What is the Difference Between Bolt and Screw?

It’s a common assumption that bolts are screws that refer to the same fastening hardware. But while they look similar and have similar characteristics, they are two unique fasteners with their own unique applications. So, what’s the difference between a bolt and screw?

Bolt vs. Screw: Definitions

Let’s begin with a couple of definitions provided by the CBP which are quoted from ANSI-ASME B18.2.1:

• Bolt: A bolt is an externally threaded fastener designed for insertion through the holes in assembled parts, and is normally intended to be tightened or released by torquing a nut.
• Screw: A screw is an externally threaded fastener capable of being inserted into holes in assembled parts, of mating with a preformed internal thread or forming its own thread, and of being tightened or released by torquing the head.

A bolt is meant to be used with a nut, and it is tightened by torquing the nut. A screw is designed to be used in either a preformed or threaded hole, or it is capable of forming its own threaded hole. A screw is designed to be tightened by torquing the head.

What are Bolts?

A bolt is an externally threaded fastener designed for insertion through holes in assembled parts. Most often, it’s intended to be tightened or released by torquing a nut.

While there are many types of bolts and their designs vary, most bolts have the same general features. They have a threaded cylindrical rod, along with a head at one end and, usually, a nut at the other.

Bolt heads are normally larger than their bodies, which is a design that helps distribute the load of the object being fastened over a larger area. The associated nut is used to secure the joined objects in place.

There are dozens of different types of bolts, some of which include anchor bolts, arbor bolts, elevator bolts, hanger bolts, hex bolts, J bolts, lag bolts, rock bolts, shoulder bolts and U bolts.

Additionally, bolts are available in a wide range of materials, including steel, stainless steel, bronze, brass and nylon. Statistics show, however, that up to 90% of all bolts are made of steel, making it the preferred choice among manufacturing companies.

Bolts are used for many applications, with construction, manufacturing, and automotive industries being the most common.

What Is A Screw?

A screw is a broad category of mechanical fasteners with a threaded shank designed to be screwed into a part.

These include wood screws and self-tapping screws, which have a tapered shank with sharp threads designed to cut matching threads into the part to which they are attached. It also includes machine screws, which are much more like bolts, but their entire shank is usually threaded.

Bolts and screws are sometimes distinguished by their use. According to these definitions, a bolt goes through unthreaded holes in parts and is secured by a nut, while a bolt mates with a threaded hole in one of the parts to be fastened.

In practice, this definition is not strictly applied. The term screw is usually used for a fastener that is not self-tapping and has only a portion of its shank threaded. Fasteners that are either self-tapping or have the entire shank threaded are usually referred to as screws.

A screw is usually installed in a threaded hole unless it is a self-tapping screw that creates its own thread. Bolts don’t need nuts as they are secured by tightening them into the hole with a screwdriver or screwdriver bit that fits into the drive recess.

Screws are generally shorter than the width of the material they are screwed into so they don’t protrude on the other side.

There are also dozens of different types of screws, including chipboard screws, particleboard screws, deck screws, drive screws, hammer drive screws, drywall screws, eye screws, and dowel screws, wood screws, twin fast screws, security head screws, and sheet metal screws.

Some of the different head shapes of the screws are pan, button, round, mushroom, oval, bulb, cheese, larynx, and flange head shapes. And like their bolt counterparts, screws are available in a range of materials.  

Bolts Vs Screws: What’s the Difference?

The difference according to a thesaurus is that there is no difference as bolts and screws are regarded as synonyms of each other. Whilst it would be easy to lump all these fixings together (it would make our job easier!) there are actually significant differences that a user needs to be aware of before making a purchase.

It is true that, to the naked eye, there does not appear to be much difference between a bolt and a screw. They are both threaded fixings and have a head for tightening the fastener but there is more to it than that.

The most obvious way of differentiating between a bolt and a screw is that a bolt is not usually threaded all the way along its shank as it has a plain portion. A screw, however, is threaded fully to the head.

Bolt vs. Screw: Four Primary Criteria

The four primary criteria are based on the root of the two definitions above. Each of the four criteria essentially attempts to confirm a key element of one definition or the other.

If a part in question clearly satisfies any of the four criteria, it can be identified without any further examination.

These are classified as bolts because you can’t turn the head during assembly.

Criterion 1 — Bolt

If the fastener in question has a head or other design feature that prevents turning during assembly and which can be tightened only by turning a nut, then the fastener is a bolt. This criterion is a process of elimination.

Screws are designed to be tightened by turning the head, so if you can’t turn the head, the part must be a bolt.

Criterion Two — Bolt

If an externally threaded fastener has an intended function that requires it to be assembled with a nut, then the fastener is a bolt.

This criterion obviously requires knowledge of the intended function of the fastener in question.

Criterion Three — Screw

If the fastener in question has a thread form that prohibits it from being assembled with a nut, then the part is a screw.

This is another process of elimination situation. If the threads of a part prevent the part from being used with a nut, then the part cannot be a bolt and must be a screw.

Criterion Four — Screw

If the fastener is designed to be torqued by its head into a tapped or preformed hole, then the fastener is a screw.

Again, knowing the intended function of a part can be tremendously helpful. Any part designed to be torqued by the head into a tapped hole is a screw.

How To Decide When to Use Bolts Instead of Screws

Before choosing a side in the bolts vs. screws debate for your next project, think about these three factors:

Application

Your project is the biggest factor in whether you use screws or bolts. Remember: bolts provide a tremendous amount of holding strength whereas screws offer a middle ground for holding power.

Light to medium construction projects, like fencing or framing, are best for screws. Your heavy-duty projects are best for bolts. Bolts are also best when the finished project is meant to be assembled and disassembled easily.

Material

Whether you’re using sheet metal, wood, concrete, or drywall for your DIY undertaking, you want to think about how it applies to bolts vs. screws. Material matters.

Materials That Typically Require Screws:

• Plywood
• Pressboard
• Drywall
• Sheet metal
• Deck planks

Materials That Are Usually Best for Bolts:

• Concrete
• Studs
• Beams

There are some materials that could require either a screw or a bolt assembly for fastening. For example, a 2” x 4” might be best fastened as a stud that is part of the framework for a wall using screws, but for an application like a deck assembly, bolts are likely a better choice.

Be sure to consider the load being put on the fastener in combination with the material when choosing between bolts and screws.

Cost

Price is always a factor when budgeting a project, especially if you’re going to need a large number of fasteners. While using bolts can be more costly, using the correct fastener will save you money in the long run.

If you end up having to do repairs on the project because you used the wrong fastener for the material or application, you aren’t really saving any money by going the cheaper route.

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