Deep holes demand reach, straightness and reliable chip clearance that a standard bit cannot deliver. The best drill bits for deep holes are extended-length designs engineered to run true and evacuate chips well beyond the depth an ordinary jobber-length bit can reach.
What to Look For
As a hole gets deeper, three problems grow together. The bit tends to wander off line, chips pack in the flutes because they have farther to travel to escape, and heat builds up because coolant cannot easily reach a tip buried in the work. A long bit also flexes under load, so rigidity and a well-ground, self-centring point matter more with every extra inch of depth you cut. Match the flute length to the depth you need and choose a bit ground for concentricity, because a long bit that starts even slightly off will drift steadily as it goes deeper.
The Best Type of Bit for Deep Holes
Extra-length drill bits provide the flute length and reach for genuinely deep holes in wood or metal while keeping the geometry of a proper twist drill. For running wire, or drilling through wall plates and studs, bellhanger bits are purpose-built — long, straight and often fitted with a hole through the tip so you can hook and pull cable back through the finished hole.
- Reach: extra-length bits give flutes that extend to the full hole depth for clean chip evacuation.
- Bellhangers: ideal for electricians and installers drilling through framing and pulling wire.
- Peck drilling: back the bit out often to clear chips and prevent binding deep in the hole.
- Start straight: begin with a shorter bit or a guide so the long bit follows a true line.
Keeping the Hole Straight
Let the bit reach full speed before it touches the work, keep the drill square to the surface, and avoid forcing the feed. Excess pressure bows a long bit and the hole drifts off target, sometimes breaking out the side of the material. Withdraw fully every so often to clear packed chips and to cool a metal-cutting tip. American-made extra-length bits are ground for concentricity and balance, which is exactly what keeps deep holes on target rather than spiralling away from your mark.
Matching Length to the Job
Choose a bit only as long as the job actually requires, because every extra inch of reach adds flex and reduces control. A hole that could be done with a moderate bit does not need the longest one on the shelf, and reaching for excess length only makes the hole harder to keep straight. It is common to start a deep hole with a stubby bit to establish an accurate entry, then switch to the long bit to finish at depth. If you are unsure of diameters, depths or how sizes convert between systems, our conversion and technical charts help you match the bit to the application before you buy.