Screw buyer's guide

Everything you need to decide before you spend money: what the size code means, which head and drive to pick, what metal to buy, how machine screws differ from self-tapping screws, and when an assortment kit beats buying singles.

1. Read the size first

A metric screw is written thread × length, for example M2 × 4 mm. The number after the M is the outside diameter of the threads in millimetres, not the head and not the hole. The length is measured from under the head for a normal head, and from the very top for a countersunk (flat) head, because that head sinks into the part.

If you already know the device, jump straight to its page and copy the size: browse what it fits. If you only have the screw in your hand, use the screw size finder or the thread pitch counter.

2. Machine thread or self-tapping?

A machine screw goes into an existing thread — a tapped metal hole, a nut or a brass insert. It has even, shallow threads and a blunt tip. Nearly every laptop, camera, rack and instrument screw in this database is a machine screw.

A self-tapping screw makes its own thread in plastic, sheet metal or wood. The threads are taller and further apart, and the tip is often pointed. Put one into a tapped metal hole and it will chew the thread out. Details and pilot-hole sizes are on the self-tapping screw pages and the pilot hole calculator.

3. Head shape and drive

• Low profile / button head — the default for laptops, covers and anywhere the screw must not stick out. This is what most people mean by "laptop screws".
• Countersunk (flat) — sits flush inside a tapered hole. Only use it where the hole is actually tapered, otherwise it will not seat.
• Pan / cheese head — general purpose, more clamping face.
• Socket cap — hex-key drive, the strongest common option for machinery.

Drive matters as much as head shape: a Phillips bit that is one size too big rounds a tiny screw on the first turn. Match it with the bit size finder and see all shapes on screw & bolt types.

4. Materials — what to buy and when

• Black carbon steel — what most electronics ship with. Strong, cheap, and it will rust if it gets damp.
• A2 stainless (304) — the safe everyday replacement. Slightly softer than hardened steel but it will not rust indoors.
• A4 stainless (316) — marine and outdoors, where salt is involved.
• Brass — soft; used for standoffs, electrical contacts and instruments, not for anything you will torque hard.
• Nylon — non-conductive, for PCBs and antenna mounts.
• Titanium — light and strong, expensive, mostly for bikes and drones.

Never mix a stainless screw with a bare aluminium part outdoors without a washer or paste — the two metals corrode each other. More in the material picker.

5. Kits vs single sizes

If you repair things regularly, one boxed assortment per thread family (M2, M2.5, M3) costs less than three individual packs and covers the lengths you will actually hit. Buy singles when the device needs an unusual length, a specific head, or a security drive. The kit builder lists exactly what to order for the devices you work on.

Also budget for the boring extras: a magnetic parts tray, a precision driver with a knurled spinner top, thread-locking fluid for anything that vibrates, and a few brass inserts for stripped plastic bosses.

6. Before you click buy

1. Thread size confirmed (M2, M2.5, M3…) — measured or taken from a fitment page.
2. Length confirmed, remembering countersunk heads are measured overall.
3. Head shape matches the recess in the part.
4. Drive type matches a bit you own.
5. Material suits the environment.
6. Quantity — buy a few spares; tiny screws disappear.