What are pilot holes? This guide covers everything you need to know—including when to drill them, why they matter, and how to choose the right size for wood, metal, and more.
What are pilot holes? They are small holes drilled into a piece of construction material, be it steel, timber, plastic, ceramic and more.
The main purpose of drilling pilot holes may be:
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 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 to 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.
Plastic materials such as polycarbonate (sheets) have 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.
(Read more: Polycarbonate Roofing Fixing Instructions: 8 Roof Fixing Tips)
When it comes to drilling and fixing through tough materials such as hardened steel or hard wood, pre-drilling pilot holes can be a blessing. This will eliminate the need for heaving duty fixings that must be used to drill through the substrate but can be totally unnecessary to fix the attached material, which is often 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.
| 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.
(Read more: Self-Drilling Screws for Metal: 7 Features that Simplify Projects)
| 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 |
Still wondering what are pilot holes used for in other materials like masonry or composite panels? Contact us or your local BDN Fasteners® distributor for tailored advice.
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