What are pilot holes?
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:
- To prevent potential damage or cracking of the material when making a large hole is required.
- To pinpoint the exact location where screws or fixtures to go on smooth or slippery surfaces where there isn’t enough space for second chances.
- Pilot holes help prevent splitting or cracking when screws or nails are driven in wood and other materials.
- Making larger holes on materials (normally plastics) which require thermal expansion holes.
- To locate, guide or clear out excess material for self-tapping screws in hard wood or plastics to prevent damaging the material or burning out the screws.
Pilot holes for larger holes
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.
Pilot holes on slippery surfaces
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.
Pilot holes on tough surfaces or hard material
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.
Pilot holes for thermal expansion
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.
Pilot holes for fixing through tough materials
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:
- Thinner steel (≤ 1/8″) requires a smaller pilot hole to ensure proper thread engagement.
- Thicker steel (> 1/8″) may need a slightly larger pilot hole to accommodate thread cutting.
- Self-drilling screws often do not require a pilot hole but may benefit from one in thicker or harder steel.
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:
- Softwood (like pine, cedar, spruce) usually requires slightly smaller pilot holes.
- Hardwood (like oak, maple, walnut) usually requires slightly larger pilot holes to avoid splitting.
- Always drill the hole slightly deeper than the screw length to allow the screw to seat properly.
- When in doubt, test pilot hole size on scrap material first.
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.
