Pilot holes are small holes drill into a piece of construction material, be it steel, timber, plastic, ceramice and more.
The main purpose of drilling pilot holes may be:
Pilot holes can be drilled to its full extent or half-way through (depth) depending on the purpose. Pre-drilling pilot holes reduces the torque required for drilling a large hole in on-go, therefore reduces risk of damaging the drill and the substrate that is being penetrated. If very large holes are required, the drilling process may be split into multiple steps until the final size is drilled.
Oftentimes, very hard or smooth surfaces such as most metals and plastics, fixtures such as self-drilling screws tend to wander during the initial drill and drive process. A center-punch can be used pinpoint the exact location prior to creating the pilot hole.
On tough surfaces, hard materials or in wood, pilot holes help prevent splitting or cracking that could result from drilling a larger hole in one go.
Plastics materials such as polycarbonate (sheets) has a high coefficient of thermal expansion compared to steel or wood, which means that the polycarbonate sheets tend to shift and move when temperature ranges are significant. Drilling a pilot hole that has a larger diameter than the fixture gauge can prevent sheets from being damaged when the shifting happens.
When it comes to drill and fix through tough materials such as hardened steel or hard wood, pre-drilling pilot holes can be a blessing. This will eliminate the need for using heaving duty fixings that has to be used to drill through the substrate but can be totally unnecessary to fix the attached material, which is oftentimes just a thin piece of sheet metal or plastic glazing. When fixing material to hard wood substrates, drilling a pilot hole can prevent the wood from splintering during the process, as often, you only get one chance at fixing the attachments correctly.
Choosing the right drill bit for a pilot hole is equally important as the pilot hole itself. Although important, it is not complicated at all once you have learned the knack of it. A pilot hole that is too small can cause the wood to split, which is exactly what you want to prevent. Conversely, a hole that is too large won’t provide enough grip for the screw threads, reducing their holding strength.
Pilot hole size chart for common screw sizes when fixing to steel substrates:
| Screw Size (#) | Screw Diameter (inches / mm) | Pilot Hole Size (Steel ≤ 1/8″) | Pilot Hole Size (Steel > 1/8″) |
| #6 | 9/64″ / 3.5 mm | 3/32″ / 2.4 mm) | 7/64″ / 2.8 mm) |
| #8 | 5/32″ / 4.0 mm | 7/64″ / 2.8 mm) | 1/8″ / 3.2 mm) |
| #10 | 3/16″ / 4.8 mm | 1/8″ / 3.2 mm) | 9/64″ / 3.6 mm) |
| #12 | 7/32″ / 5.5 mm | 9/64″ / 3.6 mm) | 5/32″ / 4.0 mm) |
| #14 | 1/4″ / 6.3 mm | 5/32″ / 4.0 mm) | 3/16″ / 4.8 mm) |
Key Considerations:
Always follow manufacturer recommendations for the specific fastener you are using.
Here’s a helpful pilot hole size chart for common screw sizes in woodworking:
| Screw Size (#) | Screw Diameter (inches / mm) | Pilot Hole Size (Softwood) | Pilot Hole Size (Hardwood) |
| #4 | 7/64″ / 2.8 mm | 1/16″ / 1.6 mm | 5/64″ / 2 mm |
| #6 | 9/64″ / 3.5 mm | 5/64″ / 2 mm | 3/32″ / 2.4 mm |
| #8 | 5/32″ / 4.0 mm | 3/32″ /2.4 mm | 7/64″ / 2.8 mm |
| #10 | 3/16″ / 4.8 mm | 7/64″ / 2.8 mm | 1/8″ / 3.2 mm |
| #12 | 7/32″ / 5.5 mm | 1/8″ / 3.2 mm | 9/64″ / 3.6 mm |
| #14 | 1/4″ / 6.3 mm | 9/64″ / 3.6 mm | 5/32″ / 4 mm |
General tips when selecting a pilot hole size:
If you need pilot hole sizing information for another specific material, such as metal or masonry, or for a particular screw type, please contact us or your local BDN Fasteners® distributor.