Screws look simple until you are stood at the rack with twenty boxes in front of you, each one covered in numbers. What does a "4.0 x 40" actually mean? Is a gauge 8 bigger or smaller than a gauge 10? And why does the same job list a Pozi drive in one box and a TX in the next?

This guide clears it up. We supply screws to UK trades and self-builders every day, so it is written the way we would explain it across the trade counter: how screws are sized, how to pick the right length, which head and drive to use, and which finish survives outdoors. There is a gauge-to-millimetre chart you can bookmark, a quick pilot-hole reference, and a short "which screw for which job" section at the end.

How a wood screw is sized

A wood screw is described by two things: its gauge (the thickness) and its length. You will see this written two ways. The traditional UK way uses a gauge number and an imperial length, like "8 x 1½". The modern way uses millimetres for both, like "4.0 x 40". They describe the same screw.

• Gauge is the diameter of the shank (the solid part of the screw, measured across the threads). A higher gauge number means a thicker screw. Gauge 8 is thicker than gauge 6, not longer.
• Length is how far the screw goes into the material. On a countersunk screw, length is measured from the very top of the head, because the head sinks flush. On a screw that sits proud, like a pan or round head, length is measured from under the head.

So "8 x 1½" is a gauge 8 screw (4.0mm thick) that is one and a half inches (about 40mm) long. Once you can read both, the wall of boxes gets a lot friendlier.

Screw gauge to mm conversion chart

This is the one to save. UK wood screws are still sold by gauge in a lot of places, but the box often prints the metric diameter as well. Here are the everyday sizes and what they convert to.

Gauge	Shank diameter (mm)	Typical use
4	3.0mm	Light trim, small hinges, thin timber
6	3.5mm	General second-fix, boards, brackets
8	4.0mm	The everyday all-rounder for most joinery
10	5.0mm	Heavier fixings, thicker timber, framing
12	6.0mm	Structural and heavy-duty work
14	6.5mm	Large structural fixings

If you only remember one, remember that gauge 8 is 4.0mm. It is the size that covers the widest range of everyday jobs, which is why it is the most common screw in most vans.

Choosing the right length

The rule of thumb we use: the screw should reach at least twice the thickness of the piece you are fixing down into the piece underneath. Put another way, aim for roughly two thirds of the screw sitting in the base material.

A worked example. Say you are fixing an 18mm shelf to a timber batten. Two thirds of the screw needs to be in the batten, so a 40mm screw works well: about 18mm passes through the shelf and roughly 22mm bites into the batten. Going much longer risks poking out the far side; much shorter and the shelf will not hold weight.

Two things that catch people out. Do not include the material you are fixing through when you count the grip; only the part in the base timber holds. And near the end of a board, drop down a gauge or drill a pilot hole, because a fat screw close to an edge is the fastest way to split the timber.

Pilot holes: a quick reference

A pilot hole is a small guide hole drilled before the screw goes in. It stops the timber splitting, makes driving easier, and gives a straighter, stronger fixing. Softwood often forgives you skipping it. Hardwood, close to edges, or anything you cannot afford to split, drill first.

Screw size	Pilot hole (softwood)	Pilot hole (hardwood)
Gauge 6 (3.5mm)	2.0mm	2.5mm
Gauge 8 (4.0mm)	2.5mm	3.0mm
Gauge 10 (5.0mm)	3.0mm	3.5mm
Gauge 12 (6.0mm)	3.5mm	4.5mm

Treat these as a starting point. In very hard timber, step up a touch; in soft pine, you can often go smaller or skip the pilot entirely on a sharp-pointed screw.

Head types, and when to use them

The head does two jobs: it sets how the screw finishes against the surface, and how much it clamps. These are the ones you will meet most.

• Countersunk: the cone-shaped head that sits flush with the surface, or just below it. The default for most joinery and the head on a multi-purpose woodscrew.
• Pan and round: sit proud of the surface with a wide bearing face, so they clamp thin sheet, straps and fittings rather than sinking in.
• Bugle: a shallow trumpet shape that self-finishes into board, used on plasterboard screws so the head pulls the board tight without tearing the paper.
• Hex flange: a six-sided head driven with a socket, used on coach screws and heavy timber screws where you need serious torque.
• Wafer: a wide, flat, low head that spreads the load, common on timber-to-metal and framing screws.

Drive (recess) types: Pozi, TX, slotted and more

The drive is the shape in the head that the bit engages. Using the right bit, snugly fitted, is the difference between a clean drive and a chewed-up head.

• Pozi (PZ): the UK workhorse. A cross with extra ticks at 45 degrees for better grip than an old Phillips. Sized PZ1, PZ2 and PZ3, with PZ2 covering most everyday screws.
• TX (Torx / star): a six-point star that resists cam-out almost completely, so it holds the bit steady under high torque. Standard on decking and premium construction screws.
• Slotted: the single straight slot. Rare on modern screws, but still used on traditional brass woodscrews for a period look.
• Square (Robertson): a square recess that grips hard and resists slipping, seen on some flooring and collated screws.
• Phillips (PH): the plain cross, still used on drywall and self-tapping screws.

If your bit spins in the head, it is usually the wrong size rather than a bad screw. Match PZ2 to a PZ2 screw, TX20 to a TX20, and keep a fresh bit in the driver.

Thread types and points

Not all threads pull the same way. A single thread is the traditional woodscrew. A twin thread has two starts, so it drives in faster. A fully threaded screw grips along its whole length; a partially threaded screw has a smooth shank under the head that lets it pull two pieces of timber tight together, which is why framing and decking screws are often part-threaded.

The point matters too. A sharp point starts fast in timber. A twin-cut or self-countersinking point cuts its own path and reduces splitting. A self-drilling point drills through light steel with no pre-drilling, which is what makes a self-drilling (tek) screw fix straight into a metal frame.

Material and finish: which screw survives where

Finish is not just colour. It decides whether a screw lasts a decade or rusts through in a winter. This is the part worth getting right.

• Zinc and yellow (ZYP) or bright zinc (BZP): plated carbon steel for interior and dry work. Fine indoors, not for permanent outdoor exposure.
• Black phosphate: the interior drylining finish on black plasterboard screws.
• Exterior organic coatings (green, silver): multi-layer coatings rated to 1,000 hours of salt spray, made for decking, timber frame and outdoor work.
• A2 stainless steel: strong corrosion resistance for outdoor and damp jobs. A good default for anything exposed.
• A4 marine stainless steel: the top grade for coastal, salt-water and pool environments where even A2 would eventually mark. See our A4 marine stainless woodscrews.
• Solid brass: corrosion resistant and decorative, for period ironmongery and marine joinery.

The short version: indoors, plated steel is fine. Outdoors, use an exterior-coated or stainless screw. By the sea, use A4. A cheap screw in the wrong place stains the timber and works loose long before the job is done.

Which screw for which job

A quick reference to point you at the right type. Each links through to the range if you want to see sizes and finishes.

• General joinery and second-fix: multi-purpose woodscrews.
• Decking: exterior decking screws with a TX drive.
• Plasterboard to timber or metal stud: drywall screws (coarse for timber, fine for metal).
• Timber frame and landscaping: hex and wafer timber screws.
• Heavy timber to timber: coach screws.
• Into masonry, concrete or brick: concrete screws that fix with no plug.
• Metal to metal or wood to steel: self-drilling screws.
• Bolting through drilled holes: machine screws and nuts.

Hiding the screw head

If the fixing shows and you want it neat, you have two options. Sink a countersunk screw slightly below the surface and fill over it, or use a screw made to be covered. Mirror screws take a screw-in chrome cap that hides the head completely, which is the tidy way to fix mirrors, bath panels and splashbacks.

A note on standards

Quality woodscrews for structural use are made to BS EN 14592, and plasterboard screws to BS EN 14566. It is worth checking the box carries the standard if the fixing is load-bearing, because a screw that meets it has verified strength and coating performance rather than just a printed size.

Frequently asked questions

How are wood screws measured?

By gauge (thickness) and length. Gauge is the shank diameter, written either as a number (like 8) or in millimetres (like 4.0mm). Length is measured from the top of the head on a countersunk screw, or from under the head on a pan or round head. So a "4.0 x 40" is 4.0mm thick and 40mm long.

What is the most common wood screw size?

Gauge 8 (4.0mm) is the everyday all-rounder, most often in 40mm to 50mm lengths. If you keep one size in stock for general work, that is the one.

Is a higher gauge number a bigger screw?

Yes, for thickness. A higher gauge means a thicker shank, so a gauge 10 is thicker than a gauge 8. Gauge only describes diameter, not length, which is why the length is always quoted separately.

What length screw should I use?

Aim for the screw to reach about twice the thickness of the piece you are fixing into the base material, or roughly two thirds of the screw length in the base. For an 18mm board into a batten, a 40mm screw is a good choice.

Do I need to drill a pilot hole?

In softwood, often not, especially with a sharp-pointed screw. In hardwood, near the end of a board, or for larger screws, drill a pilot hole to stop the timber splitting and to make driving easier.

What is the difference between Pozi and Torx (TX) screws?

Both are recess shapes. Pozi is a cross with extra ticks and is the UK standard for general screws. TX (Torx) is a six-point star that resists cam-out under high torque, so it is used on decking and premium construction screws. Use the matching bit for either.

Which screws can I use outdoors?

Use an exterior-coated screw (rated to around 1,000 hours salt spray) or a stainless steel screw. A2 stainless suits general outdoor and damp work; A4 marine stainless is for coastal and salt-water environments. Plain zinc-plated screws are for interior use only.

How do I hide a screw head?

Either sink a countersunk screw just below the surface and fill over it, or use a mirror screw with a screw-in cap that covers the head for a clean finish on mirrors and panels.