Start with the hole
Most bit-buying advice starts with the bit. It is often easier to start with the hole. Ask what the hole is for, how big it must be, how deep, and how clean the edges need to look. Once you know those answers, the choice of bit narrows quickly.
Know the kind of hole
Pilot hole. A small hole that guides a screw and gives the threads room to bite without splitting the wood. The usual idea is that the pilot is close to the diameter of the screw’s core, the solid center without the threads. Softwood tolerates a smaller pilot, while dense hardwood often needs one closer to the full core size. The anatomy behind this is covered in more depth under pilot holes.
Clearance hole. A hole in the top piece that is slightly larger than the screw’s shank, so the screw pulls the two pieces together instead of just threading into both. A clearance hole is common when fastening one board to another, or when bolting through a part. For bolts, the hole is typically a little larger than the bolt diameter, and the exact fit depends on how tight the alignment needs to be.
Through hole. A hole that passes entirely through the material. The concern here is the exit side, where fibers can blow out.
Blind hole. A hole that stops partway. It needs a depth stop and a bit that leaves a flat or acceptable bottom, which is where a Forstner or brad-point bit tends to help.
Getting the diameter right
Bits are sold in fractional inches in the United States and in millimeters elsewhere, and many sets include both. For reference, 1/8 inch is about 3.2 mm and 1/4 inch is about 6.4 mm. Be careful if a project mixes systems, because a metric bolt and a fractional bit rarely match exactly. The difference between metric versus SAE sizing is worth understanding before you buy.
Some practical points:
- Hold the bit against the screw or bolt. For a pilot, the bit should hide the core but let the threads show on both sides.
- Test on a scrap piece of the same material first. Wood species and moisture change how a pilot behaves.
- Remember that a worn or wobbling bit can cut a slightly larger hole than its stated size.
- For countersinking screws, the pilot comes first and the cone-shaped recess second, so you may need two tools or a combination bit.
Depth
Depth is limited by the length of the flutes and by how far the bit can reach into the work. Standard bits, often called jobber length, handle most tasks. Longer bits exist for deep holes but flex more, which can make them wander.
For a blind hole, set a depth stop. If your drill has no stop collar, a wrap of masking tape on the bit is a common, rough marker. A drill press with an adjustable depth stop gives the most repeatable result, which is one reason a drill press is popular for precise work.
Retract the bit occasionally on deep holes to clear chips. Packed flutes cause heat and binding.
A clean exit
Blowout on the back of a board happens when the bit pushes through unsupported fibers. Several simple habits reduce it:
- Clamp a scrap board behind the workpiece and drill into it.
- Slow down and ease the pressure as the tip breaks through.
- Drill from one side until the point just appears, then finish from the other side.
- Choose a bit with a centering point and scoring edges for visible wood surfaces.
On plywood and laminated sheet goods, thin surface layers chip easily, so a sharp bit and a backer matter even more.
Matching bit to hole
| Need | Typical choice |
|---|---|
| Small pilot in wood | Twist or brad-point bit |
| Clean dowel or hardware hole | Brad-point bit |
| Large rough hole through framing | Spade or auger bit |
| Flat-bottomed pocket | Forstner bit |
| Hole in steel | Sharp HSS or cobalt twist bit with lubricant |
| Hole in brick or concrete | Carbide-tipped masonry bit |
Check before you drill
Mark the spot, confirm what is behind the surface, and clamp the work. Wear safety glasses, and unplug the drill or remove the battery whenever you change bits. If you are drilling into walls, be careful about hidden wiring and pipes, and consult qualified help if you are unsure.
The right bit is the one that matches the hole you planned, and a quick test on scrap is a cheap way to confirm it before you commit to the real piece.