A bolt passes through a hole and is held by a separate nut — "bolt the bracket to the frame and tighten the nut". A screw threads directly into the material itself and needs no nut — "drive four screws into the joist". If there's a nut, it's a bolt; if the fastener grips the wood or metal by its own thread, it's a screw.
The core difference
Both are threaded fasteners, and both look like a shaft with a head. What separates them is what the thread grips. A screw forms or follows a thread inside the material it enters, so tightening it pulls the parts together directly. A bolt passes clean through both parts and is clamped from the far side by a nut; the bolt itself often doesn't touch a thread until the nut arrives.
How to spot each one
- Screw: usually tapers to a point, has a coarse thread running most of its length, and is turned by a driver in the head — a slot, cross, Torx or hex socket.
- Bolt: usually a uniform shaft, often with a smooth unthreaded section near the head, a blunt end, and a hexagonal head meant for a spanner. Sold with, or expecting, a nut and often a washer.
The test you can run
Ask: what do I turn to tighten it? If you turn the fastener itself and it bites into the workpiece, it's a screw. If you hold the fastener still and turn a nut, it's a bolt. That single question resolves nearly every real case on a workbench.
A second check: could you remove it and be left with a threaded hole in the material? Then it was a screw. Left with an empty round hole and a loose nut? A bolt.
Be honest — the trade names contradict the rule
Fastener catalogues and everyday speech do not obey this cleanly, and it is worth knowing before you argue with a builder:
- A lag bolt (or coach bolt in British usage) is sold as a bolt but threads straight into timber with no nut — by the definition above it is a screw, and engineers often call it a lag screw.
- A machine screw is called a screw but is very often used with a nut.
- Casually, people call almost any small threaded fastener a screw and any large one a bolt. That usage is understood and not worth correcting outside a technical document.
Formal standards define the pair by intended assembly method — a bolt is designed to be tightened by torquing a nut — but the naming of individual products predates the standards and was never revised to match.
Choosing between them
Bolts give a stronger, more repeatable clamp and can be undone and redone many times, which is why they hold engines, scaffolding and steel frames together. Screws are faster, need access from only one side, and suit wood, plasterboard and thin sheet where there is nowhere to put a nut.
Frequently Asked Questions
Is a lag bolt a bolt or a screw?
Technically a screw — it cuts its own thread into timber and takes no nut. It is sold and spoken of as a bolt because of its size and hex head, and many suppliers now list it as a lag screw for exactly this reason.
Can I use a bolt without a nut?
Only if it goes into a tapped hole, in which case it is doing a screw's job and is often called a cap screw. Left in a plain through-hole with no nut, a bolt clamps nothing and will work loose.
Which is stronger, a bolt or a screw?
For the same diameter and grade, a bolted joint is generally stronger and more reliable, because the clamping force is set by the nut and spread by a washer rather than depending on the thread's grip in wood or sheet metal.