Drill Bits Explained: Which Bit for Which Material, and What Size for Every Fixing

Get the bit wrong and the hole is wrong, and a wrong hole cannot be un-drilled. A 6mm plug in a hole that wandered out to 8mm spins uselessly instead of gripping. An HSS twist bit run in hammer mode shatters. A masonry bit pressed into a wall tile cracks the tile straight across the face. Bits are the cheapest thing in the drill case and the only part that touches the work.

Most people choose a drill carefully and then buy bits by accident, grabbing whatever 100-piece case sits nearest the till. Ninety of those pieces never leave the box. The four or five sizes that actually get used are the cheapest steel in it.

What a drill bit actually does

Every bit has three parts. The shank is the plain end the drill grips. The flutes are the spiral grooves up the body, and their only job is to carry waste out of the hole. The tip does the cutting, and the tip is what separates one family of bit from another.

Masonry bits do not really cut. A carbide tip (a hard tungsten alloy brazed onto the steel body) crushes brick and block into grit while the drill's hammer action pounds it forward. Look at one side-on and you'll see the tip is wider than the body behind it, which is deliberate: the oversize tip clears a hole slightly larger than the flutes so waste can escape.

HSS twist bits work in the opposite way. HSS stands for high speed steel, and the two ground edges at the tip shear metal into curled swarf. Those edges are precision-ground and brittle under impact, which is why hammer action ruins them in seconds.

Wood bits have a point in the middle that bites first and holds the bit on the mark. Everything else about the bit is designed to stop the surrounding fibres tearing out.

The families of bit, and what each one is for

The distinction that catches people out is that masonry bits and SDS+ bits do the same job with different shanks. A 10mm masonry bit and a 10mm SDS+ bit both drill a 10mm hole in brick. One fits a combi drill, the other only fits an SDS drill, and neither fits the other machine.

Multi-material bits (Bosch Multi Construction, Alpen Profi Multicut) are the exception worth knowing about. They have a carbide tip ground to a geometry that cuts tile, brick, timber and mild steel without changing bits. Reviewers consistently rate them as worth the money for mixed work. They are a poor buy if all you do is drill brick, because a plain masonry bit of the same size costs far less and lasts longer at that one job.

Shanks: which bits fit which drill

Round shank. Goes in any keyless chuck (the three-jaw clamp on the nose of a combi drill). Most combi drills take bits up to 13mm.

1/4in hex shank. A six-sided shank that snaps into a quick-change holder. An impact driver accepts nothing else, so wood bits for joist drilling are usually sold with hex shanks.

SDS+. A 10 mm shank with grooves that spring-loaded ball bearings lock into. The bit slides back and forth inside the chuck under the hammer blow, which is exactly what a clamped chuck cannot allow. SDS+ is the only shank an SDS drill will hold.

SDS-Max. An 18 mm shank used on breakers and heavy rotary hammers. Not a homeowner purchase.

Adaptors exist in both directions and only one of them is sensible. A chuck adaptor with an SDS+ shank lets you spin round-shank wood and HSS bits in an SDS drill, which is genuinely useful if the SDS is the only drill on site. The reverse, an SDS bit held in a keyless chuck, does not work at all.

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What size bit for which fixing

This is the question that ranks above all the others, and the answer is a lookup, not a judgement call. The diameter is set by the fixing, never by the screw and never by the hole in the bracket.

For wall plugs, the bit diameter and the plug diameter are the same number. Bit diameter = plug diameter exactly. The screw is the part that's smaller, by 1mm to 2mm. Drill 10 mm deeper than the plug length so dust falls clear.

The colours are a convention, not a standard, and the top end of the range is where brands diverge. Rawlplug's UNO range, for instance, uses its own colour assignments rather than the generic chart, so a blue plug out of one box is not always the same diameter as a blue plug out of another. Yellow, red and brown are reliable across every brand on a UK merchant's shelf. Above that, take the drill size off the packet.

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Anchor bolts are where the pattern breaks, and it breaks differently for each mechanism. Wedge anchors take a hole the same size as the bolt. Sleeve and shield anchors need a hole bigger than the bolt, because the expanding part has to sit around it. A 1mm error either way and the anchor either fails to seat or fails to expand, so read the box rather than a general chart.

Pilot holes in timber

Pilot holes are mandatory in hardwood, near edges and ends, and with brass or stainless screws. In softwood, a modern self-drilling screw will go in without one, though the joint closes better with a pilot.

Once the pilot is drilled, the screw needs the right driver bit too, and most wood screws take a PZ2. The screwdriver bits guide has the full screw-to-bit table.

Why a 160mm bit does not drill 160mm deep

SDS+ bits are sold by overall length. A bit marked 6 x 160mm is 6mm diameter and 160mm long from tip to the end of the shank, but the shank and the plain stem behind the flutes take up roughly 60mm of that. Usable drilling depth is about 100mm, which happens to be the thickness of a single skin of blockwork.

Plan the length from the hole, not the packet. Fixing a frame fixing through a 70mm window frame to the minimum of 40 mm into masonry needs 110mm of drilling, so a 160mm bit is already marginal. Passing right through a cavity wall, about 300mm of brick, cavity, block and plaster, needs a 400mm long-series bit. Long bits also flex, which is why you start the hole with a short bit and swap to the long one once the entry point is established.

How to drill without ruining the hole

Masonry

Mark the centre with a pencil cross and tap a starting dimple with a nail or centre punch. Start at low speed with light pressure until the tip stops skating, then increase speed and let the hammer work. Leaning on the drill does not speed anything up; it overloads the hammer and overheats the tip.

Pull the bit back out every 20-30 mm of depth to clear the flutes. Packed flutes mean the bit is grinding its own dust instead of cutting fresh material, and that is what cooks a carbide tip.

Mortar joints are the classic cause of oversize holes. The bit meets soft mortar, drifts sideways, and a 6mm hole opens out to 8mm before you notice. Drill a 4mm pilot first, then open up to 6mm with the correct bit. The pilot guides the larger bit and stops the wander.

Metal

Use rotation only. Never hammer. Big HSS bits want slow speed: a 3mm bit can run flat out, a 13mm bit in steel wants the drill barely turning, because the rim of a wide bit travels much faster than the speed dial suggests.

Punch the centre first or the bit will skate across the steel and score it. Drill a 3mm pilot, then step up. A drop of cutting fluid, or three-in-one oil at a push, doubles the life of the bit and keeps the edge from bluing. Ease off as the bit breaks through, because that's the moment it grabs and snaps.

Timber

Brad point bits give clean holes and the centre spur stops them wandering, so use them anywhere the hole will be seen. Spade bits are fast and rough, fine inside a stud wall where nobody will look.

For joists, an auger bit pulls itself through on a screw thread at low speed. Stop before the tip breaks out the far side, then finish the hole from the other face, and the exit stays clean instead of blowing out a lump of timber. Holes through joists must stay within the middle third of the joist depth and away from bearings.

How to tell when a bit is finished

A blunt bit is dangerous rather than merely slow, because the natural reaction is to press harder, and pressing harder is what snaps bits and wrenches wrists.

Masonry bits. The tell-tales are smoke, a squeal, and fine grey powder instead of gritty chippings coming out of the hole. Check the carbide tip for chips. The best field test is the fixing itself: if a 6mm plug that used to push in with thumb pressure now needs a hammer, the tip has worn below 6mm and the bit is done. Brick hardness varies wildly inside one house, with soft reds in one wall and near-black hard-fired bricks in the next, so a bit can perform perfectly on Monday and die on Tuesday.

HSS bits. Blue or straw-coloured discolouration at the tip means the steel has been overheated and softened. That damage is permanent. A sharp HSS bit produces two curling ribbons of swarf; a blunt one produces dust and heat.

Wood bits. A bent brad point drills oval holes. Dull spurs on a brad point or auger tear the fibres around the rim instead of slicing them.

Sharpening is possible but rarely sensible. HSS bits can be reground on a bench grinder or a dedicated sharpener if you have the knack. Carbide masonry tips need a diamond wheel or stone, and the kit costs more than a full set of replacement bits. Buy new ones.

What to buy

Skip the 100-piece mega case. The consistent view across UK trade forums is that most people only ever use a handful of sizes, and that a few good bits in those sizes outlast a bargain assortment several times over.

A working shopping list for an extension or renovation looks like this: masonry bits in 5mm, 5.5mm, 6mm, 7mm and 10mm for plug work; SDS+ bits in 6mm, 8mm, 10mm and 12mm for anchors and frame fixings; one 400mm long-series bit if anything passes through an external wall; a small HSS set; and one 20mm or 25mm auger for joist runs.

Entry level. A straight-shank masonry set in 4-10mm costs £7.49 – £14.72, which covers the DeWalt carbide set at Screwfix and the Bosch CYL-3 concrete set. Buying singles is worse value: one 6mm bit on its own is £3.86. The cheapest SDS+ set, steel shank in 6-10mm, is £6.49 and will get you through soft brick without lasting long in anything harder.

The sensible buy. A carbide-tipped SDS+ set covering 5-12mm, such as DeWalt's 7-piece Extreme 2 set, at £24.95 is the one to put in the case. It covers every fixing size on the table above, and carbide holds its edge in dense block where the plain steel-shank sets give up.

Premium. DeWalt Extreme 2, Bosch blue, Heller and Alpen are the names that come up again and again on DIYnot and BuildHub when people talk about what survives concrete. A larger premium SDS+ set, 7 to 10 bits reaching 12mm with four-cutter tips such as Bosch's Expert range, runs £43.61 – £49.99. The money goes into carbide grade and the quality of the braze holding the tip on. Better flute geometry clears dust faster, which is what keeps the tip cool enough to survive a day of concrete. Individual premium SDS+ bits cost £11.45 – £19.84 depending on diameter and length, and a combined bit and chisel set of 15 pieces is around £30.

For everything that isn't masonry: a full HSS twist set in 1-13mm is £34.97, which surprises people, and a 1-10mm set covers nearly all domestic metal work for less. The DeWalt DT4535-QZ brad point set in 4-10mm is the common choice for timber. Single augers run £6.40 – £10.82 each, climbing with diameter. A Forstner set is £7.84 – £44.99, and if you're only hanging cabinet doors, a single 35mm Forstner for the hinge cups is all you need.

External resource

Screwfix drill bits

Full masonry, SDS+, HSS and wood bit range. Filter by diameter, length and shank type.

screwfix.com

External resource

Toolstation drill bits

Erbauer, Bosch, DeWalt and Milwaukee bits, including long-series masonry and step drills.

toolstation.com

The bits covered elsewhere

Four jobs need a bit that isn't on the table above. Tiles, glass and porcelain need a spear-point or diamond bit run without hammer, covered on the glass and tile drill bit page. Holes of 20mm to 150mm in timber, plasterboard or sheet metal are hole saw territory. A clean round hole through brick or block much bigger than a masonry bit, such as 107mm for an extractor duct or a soil pipe opening, needs a core drill, or the ring-of-holes method above for a one-off. Shaping a timber edge is a job for a router bit in a router, never a drill.

Where you'll need this

Bits get consumed on nearly every phase of any extension or renovation project:

• Steels and lintels - SDS+ bits for anchor bolts and resin fixings into padstones and blockwork
• Windows and doors - long-series masonry bits through frames into the reveal
• First fix electrics - augers and spade bits through joists, masonry bits for back boxes
• First fix plumbing - augers for pipe runs through joists and studs
• Extractor and ventilation - masonry bits for duct brackets and external cowl fixings
• Second fix electrics - small masonry bits for faceplates, clips and pattress boxes
• Kitchen installation - 7mm masonry for wall unit brackets, pilot bits and a 35mm Forstner for hinges
• Garden and external works - masonry bits for downpipes, lights and fence brackets

Buy the four sizes you'll use most at the start of the build and keep a spare 6mm and 7mm in the case. Running out of the right bit halfway through fixing a run of units costs an afternoon.

Safety

Gloves are a judgement call rather than a rule. They protect your hands from hot bits and sharp swarf, but a loose glove caught by a rotating auger will pull your hand into the work. Keep them snug or take them off for high-torque drilling in timber.

Let the bit cool before you touch it. A masonry bit that has just drilled 300mm through a wall will blister a fingertip, and a hot HSS bit dropped into a plastic case melts its way into the moulding.

Where to buy Drill Bits

DeWalt DT9701-QZ Extreme 2 SDS Plus masonry bit set, 7 pieces, 5-12mm Carbide-tipped SDS+ bits in 5, 6, 8, 10 and 12mm, the fixing sizes the page's shopping list calls for.

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