What is the difference between a bolt and a screw? This is a surprisingly common question with a complex answer. There is a surprisingly large amount of wrong information out there.
Before we start, please remember there are legal definitions of fasteners that vary from country to country – this article discusses the proper terminology as used in the industry and in industry standards books and not the frequently erroneous taxing and tariffing regulation definitions.
There are two three-step criteria to go through for final determination. The first will tell you if it qualifies as either a bolt or screw and the second set tells you which one of the two it is.
Is it a bolt, screw, or something else?
The first three-step criterion:
- First, and foremost, you cannot tell by the thread length, diameter, or whether or not it takes a nut. You can’t even tell by the shape of the head or whether it even has a head or not. The many websites and reference books that make this claim1 are all wrong and they shouldn't be trusted if they can't get something this basic correct.
- Secondly, it must be externally threaded, and it must have a drive mechanism – either a head or a drive (or both). So, a fully threaded rod (or stud) is neither a bolt nor a screw because it has no head and no drive mechanism. The commonly used term "stud bolt" is erroneous and must not ever be used. A stud is not a bolt – it’s part of the threaded rod family. Note that Cap Screws are really Machine Bolts with a chamfered washer face, so are properly bolts and not screws.
- Thirdly, it must have continuous threads that circumnavigate the shaft of the part at an angle to the appropriate length – the tips of some screws have notches and cuts and special points but that doesn't affect the determination. If it doesn't have external threads, it's neither a bolt nor a screw. Some items that are threaded both inside and outside, such as bushings, are neither screws nor bolts. If it's got internal threads, it's never a bolt or screw.
Identifying a Bolt vs a Screw
Once you've gone through the first three steps and met them all, then you know it's either a bolt or a screw and not some other fastener. Then you have….
The second three-step criterion.
- If you need to put it in with a wrench or some other non-drive tool, it's probably, but not always a bolt.
- If it can't take a nut – or some other matching thread or tapped mating hole – then it's a screw. Period. Without exception. (And, before you suggest Lag Bolt, those are properly called Lag Screws though the incorrect name is commonly used.)
- If it has a drive (slotted, Phillips2, square3, Torx®4, Allen/Socket/Hex, etc.) then it's usually a screw. A knowledgeable, experienced, well-trained, person can tell by looking at a part.
Those pair of three-step criteria are all you really need to make the determination. Are there exceptions? Not many, but some parts can be very confusing. For example, the parts listed below are some that might cause confusion. Sometimes you have to accept what the market calls a product even if it's completely wrong.
- Anchor Bolts (J Bolts, L Bolts, U Bolts) are not These are really just bent and/or cut threaded rods. Remember, for something to be a bolt or screw it must have a drive mechanism. (Incorrect Terminology)
- Hanger Bolts are dual-use parts, but they are classified as bolts because it takes a nut and has no drive mechanism. (Correct Terminology)
- Toggle Bolts look like screws, but really are anchors once you add the wing to them and don't qualify as a bolt or screw. (Incorrect Terminology)
- Tapcons® (including but not limited to Ultracon®, LDT®, Wedge Bolts®, Titen®, and Screw-Bolt+®) are screws, and can be also classified as anchors. They are not bolts, even if the word bolt is in the name. (Incorrect Terminology for some parts)
- Shoulder Bolts are really screws, but they are not commonly called screws; they are part of the socket family. (Incorrect Terminology)
- Square Head Set Screws are really bolts, but are not called bolts because they are part of the socket family. Since they have no drive mechanism, they are bolts. (Incorrect Terminology)
- Dowel Screws are dual-use parts, but they are classified as screws because it neither a nut nor has a drive mechanism. (Correct Terminology)
- Type-U drive screws qualify as screws on the barest of technicalities, but they're really friction fasteners – closer to a rivet or nail – and are not a true screw as they are installed with a hammer. They are a subcategory of tapping screws. (Incorrect Terminology)
- Eye Bolts are bolts because the head itself is the drive mechanism. You turn it with the eye. (Correct Terminology)
- Thumb screws or wing screws are technically bolts since you have to turn them by hand as there is no drive mechanism. (Incorrect Terminology) But please note, even as pedantic as I still call them screws.
Why would you pick a screw over a bolt?
There’s no general answer to this question. Every application is unique. In general, wood-only applications still use screws because of thread engagement issues – but even then you might wish to bolt two pieces of wood together instead of using screws (such as when you’re building a dock). Most concrete applications use concrete screws or anchors (or even special epoxy) and not bolts. But again, that is no longer written in stone. Almost any provided application can be fastened with bolts or screws. For example, give me an application and I can provide multiple ways to hold it together.
If you went back 50 years, this question would have an answer – but with the advent of modern fasteners screws are made for wood, metal, and concrete. Bolts can fasten pretty much anything together. (That being said any application with a vibratory load a bolted application with a locking mechanism is usually still called for.) From a personal standpoint, I always prefer bolts because they can be re-used without damaging the items being fastened and also allow for easier maintenance.
We’d really be happy for our customers (or would-be customers) to submit questions as to whether something qualifies as a bolt or screw. Please enclose a photo or link to a photo so we can see what you’re talking about. We’ll update the blog accordingly.
Remember, many fasteners have multiple names which you can debate. Whether or not a particular fastener is a screw or a bolt (or something else entirely) isn’t a debate – it’s a matter of the rules. Sure, I’ve simplified them a bit for this blog, but the goal was to make it easy for the layperson to understand.