Pilot hole chart: drill bit size for every screw

A pilot hole is the hole you drill before driving a screw, sized so the threads bite without splitting the wood and the screw drives without its head twisting off. The right size depends on two things only: the screw and the wood. This page gives the charts for standard wood screws by gauge, lag screws and lag bolts, and metric screws, plus the clearance-hole sizes that make two-board joints pull tight. Where published references disagree, both values are shown with the reason, because a pilot hole chart that hides its disagreements is how oak gets split.

Wood screw pilot hole chart by gauge

The engineering basis is thread geometry: the pilot should sit near the screw's root diameter, the solid core under the threads, not the outside of the threads and not the shank. The NDS (the US wood design standard) puts that at 70 percent of root diameter for softwoods and 90 percent for dense hardwoods; the Wood Handbook carries the same practice. Two independent implementations of that basis, The Planstack's NDS-derived chart and the DIY-standard values published by BuildToolHQ and Power Tools Insider, land one drill size apart on several gauges. Both columns are printed below; when in doubt, drill the smaller, test on scrap, and step up if the screw fights.

Sources per row: The Planstack (NDS 2018 Table L3 dimensions, 70/90 percent of root per NDS 12.1.5.2) + BuildToolHQ and Power Tools Insider for the DIY-standard columns, which agree with each other on every gauge listed. Shank and root diameters are identical across all three references.

Reading the two bases: the NDS percentages are for withdrawal strength, and their rounding-down rule keeps grip. The DIY-standard values trade a little holding power for a wider margin against splitting and snapped screws, which is the safer default for furniture and finish work in dense stock. Neither is wrong; know which one your other chart is quoting.

Clearance holes: the half of the job most charts skip

When you join two boards, the screw should thread only into the second one. A clearance hole through the top board, at the shank diameter (last column above), lets the screw slide through freely so the head pulls the joint closed. Skip it and the threads grip both boards, locking in whatever gap existed when they first bit; tightening harder just strips the top board or snaps the screw. This is also why screw length matters: the unthreaded shank should span the top piece, so the threads cross only the joint line into the receiving board.

Lag screw and lag bolt pilot hole chart

Lag bolts and lag screws are the same fastener: a hex-head screw that threads into wood, used for ledger boards, posts, and brackets. Lags need two holes. A clearance hole through the top piece at the full shank diameter, and a lead hole into the receiving member, which the NDS sizes by wood density group. Those group values, verified by WhatDrillBit against Portland Bolt, Mudge Fasteners, and Triangle Fastener spec sheets, are the left pair of columns; the larger one-bit-up values printed by MISUMI and Power Tools Insider are alongside. A washer under the head is always required; without one the head crushes the fibers and loses clamping force.

Sources per row: WhatDrillBit (NDS wood-group values, verified against Portland Bolt, Mudge Fasteners, Triangle Fastener) + MISUMI and Power Tools Insider for the one-size-up columns. WhatDrillBit also publishes a medium-wood column (Douglas fir, southern yellow pine) between the two: 5/32, 3/16, 15/64, 5/16, 13/32, 1/2 in for the same six sizes.

For structural connections, deck ledgers especially, the chart is not the authority: the lag manufacturer's specification and your code are. The IRC, for example, sets specific lag sizes and spacing for ledger boards, and an engineer's or manufacturer's table overrides any generic chart including this one.

Metric screw pilot hole chart

For metric wood screws (and coach screws, the European lag equivalent), the published guidance is a ratio of the thread diameter: roughly half in softwood and three quarters in hardwood. FixABolt's DIN 571 coach screw chart states that rule with exact values, and Power Tools Insider publishes a parallel metric wood-screw table; where they overlap the values are close but not identical, so both bases are shown.

Sources per row: FixABolt (half / three-quarters of thread diameter rule for DIN 571 coach screws) + Power Tools Insider (metric wood-screw pilots and clearances). FixABolt's own published table starts at M6; its M3 to M5 values here are its stated ratio rule applied to those diameters, cross-checked against PTI's full M3 to M12 table. The two bases bracket the same territory the inch charts do: smaller holes hold harder, larger holes protect the wood and the screw.

Cross-system note for a metric index and imperial screws: 5/64 in is close to 2.0 mm, 3/32 in to 2.4 mm, 7/64 in to 2.8 mm, and 1/8 in to 3.2 mm. If you tap threads rather than drive screws, machine screws are a different regime entirely; that is our tap and drill size chart.

Material notes that change the answer

• Hardwood (oak, maple, cherry, walnut): always pilot. Even a #6 will split oak near an edge. In very dense species like ipe, go one bit size above the chart.
• Softwood (pine, spruce, fir, cedar): the NDS waives the lead hole entirely for withdrawal in wood with specific gravity under 0.5, but drill anyway near edges and ends, and always in cedar.
• Plywood: behaves closer to softwood; the cross-grain layers resist splitting. Use softwood sizes, pilot near edges.
• MDF: no grain to resist splitting, cracks unpredictably, and mushrooms around the head. Always pilot at the hardwood size and always countersink.
• Pressure-treated lumber: denser and more split-prone than untreated, particularly at board ends. Pilot deck screws; use hot-dip galvanized or stainless fasteners.
• End grain: the NDS assigns withdrawal from end grain a factor of zero, not a reduction. Never hang a load from a screw threaded into end grain; change the joint instead.

Depth, wax, and the rest of the technique

Drill the pilot at least as deep as the threads will penetrate; a shallow pilot just moves the splitting moment down the hole. A strip of tape on the bit makes a depth stop. For hardwood, a rub of wax or soap on the threads cuts driving torque noticeably, and the Wood Handbook explicitly permits it with no loss of holding strength. Countersink flat heads so they seat instead of crushing their own crater; the countersink diameters by gauge are in our countersink size chart. And the drill doing this work wants a clutch once screws go in, which is covered in torque settings explained; for which bit shape belongs in which material before any of this, see drill bits by material.

Quick answers

• What size pilot hole for a #8 screw? 5/64 in softwood and 7/64 in hardwood on the NDS basis; 7/64 and 1/8 on the common DIY basis. In oak, start at 7/64 and be ready for 1/8.
• What size pilot hole for a 1/2 inch lag? 15/64 in softwood and 11/32 in hardwood per the NDS group values; up to 3/8 and 7/16 in the one-size-up charts, plus a 1/2 in clearance hole through the top board.
• Lag bolt vs lag screw? Same fastener, two names. Same chart, same washer requirement.
• Pilot hole for M6 coach screw? 3 mm in softwood, 4.5 to 5 mm in hardwood per the DIN 571 rule.
• Do deck screws need pilot holes? In pressure-treated lumber, near ends and edges, yes. Modern coated deck screws self-drill in field areas of sound lumber.
• Does screw length change the pilot size? No. The pilot diameter follows the gauge only; the depth follows the length.

Related guides

Also on this site: the tap and drill size chart for cutting threads in metal, drill bits by material for choosing the bit itself, drill torque settings explained for driving without stripping, screwdriver bit types for the driving end, and the bolt and screw extractor size chart for when the driving already went wrong. For sheet-metal screws that tap their own threads, self tapping vs self drilling screws sorts the two types and their pilot-hole rules. In masonry instead of wood, concrete anchor bit sizes by fastener is the reference.