You handy-man-people might be laughing at me for not knowing this, but I'm not the only one who has trouble discerning the difference between bolts and screws. Apparently the terms "bolt" and "screw" have been used interchangeably, or confusingly, for a long time. In fact, some people say that there is not A universally accepted method of distinguishing the two. So I'm not the only one. Besides, 8th grade Shop with Mr. I-Love-the-Feel-of-Wood-Chips-in-My-Chest-Hair Benson was a long time ago.
But now, after having looked around online at various sites, I'm going to tell you what generally seems to be the case about bolts versus screws.
SCREWS VS. BOLTS
- Whether it's a screw or a bolt is not determined by the head where you drive the thing, but by the end that gets driven into the wood or other material.
- Basically, screws have pointy ends and bolts have blunt ends.
Fairly typical screw. Note the pointy end.
(Photo and screw from Fastenal)
Fairly typical bolt. Note the blunt end.
Confusing matters, Fastenal calls these "heavy hex bolts" on their main page. Once you drill down to the pages about the individual items, however, they change the name to "heavy hex cap screws." There is actually a slight difference between a hex bolt and a hex cap screw. But both seem to have blunt ends. I'm sure if I'm wrong some generous reader will be quick to correct me.
(Photo and bolt from Fastenal)
- Bolts also usually require a nut to be attached to the business end in order to keep the bolt from unscrewing itself. I say "usually" because not all bolts need nuts.
Nuts are threaded internally and are designed to marry -- that is the technical verb for it -- with bolts that are threaded externally to similar specifications.
(Nut and photo from Home Depot)
- Sometimes, though, a thing that has a pointy end and does not require a nut still gets called a bolt. In those cases, the reason seems to be that it's fairly thick in diameter, larger than most household screws. But that only generally seems to be true.
- But let's pretend that I have successfully separated the bolts from the screws. Within the worlds of screws and bolts, there are all sorts of variations. I'll describe some of the more common variations within each. In addition to what I'm going to show you here, there are many more types, and within each type, there are further variations in terms of what the head might look like, or what dimensions the threads have, or what material the thing is made out of or coated with.
(Okay, yes, I'm aware of the pun on "screw" and in some situations on this page, that pun is pretty funny. Rest assured, any time you notice it, I've noticed and chuckled at it, too.)
- Concrete screws are usually blue and they are often coated to resist corrosion. They were designed to secure other materials to concrete or masonry. These screws are sometimes also called Tapcons, after the brand name of concrete screws often sold in the US. They have a unique set of threads that will cut threads into the base material as it is being driven.
A Tapcon flat concrete screw
(Photo and screw from Tapcon)
- Deck screws are very similar to drywall screws (next item). The main difference is that they usually have additional coating to make them resistant to corrosion, since they are more exposed to the elements, and they're also usually larger.
Deck screw and photo from Fastenal
- Drywall screws have very pointy ends so they'll pierce the drywall material easily. The threads are "coarse" or spaced fairly wide, since drywall may crumble easily. It also has what's known as a bugle head which helps to keep the drywall attached to the studs.
This particular drywall screw has a Philips head.
(Photo and screw from Fastenal)
- Self-drilling screws are so-called because the funny-looking smooth end creates a starter hole, rather than you having to drill one. The pointy tip pierces through soft metals and the smooth starter portion allows the screw to sink a ways into the material, and then the threads begin to bite.
This particular variety of self-drilling screw has a hex head with a built-in washer beneath it.
(Photo and screw from Fastenal)
- Sheet metal screws were originally designed to work with sheet metal. The threads are sharp enough to cut through the sheet metal. But those sharp threads make this type of screw popular with other materials like plastic or wood. These make a good all-purpose screw.
This variety of sheet metal screw has a hex head plus washer, and the head also has a slot for a screwdriver. Can't get much more all-purpose than that.
(Photo and screw from Fastenal)
- Thumb screws. You know that phrase, "putting the thumb screws to a person?" That used to refer to applying a particular method of torture to a person's thumbs. This is a different type of thumb screw. It's pretty obvious where you're supposed to put your thumb here.
Ignoring the fact that it has a blunt, bolt-like end, this is a thumb screw, designed to be turned by hand.
(Photo and thumb screw by Fastenal)
There are lots and lots of different kinds of bolts. I'm going to talk about only a few of the most commonly used ones.
- Carriage bolts have a domed head and a square hunk beneath the domed head. These are designed to drop into holes that have already been drilled into wood. The end of the bolt where the threads are sticks out on the other side of the wood and the nut is tightened onto the threads. As it is tightened, the bolt is pulled farther into the wood and the edges of the square bite into the wood, keeping it snug.
Carriage bolt with domed top and square section just beneath it.
(Image from Bolt Products, Inc.)
- Stove bolts are bolts, but they are defined as a type of machine screw. They may have either a round head or a flat head, and the head may fir a Philips head screwdriver or be a single slot. They are typically made of lower grade steel. Home Depot, Lowe's, Fastenal, and Bolt Products, Inc. don't sell them. This was the creature I was trying to un-stick. Perhaps now you can understand why I wasn't sure whether to call it a bolt or not. The fact that the store where I went for help doesn't sell them may also explain why the clerk seemed to be very confused when I said I was using a screwdriver and a wrench simultaneously to try to loosen it.
Drawing of stove bolt from Milspecfasteners
- Tap bolts are threaded from the blunt end all the way to the head. These are designed to be dropped into holes that have already been drilled to the same diameter as the bolt and "tapped" into place. They are used often in boat-building and woodworking.
Tap bolt and photo from Fastenal
- Toggle bolts are used when you can't get to the other side of the material where the end of the bolt will stick out, such as the other side of a wall or a ceiling. Toggle bolts have a flaring hinged wing on the business end. The wing is folded up as the bolt is screwed into the material. Once the end of the bolt clears the other side of the material, the wing pops open and acts as a nut to keep the bolt in place.
This particular variety of toggle bolt has a slot on the head to allow the bolt to be turned with a screwdriver.
(Toggle bolt and photo from Ameribest)
- U-bolts look like their name suggests. They're used to attach pipes to other things. U-bolts that have squared-off corners are used for attaching lumber.
U-bolt and photo from Fastenal
SOMETIMES BOLTS OR SOMETIMES SCREWS?
There are some fasteners with names that still muddy the waters.
- Eyebolts have a loop for a head where you could tie or attach something like the strings of an awning, for example. Some of them have pointy ends like screws and others have flat ends like bolts.
Home Depot calls this a "stainless steel screw eyebolt."
Here is an eye bolt sold by Fastenal.
- Hex cap bolts have heads, or caps, with hexagonal sides. These are designed to be turned with wrenches or sockets that have hexagonal openings. There are also hex cap screws. Hex cap screws are slightly smaller in diameter than their bolt counterparts, but often the terms are used interchangeably.
Hex cap screws and bolts are also supposed to have ends that do not taper, but on this particular one, the end is chamfered, or beveled, in order to make the initial insertion easier.
(Hex cap bolt and photo from Fastenal)
- Lag screws are sometimes also called lag bolts, though the folks at Wikipedia say these are "clearly screws." My dad loves these things. The pointy end means he doesn't have to drill a hole first and he doesn't need a nut, the hex head means they're turned with a wrench and he prefers using wrenches over screwdrivers which can slip off the head more easily, and the "lag" or smooth space where there are no threads provides all sorts of benefits. One is that it allows metal to be joined to wood, or if you want to join wood to wood, the head will sink below the surface of the wood and not stick up. If you give my dad a bag of lag screws for Christmas, he will say, "Ohhh, lag screws!" and chuckle with delight.
Again, Fastenal muddies the waters. On their main page they call these lag bolts. But on the individual pages, they call them lag screws.
(Photo from Fastenal)
- Machine screws. All sorts of things seem to get classified as machine screws. Even though they're called "screws," some of them are used with nuts. Sometimes, like bolts, they're driven into driven into tapped or pre-drilled holes. But sometimes the phrase is used to indicate smaller-size, run-of-the-mill screws. They might have hex heads or they might have slot or Philips heads.
Philips head machine screw from Fastenal. Note its bolt-like blunt end.
Home Depot classifies this as a type of machine screw, but the page where it's listed individually describes it as a durable lag screw. Note the pointy, screw-like end.
There. I feel better. At least I know what the thing I was trying to loosen is called. And at least I know I was not an ignoramus for being somewhat confused by bolts and screws all this time.
Danny Lipford, How to join wood using a carriage bolt video
Bolt Products, Inc. Bolts and Screws
Fastenal Fasteners pages
American Society of Civil Engineers, Task Committee on Fasteners, Mechanical connections in wood structures, Chapter 3: Lag Screws and Wood Screws
Wikipedia, Differentiation between bolt and screw