What Is The Difference between A Bolt And A Screw?

Think all bolts are screws? Think again.Many people mix them up—and it can lead to costly mistakes.Using the right fastener matters in construction, engineering, and DIY projects.In this post, you'll learn the real difference between bolts and screws.We'll explain how to identify them, when to use each, and why it matters.

Basic Definitions

What is a Bolt?

A bolt is an externally threaded fastener. But unlike screws, bolts don't cut their own threads.

Instead, bolts go through unthreaded holes and are secured using nuts or threaded inserts. This makes them ideal for joining multiple parts together, especially when strength and reliability are key.

Bolts are usually designed to be tightened by turning the nut, not the head. You'll often see them with hex or square heads for use with wrenches or spanners.

Here are a few common types of bolts:

They're often partially threaded, and materials range from steel to bronze.

What is a Screw?

A screw, like a bolt, has external threads. But screws don't need nuts to stay in place.

Instead, they form their own threads as they're driven into a surface—like wood, plastic, or even metal. Some screws mate with pre-threaded holes, but many are designed to cut into the material directly.

Screws are tightened by turning the head, typically with a screwdriver or bit.

Common examples of screws:

Screws are usually fully threaded. They're often used where access to only one side of the workpiece is possible.

Key Differences Between Bolts and Screws

Threading

This is where it starts to get real.

● Bolts usually have a partially threaded shank. The smooth section adds shear strength.

● Screws are fully threaded from tip to head. That gives them better grip in materials like wood.

Bolts handle structural loads better. Screws hold fast in soft or light-duty jobs.

Installation Method

Bolts need a nut or threaded hole to function. That means you'll likely need access to both sides of your workpiece.

Screws? Much easier. You just drive them into place from one side. That's great for quick tasks.

● Bolts: Insert through holes, tighten with nut

● Screws: Drive directly into material

So, bolts are common in metal fabrication, while screws dominate in woodwork and home fixes.

Tools Required

Here's what you'll need to tighten each type:

Bolts require torque from the side (via nut), while screws are tightened head-on.

Holding Strength

Now let's talk strength.

● Bolts are generally stronger. That's thanks to the nut + washer combo, which distributes the load and resists pullout.

● Screws, while not as strong overall, grip better in soft or fibrous material. Perfect for wood and plastic.

So if you're building a deck? Go for bolts. Hanging drywall? Screws are your friend.

Common Misconceptions

Size Does Not Define a Bolt or Screw

It's easy to think big = bolt, small = screw. But that's false.

You can have large screws and small bolts. Size doesn't matter. It's the thread behavior and installation method that count.

You might hold a heavy bronze fastener and think, "definitely a bolt." But if it cuts its own threads—it's a screw.

Drive Type is Not a Defining Feature

Another myth? “If it uses a wrench, it's a bolt.” Nope.

Many screws have hex heads. Some bolts use slotted heads. The tool needed does not determine the type.

Examples:

● A lag screw has a hex head, but it's still a screw.

● A fully threaded fastener with a Phillips head? Still could be a bolt—if it's used with a nut.

So, never judge a fastener by its head alone.

Classification Confusion: What About Hybrid Fasteners?

In the fastener world, names don't always match technical definitions. That's where things get tricky. Many fasteners blur the lines between bolts and screws—making classification confusing, especially for beginners or even seasoned DIYers. Let's break down three common examples of hybrid fasteners and explain why they're not always what they seem.

Are Machine Screws Really Bolts?

This one surprises a lot of people.

Machine screws look like screws, behave like bolts, and live in a gray zone. Technically speaking, if a fastener is externally threaded and used in a pre-tapped hole or with a nut, it qualifies as a bolt—not a screw.

Most machine screws don't create their own threads. Instead, they're installed into pre-threaded holes, just like bolts. Some can also be paired with nuts to fasten materials. That checks off two “bolt” boxes.

So why do we still call them screws? Mostly tradition and industry habit. You'll find them labeled as “machine screws” in catalogs and hardware stores, even though they're mechanically bolts.

In short, machine screws are bolts in disguise—technically bolts, culturally screws.

Are Lag Bolts Actually Screws?

Despite the name, lag bolts behave nothing like true bolts.

● They don't go through pre-drilled holes.

● They don't use nuts.

● They cut their own threads into wood.

These are all characteristics of screws. The only reason they're called “lag bolts” is because of their large hex heads and the need to use a wrench during installation.

But that's a red herring.

Tool type and head shape don't define the fastener class. Threading behavior does.

So when you install a lag bolt by driving it into wood—creating threads as it goes—you're technically installing a lag screw.

The takeaway? Lag bolts are really screws, despite their bulky build and misleading name.

What If a Bolt Has a Thread-Cutting Tip?

This one is extra tricky.

Sometimes, you'll find a fastener that looks exactly like a bolt—hex head, sturdy body, threaded shaft—but the tip is specially designed to cut threads as it enters the material.

That tip is the key.

Even though the fastener looks like a bolt, if it cuts its own threads, it behaves as a screw. True bolts don't form threads—they rely on nuts or pre-threaded holes.

These types of fasteners are often called thread-cutting bolts, but they're technically screws because of how they function.

In fact, many manufacturers list them under “screws” but include “bolt” in the name just for familiarity.

So, again, appearance doesn't always tell the truth. Function does.

Conclusion

Need a nut to hold it? Use a bolt. Driving it directly into wood or metal? That's a screw. Need strong, reusable fastening? Choose a bolt. Want a tight grip with fewer parts? Go with a screw. Use this guide when picking fasteners for your next project. It saves time—and helps you build better.

FAQ

Q: Can you use a screw instead of a bolt?

A: Sometimes. Screws work well when only one side is accessible, but bolts offer stronger, reusable fastening for heavy-duty tasks.

Q: Are bolts always stronger than screws?

A: Yes, generally. Bolts use washers and nuts, spreading force and offering greater strength in structural applications.

Q: What happens if you use the wrong fastener?

A: The joint may fail, loosen, or strip the material. Using the right fastener ensures durability and safety.

Q: How can I tell by looking whether it's a bolt or a screw?

A: Bolts need nuts or pre-threaded holes. Screws cut or match threads and are driven by the head.