Plywood is built from cross-laminated veneer layers with alternating grain direction, which can cause uneven resistance as a screw passes through multiple layers, and face drilling is generally more forgiving than edge drilling. MDF and particleboard have no long grain fibers, so they crush and strip differently than solid wood or plywood; overdriving compresses the panel around the head and reduces holding strength. Do not apply a solid-wood pilot hole chart to these materials without qualification, and always follow product-specific pilot hole guidance for cabinetry-grade panel screws.

Composite and PVC decking are engineered products that can mushroom or crack near board ends without a pilot hole, particularly in cold weather when the material is more brittle. Some modern deck screws are engineered to avoid predrilling in standard wood decking, while composite and PVC boards frequently still call for a pilot hole depending on the product and installation temperature. For material quantity planning, see the Decking Calculator.

Lead hole ranges calculated from AWC 2018 NDS Section 12.1.4.2: 65 to 85 percent of shank diameter for G greater than 0.6, 60 to 75 percent for 0.5 less than G up to 0.6, and 40 to 70 percent for G at or below 0.5. Use the larger percentage in each range for larger lag screw diameters. The shank clearance hole always equals the nominal shank diameter, per NDS 12.1.4.2(a). NDS Section 12.1.4.3 notes that lead and clearance holes are not required for 3/8 inch and smaller lag screws loaded primarily in withdrawal in wood with G at or below 0.5, provided edge and end distances are sufficient.

Rather than relying only on the labels softwood and hardwood, the NDS ties lag screw lead-hole sizing to specific gravity (G), a direct measure of wood density, because species within the same softwood or hardwood category can have meaningfully different densities. A dense softwood species and a lighter hardwood species can have overlapping specific gravity values, which is why a lead hole based purely on the softwood/hardwood label can be undersized or oversized for a particular species. For lag screw dimensional specifications rather than hole sizing, see the Screw Size Chart.

Diagram is schematic and illustrates the NDS clearance hole and lead hole concept for lag screw connections; it is not to scale for any specific lag screw diameter.

The clearance hole is drilled through the member the unthreaded shank passes through, sized to match the shank diameter exactly. The lead hole is drilled into the member that receives the threaded portion, sized smaller than the shank so threads can engage and grip the wood. Any structural ledger, post, or beam connection using lag screws should be verified against the applicable design standard, local code, and, where the connection is load-bearing, a qualified engineer.

Structural wood screws often use specialized cutting or self-drilling tip geometry, serrated or reversed threads near the head to reduce splitting without a pilot hole, and manufacturer-specified drill sizes for certain materials. Manufacturers such as Simpson Strong-Tie publish technical notes stating that certain structural wood screws require no predrilling for specific applications, which is why a generic lag-screw lead-hole table should not automatically be applied to a proprietary structural screw. Always consult the specific product’s code report or installation guide rather than a generic chart.

Values represent commonly published thread-forming/thread-cutting sheet-metal screw pilot hole references for Type AB and Type B style screws. Confirm against the specific manufacturer’s data, since thread form and material hardness both affect the correct size.

Self-tapping screws cut their own internal threads as they are driven into a pre-existing pilot hole. Type A, Type AB, and Type B screws differ in thread pitch and point design, changing the correct pilot hole diameter for the same nominal size; do not mix these categories. Self-drilling screws include a drill-point tip that cuts its own starting hole, so a separate pilot hole is often unnecessary within the material thickness range the drill point is rated for; exceeding that range can cause the drill point to fail before threads engage. Always check the manufacturer’s rated thickness before skipping a pilot hole.

• Sheet-metal screws generally need a pilot hole close to the screw’s minor (root) diameter for adequate thread engagement.
• Thread-cutting screws physically remove material to form threads and typically need a slightly larger pilot hole than thread-forming screws of the same size.
• Thread-forming screws displace material rather than cutting it, generally needing a smaller pilot hole for the same nominal size.
• Steel versus aluminum: aluminum is softer and more forgiving of pilot hole variance; hardened steel requires more precise sizing.
• Thin versus thick sheet: thin sheet metal strips more easily and may need a slightly smaller pilot hole than thick stock for the same screw.

Do not mix metal pilot-hole values with wood-screw values; the two follow entirely different engineering logic. See the Self-Tapping Screws chart above for representative sheet-metal pilot hole values by screw size.

Values reflect widely published manufacturer technical data for concrete screw anchors. Exact drill bit size, minimum embedment, and hole depth vary by manufacturer and product line, so always confirm against the specific fastener’s installation instructions.

• Carbide masonry bits are required; standard twist bits will not cut concrete or block efficiently and wear out quickly.
• Hole depth should always be drilled deeper than the required thread embedment, commonly 1/4 to 1/2 inch or more, to prevent the screw from bottoming out on drilling dust.
• Cleaning the hole of dust and debris before installation matters, since packed dust can prevent full embedment.
• Manufacturer requirements always take precedence over general guidance.

A pilot hole guides a wood, lag, or self-tapping fastener that forms or cuts its own thread engagement as it is driven. A tap drill hole is prepared specifically so a tap can cut precise internal machine threads, which a machine screw is then threaded into. For internal machine-thread sizing, see the Tap Drill Chart.

A common misconception is that the pilot hole should equal the screw’s outside (major) diameter. For a threaded wood screw or lag screw driven into a lead hole, this is not correct: the threads need surrounding material to engage and grip, so the lead hole is sized smaller than the major diameter, closer to a percentage of the root or shank diameter depending on the fastener and the NDS provisions covered earlier on this page. For a clearance hole, the relationship is different: the hole is intentionally sized to match or very slightly exceed the shank diameter, so the shank passes through without engaging the material at all.

• Blind holes should be drilled a bit deeper than the screw’s tip length to avoid bottoming out before the screw is fully seated.
• Through holes do not have a bottoming-out risk, but debris clearance is still worth accounting for.
• Additional depth for debris matters most in masonry; manufacturer instructions for concrete screws commonly call for drilling 1/4 inch or more deeper than the required embedment specifically to leave room for dust.

Metric values are approximate equivalents, not necessarily interchangeable drill sizes. For precise fits, use calipers to compare the actual bit diameter to the required decimal size.

To match an unmarked bit to this chart: check the shank or packaging for a printed size first, use a drill-bit gauge plate with stamped holes for fractional, numbered, and letter sizes, or measure directly with digital calipers for the most precise reading.

This is why two published pilot hole charts for the same screw size can show different numbers. The chart on this page uses straight-bit values throughout for consistency with standard construction practice, and clearly distinguishes them from tapered-bit values where both are referenced.

• Countersinking cuts a conical recess so a flat-head screw sits flush with or slightly below the surface.
• Counterboring cuts a flat-bottomed recess, typically for a pan-head or hex-head screw, sometimes deep enough for a wood plug.
• Head angle (commonly 82 degrees for U.S. flat-head screws) determines the correct countersink bit angle.
• Material thickness limits how deep a countersink or counterbore can go without weakening the piece.

The screw head type generally does not change the pilot or lead hole diameter, which is primarily determined by the screw’s shank or thread diameter, thread design, and material. What the head type affects is whether you also need a countersink, counterbore, or recess preparation. For screw head geometry and matching drive types, see the Screw Size Chart.