a wrench or socket. It gives an inspector a clear visual check that the fastener is seated, which is part of why lag screws appear in prescriptive code tables.
• The shank — the smooth, unthreaded section under the head. This is the part people forget: it is meant to pass freely through the piece being attached, which is why that piece gets a clearance hole at full shank diameter.
• The threads — coarse and tapered, designed to cut into wood fibre. They only engage the receiving member.
• The point — a gimlet point at roughly a 60° angle, which starts the thread cleanly in a pre-drilled lead hole.

The shank-and-thread split is the whole mechanism. Threads gripping the far member while the smooth shank slides through the near one is what pulls the joint tight. If the threads bite in both pieces, the joint jacks apart instead of closing — which is exactly what happens when someone drills one undersized hole straight through. The two-hole drilling rule and lead hole sizes are covered in detail separately.

Head and Body Types

Two variations are worth knowing about because they show up in older work and in spec sheets:

For ordinary construction you will only ever meet the hex, full-body version. Square-head lags are still made, and matter if you are matching existing hardware on an old building.

Lag Screw vs Hex Bolt: The Comparison That Actually Matters

If you came here asking “lag bolt or lag screw”, the useful version of that question is lag screw or hex bolt — because those genuinely are different fasteners that look similar.

The practical rule: if you can reach the back of the connection, a bolt is usually the stronger and more durable choice. If you cannot, a lag screw is what the job needs — and access, not strength, is what decides it.

Sizes and Lengths

Standard diameters run from 1/4" to 3/4", with heavier sizes made for timber work. Lengths run from about 1" up to 16".

The head size matters more than people expect: a 1/2" lag needs a 3/4" socket, and driving a run of them with the wrong size or a worn socket rounds heads fast.

How Long Does It Need to Be?

Length is set by how far the threads must reach into the receiving member, not by the thickness of the piece you are attaching.

• Full design value: NDS Section 12.1.4.6 calls for eight thread diameters of penetration into the main member. For a 3/8" lag that is 3" of thread.
• General work: five thread diameters is a common practical minimum — 1-7/8" of thread for a 3/8" lag, 2-1/2" for a 1/2".
• Then add the top piece. A 3/8" lag through a 1-1/2" bracket needs at least 3-3/8" total, so a 3-1/2" or 4" lag is the real pick.

Sizing the lag to the top board alone is the single most common length mistake, and it leaves a fastener that feels tight while barely engaging the member that is supposed to carry the load.

Hot-dip galvanized is the requirement outdoors and in pressure-treated lumber, not an upgrade. Washers matter too — the head crushes into softwood without one, and the clamping force you lose does not come back. This kit ships both, and the listing covers 3/8" and 1/2" in several lengths so you can match the penetration figure above.

When You Need a Real Bolt Instead

Related Guides

• Lag Bolts vs Carriage Bolts — pilot hole and clearance hole sizes, and which to use where
• Structural Screws vs Lag Bolts — when an engineered screw beats a lag
• Pilot Hole Size Chart — drill sizes for every screw type
• Deck Ledger Attachment — code fasteners, spacing and flashing
• Bolt and Wrench Size Chart — wrench size for every bolt, SAE and metric

Frequently Asked Questions

Is a lag bolt the same as a lag screw?

Yes. They are two names for one fastener. ASME B18.2.1, the dimensional standard that also covers hex bolts, calls it a lag screw. “Lag bolt” is the common shop and retail name and is understood everywhere, so neither term will get you the wrong part.

Which term is technically correct, lag bolt or lag screw?

Lag screw. The distinction between a bolt and a screw is how it is secured: a bolt passes through both members and is tightened with a nut, while a screw threads directly into the material. A lag threads into wood and takes no nut, so it is a screw.

What is a coach screw?

The same fastener again. “Coach screw” is the standard term in the UK and much of the Commonwealth for what North America calls a lag screw or lag bolt. Metric coach screws are covered by DIN 571.

Do lag screws need a nut?

No. If a connection needs a nut on the far side, a lag screw is the wrong fastener — use a hex bolt or a carriage bolt instead. The lag screw’s threads cut into the receiving member and that is what holds it.

How strong is a lag screw?

Commodity lag screws are usually supplied to ASTM A307 Grade A, a low-carbon steel with a 60,000 psi minimum tensile strength. In practice the wood governs long before the steel does — withdrawal capacity depends on thread engagement and the density of the receiving member.

Can you use a lag screw in metal?

No. Lag screws have coarse, tapered wood threads with a gimlet point and are designed to cut into wood fibre. Fastening into steel needs a machine-threaded bolt or a self-drilling screw rated for metal.

How deep does a lag screw need to go?

NDS Section 12.1.4.6 calls for eight thread diameters of penetration into the main member to develop the full design value. Five thread diameters is a common practical minimum for general work. Total length adds the thickness of the piece being attached.