Pilot Hole Size Chart for Wood Screws: Softwood, Hardwood and the Species That Split Anyway
For softwood, drill just under the screw's root diameter; for hardwood, drill the root diameter exactly; for dense species like white oak, hickory or ipe, drill the root plus a 64th. Root diameter is close to 0.68 times the screw's major diameter, which is why a single-column chart works in pine and splits oak.
Softwood pilot: just under the screw's root diameter. Hardwood pilot: the root diameter. Dense hardwood pilot: root diameter plus about 1/64 in. Those are the three numbers, and everything below is that rule applied to every gauge from #4 to #14.
Root diameter is the shaft between the threads, and on a traditional tapered wood screw it sits close to 0.68 times the major diameter you buy the screw by. A #8 screw measures 0.164 in across the threads, so its root is about 0.112 in, so a 7/64 in bit at 0.109 in is your hardwood pilot.
The reason most published charts fail is that they give one hardwood column, derived from something around the density of oak, and then people use it in hickory, jatoba and ipe. Those species need the next size up and sometimes a waxed thread as well. That is why the third column exists here.
The rule the whole chart is built from
Wood does not grip a screw with the hole. It grips with the fibres that the thread cuts into after the hole ends. The pilot's job is to remove exactly the material the shank would otherwise have to displace, and no more.
Drill it too small and the shank wedges the fibres apart, which is a split. Drill it too big and the thread has nothing to bite, which is a stripped hole that spins forever.
Where species density comes in is that dense timber does not compress. Pine will happily absorb 0.015 in of displacement around the shank with nothing worse than a slightly proud fibre. White oak at a specific gravity of 0.68 has nowhere to put it, so it releases the stress along a ray and you get a crack. Same screw, same hole, completely different outcome.
Pilot hole sizes, #4 to #14
Screw gauge Major diameter Root diameter (approx) Softwood pilot Hardwood pilot Dense hardwood pilot Metric pilot (soft / hard / dense) #4 0.112 in 0.076 in 1/16 in (0.0625) or #52 5/64 in (0.078) or #47 #44 (0.086) 1.6 / 2.0 / 2.2 mm #5 0.125 in 0.085 in 5/64 in (0.078) #43 (0.089) 3/32 in (0.094) 2.0 / 2.2 / 2.4 mm #6 0.138 in 0.094 in 5/64 in (0.078) 3/32 in (0.094) or #42 7/64 in (0.109) 2.0 / 2.4 / 2.8 mm #7 0.151 in 0.103 in 3/32 in (0.094) #38 (0.1015) 7/64 in (0.109) 2.4 / 2.6 / 2.8 mm #8 0.164 in 0.112 in 3/32 in (0.094) 7/64 in (0.109) or #34 1/8 in (0.125) 2.4 / 2.8 / 3.2 mm #9 0.177 in 0.120 in 7/64 in (0.109) #31 (0.120) 1/8 in (0.125) 2.8 / 3.0 / 3.2 mm #10 0.190 in 0.129 in 7/64 in (0.109) 1/8 in (0.125) or #30 9/64 in (0.141) 2.8 / 3.2 / 3.5 mm #12 0.216 in 0.147 in 1/8 in (0.125) 9/64 in (0.141) or #28 5/32 in (0.156) 3.2 / 3.5 / 4.0 mm #14 0.242 in 0.165 in 9/64 in (0.141) 5/32 in (0.156) or #22 11/64 in (0.172) 3.5 / 4.0 / 4.4 mm Pick the column by the species you are driving into, not the species of the piece on top. The holding member decides the pilot; the top piece gets a clearance hole instead. Numbered drill equivalents are given where they land closer than the nearest 64th. A set of numbered bits from #1 to #60 costs less than a decent chisel and takes the guesswork out of the middle column permanently.
Metric figures are rounded to bits that exist. Nobody sells a 2.7 mm bit in a general set, so the chart uses 2.6 and 2.8.
Clearance and countersink, which are the two most-skipped holes
The piece on top gets a hole the screw passes through without touching. If the threads bite the top piece, the two boards cannot pull together and you get a gap you will then try to close by driving harder, which strips the bottom hole.
Clearance diameter below is simply the next fractional bit above the screw's major diameter. Countersink diameter is the nominal head diameter of a flat-head wood screw, which increases by a consistent 0.0265 in per gauge number.
Screw gauge Major diameter Shank clearance hole Flat head diameter Countersink cutter, minimum Metric clearance #4 0.112 in 1/8 in (0.125) 0.225 in 1/4 in 3.0 mm #5 0.125 in 9/64 in (0.141) 0.252 in 9/32 in 3.5 mm #6 0.138 in 9/64 in (0.141) 0.279 in 5/16 in 3.5 mm #7 0.151 in 5/32 in (0.156) 0.305 in 5/16 in 4.0 mm #8 0.164 in 11/64 in (0.172) 0.332 in 3/8 in 4.5 mm #9 0.177 in 3/16 in (0.188) 0.358 in 3/8 in 4.5 mm #10 0.190 in 13/64 in (0.203) 0.385 in 13/32 in 5.0 mm #12 0.216 in 7/32 in (0.219) 0.438 in 7/16 in 5.5 mm #14 0.242 in 1/4 in (0.250) 0.491 in 1/2 in 6.5 mm Drill the clearance hole all the way through the top piece before you drill the pilot, then clamp the parts and drill the pilot through the clearance hole so the two are automatically concentric. A countersink cut too deep is worse than one cut too shallow. Sink the head below the surface in hardwood and you have removed the material that was going to resist the head crushing in, and the screw ends up sitting in a dished crater.
Counterboring is a different operation: a flat-bottomed hole sized for a plug, drilled first, with the clearance and pilot holes drilled through the bottom of it afterwards. Use a Forstner, not a spade bit, and cut the plug from the same board so the grain matches.
Species density index: which column your timber belongs to
Specific gravity predicts splitting better than Janka does, because Janka measures resistance to a ball being pressed in and splitting is about how the fibres release stress. Both are listed because Janka is the number most timber suppliers quote.
Species Specific gravity Janka (lbf) Pilot column to use What actually goes wrong Western red cedar 0.32 350 Softwood Head crushes into the surface before the thread grips; use a wide-head screw Eastern white pine 0.35 380 Softwood The species every single-column chart is secretly written for Spruce / SPF 0.42 510 Softwood Nothing much; forgiving in every direction Poplar 0.42 540 Softwood Splits at end grain more readily than its density suggests Douglas fir 0.48 620 Softwood, hardwood near ends Hard latewood bands deflect the bit off centre Cherry 0.50 950 Hardwood Burnishes rather than splits; watch heat on deep pilots Soft maple 0.54 950 Hardwood Predictable; the easiest hardwood to screw into Walnut 0.55 1,010 Hardwood Fine, but the dark dust hides a starting split White ash 0.60 1,320 Hardwood Splits along the ring boundary, not the ray Red oak 0.63 1,290 Hardwood; dense column within 2 in of an end Ray splits appear an hour after you finish Hard maple 0.63 1,450 Hardwood; dense column above #10 Strips the pilot if you overdrive; use a clutch Beech 0.64 1,300 Hardwood; dense near ends Moves seasonally, so a tight pilot cracks later White oak 0.68 1,360 Dense The classic case of a pine setting failing Hickory 0.72 1,820 Dense Snaps small brass and stainless screws outright Purpleheart 0.86 2,520 Dense, plus wax Gums the flute and heats the bit; peck-drill it Jatoba 0.91 2,350 Dense, plus wax Interlocked grain grabs the shank halfway in Ipe 1.05 3,510 Dense, plus wax, plus full-depth pilot Snaps stainless screws; use a hardened deck screw If your species is not listed, place it by specific gravity: under 0.50 use the softwood column, 0.50 to 0.65 the hardwood column, above 0.65 the dense column. End grain, edges and the two inches that ruin everything
Distance from a free end matters as much as species. The published pilot works in the middle of a board and fails 3/4 in from the end, because the fibres there have an unrestrained path to open.
Working rule: within 2 in of an end in hardwood, move one column right. Within 1 in, move one column right and add a 64th on top. Within 1/2 in, do not screw into it at all without a clamp across the end while you drive.
Edge distance in solid timber wants at least 3 screw diameters from the edge, so a #8 needs about 1/2 in. Closer than that and the screw does not split the board so much as blow the edge off it.
End grain itself holds badly regardless of pilot size, typically around 60 to 75 percent of face-grain withdrawal, and it gets worse over time as the joint cycles with humidity. A screw into end grain is a temporary arrangement unless there is a cross-dowel or a glue block doing the real work.
Sheet goods play by different rules
MDF edges are the worst-behaved material in the shop for screws. The face holds tolerably; the edge is compressed fibre with no long grain at all and it delaminates in a mushroom around the screw. Pilot the edge at full root diameter or slightly over, keep at least 1 in from a corner, and use a straight-shank confirmat-style screw rather than a tapered wood screw. Better still, use a threaded insert.
Particle board and melamine want a straight-shank screw and a pilot at root diameter. Tapered screws act as wedges in a material with no fibre to resist them.
Plywood edges hold far better than MDF, but only if you hit the plies rather than a void. Baltic birch with 13 plies in 3/4 in is genuinely good edge-screwing material; a five-ply construction-grade sheet with voids is not.
For all three, the clearance hole in the top piece matters more than usual. Sheet goods have no give, so any thread biting the top layer holds the joint open permanently.
Modern production screws break the old chart
The chart above is built on traditional tapered wood screws. A lot of what you buy now is not that. Production screws in the GRK, Spax and similar families have a straight shank, a deeper and sharper thread, a type-17 auger point that clears its own chips, and often a set of nibs under the head that cut their own countersink.
In softwood these frequently need no pilot at all, which is the whole point of them. In hardwood they still do: the auger point clears chips but it cannot stop the shank displacing material.
For a straight-shank production screw, the pilot is the shank diameter itself rather than 0.68 of the major. Measure the smooth part of the shank with calipers and use that. A screw sold as 8 by 2 1/2 in that family might have a 0.105 in shank under a 0.160 in thread, which puts its hardwood pilot at 7/64 in, close enough to the chart, but plenty of them are thinner and you will over-drill if you assume.
Skip anything with a lubricant coating in oak. The wax makes driving easier and does nothing for splitting, and it complicates gluing anywhere near the joint.
The drilling sequence that stops splits
- Drill the pilot to at least the full length of the threaded section, and in dense species, the full length of the screw. A pilot that stops short turns the last half inch into a wedge.
- Peck-drill anything over about 1 in deep in dense timber. Two or three withdrawals clear the flute and stop the bit heating enough to burnish the hole closed.
- Wax the threads for hardwood above 0.65 specific gravity. Paraffin, beeswax or a candle stub. Not soap, which holds moisture against steel.
- Drive to seated, then back the screw out one full turn and drive it home again. It clears the compressed chip ahead of the thread and reduces the stress that becomes a split.
- Use a clutch setting rather than a trigger and hope. Stripping a hardwood pilot is easier than splitting it and just as annoying.
- Clamp across the end grain before driving anything within 2 in of a board end. A pipe clamp with a scrap caul takes ten seconds and saves the part.
One last check that catches most failures before they happen: hold the screw up in front of the drill bit against a light. You should see the thread crests standing proud on both sides of the bit and the root of the thread hidden behind it. If you can see daylight either side of the root, the bit is too big. If the bit is invisible behind the root, it is too small.