How screw sizes are written
In the UK, screws are mostly sold in metric sizes written as diameter x length in millimetres. A 4.0 x 40mm screw has a 4mm thread diameter and is 40mm long. Length is measured from the tip to the surface of the work once the screw is fully driven, which for a countersunk screw includes the head.
You may also see imperial gauge numbers such as No. 8 or 8g, along with lengths in inches. Gauge refers to the diameter: the higher the number, the thicker the screw. The table below shows common approximate equivalents. Packs vary slightly between manufacturers, so use it as a guide.
| Gauge | Approximate metric diameter | Typical uses |
|---|---|---|
| No. 4 | 2.5 to 3.0mm | Small hinges, light brackets, model work |
| No. 6 | 3.5mm | Cabinet hardware, light joinery |
| No. 8 | 4.0mm | General purpose: shelves, brackets, wall plugs |
| No. 10 | 5.0mm | Heavier brackets, timber framing |
| No. 12 | 5.5 to 6.0mm | Heavy fixings, gate hardware |
Choosing the right length
A screw holds by its thread gripping the material behind the item you are fixing. As a rule of thumb, aim for at least half and ideally around two-thirds of the screw length to be in the base material. When fixing into a wall plug, the screw needs to pass through the item and the full length of the plug so the plug expands properly. Avoid screws so long that they break through the far side or could reach hidden cables and pipes.
Common screw types and what they are for
Multi-purpose wood screws
The everyday screw for timber, MDF, chipboard and wall plugs. They usually have a sharp point, a coarse thread and a countersunk head, and many are hardened so they can be driven without a pilot hole in softwood.
Chipboard screws
A coarse, deep thread designed to grip chipboard and other particle boards. Many multi-purpose screws are sold as chipboard screws and vice versa; check the description.
Drywall screws
Designed for fixing plasterboard to timber or metal studs, with a bugle head that sits just below the paper surface without tearing it. They are not intended as general-purpose wood screws for structural or outdoor work.
Decking and exterior screws
Coated or stainless steel to resist corrosion outdoors, often with a self-countersinking head. Use them for decking boards, fences, sheds and garden furniture.
Masonry screws
Hardened screws with a special thread that cuts into a pre-drilled hole in concrete, brick or block, with no plug needed. They are useful for battens, brackets and frames. Follow the drill size and embedment depth on the pack.
Coach screws
Large, heavy screws with a hexagon head, driven with a spanner or socket. Used for heavy timber work such as gate posts and frames. Drill a pilot hole first.
Self-tapping and metal screws
Hardened screws that cut their own thread in sheet metal or plastic. Self-drilling versions have a drill-shaped tip for thin steel.
Head types
- Countersunk: sits flush with the surface; the most common type for woodwork and hinges.
- Pan or round head: sits above the surface; used for fixing metal brackets, plastics and fittings without a countersink.
- Hex or flange head: for driving with a socket; common on coach and roofing screws.
Drive types: Pozidriv, Phillips, Torx and slotted
The drive is the shape in the head that the screwdriver fits. Using the wrong bit is the main cause of stripped screw heads.
- Pozidriv (PZ): the most common drive on UK wood screws, recognisable by small extra lines between the cross. Use a PZ bit, usually PZ2 for general sizes.
- Phillips (PH): a cross without the extra lines, common on electrical fittings and imported products. Use a PH bit.
- Torx (TX or star): a six-point star that grips very well, increasingly used on decking and structural screws.
- Slotted: a single slot, mostly found on traditional brass screws and older fittings.
Keep a set of quality screwdrivers and bits so you can match the bit to the head.
Materials and finishes
Zinc-plated and yellow passivated screws suit indoor use. Outdoors or in damp areas, use stainless steel or screws with an exterior-grade coating to reduce rust and staining. Oak and some other hardwoods contain tannins that react with plain steel, so stainless steel is a better choice. Brass screws are mainly decorative and are relatively soft, so pre-drill and drive them by hand.
When to drill a pilot hole
Drill a pilot hole when screwing into hardwood, near the end or edge of a board, into MDF, or with large screws. The pilot should be roughly the diameter of the screw's core (not the thread). A clearance hole the size of the full screw diameter through the top piece helps pull two pieces of timber tightly together. Use a countersink bit so countersunk heads sit flush. You will find these in our drill bits range.
Browse all screws or, if you are fixing into a wall, read which wall plug for which wall and see our wall plugs and anchors.