How do I stop a drill bit going deeper than I want?

Two things go wrong without a depth stop: the hole is too shallow, so the plug or fixing does not seat, or the bit goes through into something on the other side. The second is the expensive one. Setting a depth takes ten seconds by any of three methods, and the diagram compares what each is actually good for.

Step by step

1. Work out the depth you need.For a wall plug, the plug's length plus a few millimetres for dust. For a fixing into timber, the screw's length minus the thickness of what it is holding. Write it down rather than holding it in your head.
2. Measure from the tip of the bit.Not from the shoulder, not from the start of the flute. Lay a rule alongside the bit with its end at the point, and mark the depth along the flutes.
3. Use tape for a quick job.A wrap or two of masking or insulating tape with its trailing edge at the mark. Free, instant, and accurate enough for wall plugs. It creeps back up as dust packs against it, so check it every few holes.
4. Use a stop collar for a positive stop.A steel collar with a grub screw clamped to the bit. It physically prevents the bit going further, does not move, and is the right choice for repeat work. It will mark a finished surface if you drive it hard against one.
5. Use the side-handle rod for a run of holes.Most combi and SDS drills come with a metal rod that clamps in the auxiliary handle. Set it so its end touches the wall as the tip reaches depth. It is the most repeatable method and the one most people leave in the box.
6. Add a few millimetres for masonry.Dust falls to the bottom of a masonry hole and takes up space. A hole drilled exactly to the plug's length leaves the plug standing proud. Go a little deeper and blow the hole out.
7. Check the depth achieved, not just the setting.Poke the plug or a marked rod into the hole. A bit that stopped cutting or a hole packed with dust can be shallower than the stop suggests.
8. Remember what a depth stop cannot do.It limits how far the bit travels. It does nothing about what is behind a plasterboard wall or on the far side of a cavity leaf. Knowing what is there is a separate job that comes first.

Tips

• For repeated holes at the same depth in timber, a stop collar plus a scrap block with a hole through it gives both depth control and a guide for perpendicularity.
• Blow masonry dust out of the hole with a puffer, a straw or a squeeze bottle rather than by mouth — the dust comes straight back at your face.
• On an SDS, some bits are sold with a marked scale printed on the flute, which is enough for rough depths and saves fiddling with tape.

Common mistakes

• Measuring the depth from the chuck — The amount of bit sticking out of the chuck varies every time it is fitted. Only measurement from the tip is repeatable.
• Trusting tape after a dozen masonry holes — Dust packs against it and pushes it back, so the holes get progressively deeper. Check and reset it.
• Drilling a plug hole exactly to the plug's length — Dust in the bottom stops the plug going fully home, and a proud plug means the fixing never pulls up tight.

Questions people ask