How to Choose the Right Drill Bit Size

The short answer: it depends on what you're drilling into and what you're fixing, but there are a few reliable rules that cover most jobs without needing to guess.

Drilling for a wall plug

Wall plugs are sized to match a specific drill bit diameter, printed on the packaging — a 6mm plug needs a 6mm masonry bit, an 8mm plug needs an 8mm bit, and so on. Match the bit to the plug, not to the screw that eventually goes into it. Using a bit that's slightly too large means the plug won't grip the sides of the hole; too small and you risk cracking the plug on insertion or not being able to push it fully home.

Drilling a pilot hole for a screw (no wall plug)

When screwing directly into timber without a wall plug, a pilot hole prevents the wood splitting and makes driving the screw considerably easier and more accurate. As a rough guide, use a bit slightly narrower than the screw's core (the solid metal shaft, not including the thread) — roughly 60-70% of the screw's diameter is a reasonable starting point for softwood, and closer to the full core diameter for hardwood, which splits more easily under a tight-fitting screw.

Masonry vs wood vs metal bits — they're not interchangeable

• Masonry bits have a distinctive wide, flat carbide tip and are for brick, block, and concrete — using a wood or metal bit on masonry will blunt it almost immediately and won't drill effectively, however much pressure you apply.
• Wood/HSS bits for timber — a pointed tip that centres itself and cuts cleanly without excessive heat build-up.
• Metal (HSS) bits for drilling into metal, and usable on wood too, though a dedicated wood bit cuts more cleanly and leaves a neater hole.

Check the bit's packaging or shank markings if you're not sure which type you're holding — masonry, wood, and metal bits are usually colour-coded or clearly labelled by most manufacturers, and mixing them up wastes both the bit and your time.

A practical sizing reference

As a rough, commonly used reference for masonry wall plugs (always check the specific plug's own packaging, as sizing varies slightly by manufacturer and plug design):

• Small picture hooks, light fixings: 5-6mm plug and bit
• Curtain poles, light shelving: 6-7mm plug and bit
• Medium shelving, mirrors: 7-8mm plug and bit
• Heavy items — TV brackets, kitchen units: 8-10mm plug and bit, or a rated heavy-duty anchor

Don't forget depth, not just width

Most masonry drill bits have depth markings, or you can wrap a piece of tape around the bit at the required depth as a visual guide — drilling deeper than needed wastes effort and risks hitting something behind the wall unnecessarily; drilling too shallow means the plug won't seat fully and the fixing won't reach its full holding strength.

Keeping bits in good condition

Masonry bits blunt faster than most people expect, especially on dense modern concrete or engineering brick — a blunt bit drills slowly, generates excessive heat, and produces a rougher, less accurately sized hole. Keeping a spare bit or two of the common sizes on hand avoids the job stalling mid-way because the only bit you have has gone dull.

A quick technique note

For masonry in particular, drilling in short bursts with the hammer action engaged (on a combi drill) rather than pushing continuously tends to work better and generates less heat — let the bit's own action do the cutting rather than forcing it. For a large-diameter hole, drilling a smaller pilot hole first and following up with the full-size bit is often quicker and more accurate than trying to drill the final size in one pass, particularly in hard masonry.

SDS vs standard masonry bits

Standard round-shank masonry bits work fine in an ordinary drill or combi drill for most household jobs — wall plugs, shelving, curtain poles. SDS bits have a fluted, locking shank designed for an SDS drill or SDS rotary hammer, which delivers a genuine hammer action rather than the lighter percussion action of a combi drill. For occasional DIY use, a good combi drill with standard masonry bits is usually all you need; SDS is worth it if you're regularly drilling into dense concrete, engineering brick, or doing enough masonry work that speed and reduced effort justify the extra cost of the drill and bit set.

Picking the right drill bit is only half the job — see our guide to choosing the right screw or fixing for the other half.

Frequently asked questions