Wood Screw Pilot Hole Chart

What size drill bit for a #8 wood screw?

For a #8 wood screw drill a pilot hole of 1/8″ (3.2 mm) in hardwood or 3/32″ (2.4 mm) in softwood, plus a clearance hole of 11/64″ (4.4 mm) through the top board. That is the chart’s #8 row, the one pre-selected above. The table gives the same three numbers for the other common gauges.

Pilot hole vs clearance hole: three holes, one strong joint

A properly drilled screw joint has a clearance hole in the top board (the shank spins freely), a pilot hole in the bottom board (sized so threads bite without splitting), and — for flat-heads — a countersink. The chart’s hardwood pilot tracks the screw’s root diameter; softwood gets one size less because the fibers compress out of the way.

The two holes do different jobs, and Fine Woodworking spells out why the clearance hole is not optional: without it the threads grab in the top board and the screw cannot pull the boards tight, however hard it is driven. Size it to the shank. A #8 screw has a shank diameter of 0.164″ (Table 8-9 of the USDA Wood Handbook), and the chart’s 11/64″ bit is 0.172″, 0.008″ above it, so the screw slides through. The pilot is sized to the root, or core, which the Handbook puts at about two-thirds of the shank diameter: roughly 0.109″ for a #8.

For a flat-head screw, countersink after the clearance hole, in short passes, test-fitting the screw until the head sits flush with the surface, or a hair below it if you plan to plug or fill.

Hardwood vs softwood pilot holes: how the sizes are chosen

Both magazines tie the pilot to the screw’s root diameter, then adjust for the wood. Fine Woodworking says to pick a bit slightly larger than the root for hardwood and slightly smaller for softwood. Popular Woodworking puts a number on “slightly”: in hardwood the pilot should be at least the root (minor) diameter, with another 1/64″ if the screw has deep threads or the wood is very hard; in softer woods it can be 1/64″ smaller. The USDA Wood Handbook gives percentages instead, about 70% of the root diameter in softwoods and about 90% in hardwoods. Here is each rule worked for a #8:

For a #8 the chart matches the Popular Woodworking rule, with hardwood at the top of its range. The Handbook’s figures are smaller: its percentages describe the lead holes behind its screw-withdrawal (holding-power) equation, and the chart’s #8 pilots are 0.017″ and 0.027″ wider. All the rules agree on the direction. The Handbook’s hardwood pilot is 29% wider than its softwood one, and the chart’s hardwood pilots run 20–33% wider than its softwood ones across every gauge.

Charts also differ from each other. An independent fastener distributor’s chart lists 7/64″ for a #8 in softwood where this chart lists 3/32″, though it agrees on 1/8″ for hardwood and 11/64″ for the clearance hole. Treat any chart as a starting point: drill a scrap of the board you are actually using, step down a size if the screw spins or strips out, and up a size if it binds or the wood splits.

One caution: “hardwood” and “softwood” are botanical labels, not hardness grades. The Wood Handbook’s lead-hole guidance for lag screws is organized by density, with separate ranges for light softwoods such as cedar and white pine, for Douglas-fir and Southern Pine, and for dense hardwoods such as oak. If you are unsure where a board falls, look up its hardness in the Janka hardness chart and pilot the dense ones like hardwood.

Do you need to drill a pilot hole?

For ordinary wood screws, yes. Popular Woodworking’s answer is that the pilot hole is what keeps the screw from breaking off and the wood from cracking; skip it, or drill it too small, and you can get hairline cracks in solid wood and MDF, and even cracks too small to see can eventually widen and cause the joint to fail.

Self-drilling and structural screws follow the maker’s rules instead. GRK’s installation bulletin says its RSS structural screw does not require predrilling the wood, and suggests a pilot only when the screw will not drive fully in very hard, dense wood such as mahogany, or when torque splits the wood or strips the recess. Its pilot is about 70% of the screw’s minor diameter for screws loaded in tension (90% in the side member and 70% in the main member for lateral loads), drilled to 50–70% of the thread length. That is a different recipe from the wood screw chart above, so use whichever matches your fastener.

How deep should a pilot hole be?

The two holes have different depths. The clearance hole goes through the top board only, at full thickness. The pilot continues into the bottom board, and one fastener distributor’s guide advises drilling pilots slightly shorter than the screw length. Don’t drill shallow to save effort: the Wood Handbook says a screw’s withdrawal load rises with the depth its threaded part penetrates.

Example: a 1-1/2″ screw through a 3/4″ top board leaves 3/4″ of screw in the bottom board. Drill the 3/4″ clearance hole through the top board, then a pilot into the bottom board deep enough to take that 3/4″ of threads, stopping just short of the point.

When the chart bends

• Very hard stock (hard maple, hickory, most exotics): one drill size up, and wax the screw.
• Near an edge or end: pilot generously — splits start at fibers with nowhere to go.
• Long screws: pilot the full length; a half-depth pilot concentrates all the strain at the tip.