Screw size chart: gauges, diameters and pilot holes

Wood screw size chart

Shank diameters are from Wood Handbook Table 8–9 and agree with the D values in NDS Table L3 for every gauge the NDS lists. Root and head diameters are from NDS Table L3, which is based on ANSI/ASME B18.6.1. Pilot and shank holes are calculated, then rounded to the nearest 1/64 in. drill (shank holes round up so the screw passes freely).

Gauges #0 to #3 are not listed: Wood Handbook Table 8–9 starts at gauge 4 and NDS Table L3 at gauge 6. ANSI/ASME B18.6.1 covers the smaller gauges but was not available for verification. Head style and driver recess (slotted, Phillips, square, Torx) vary by manufacturer and are not covered by these sources, so they are not tabulated.

How to read screw gauge numbers

A screw described as "#8 × 1-1/2" is a gauge 8 screw, 1-1/2 in. long. The gauge sets the shank diameter. Check how a maker measures length for raised-head screws, as it is not covered by the sources used here.

Gauge numbers rise in even steps. In Table 8–9, #4 is 0.112 in. and every gauge above it adds 0.013 in., so #8 is 0.164 in. and #10 is 0.190 in. That makes a #10 about 16% thicker than a #8, not 25%. Screw gauge runs the opposite way to the wire gauge used for nails, where a larger number is a thinner wire.

The Wood Handbook describes the parts of a wood screw as the head, shank, thread and core, and notes that the root (core) diameter of most sizes averages about two-thirds of the shank diameter. The NDS values are larger: the #8 root of 0.131 in. is 80% of its shank and the #14 root of 0.196 in. is 81%. NDS Table L3 takes Dr as the average body diameter of rolled-thread wood screws (its footnote 4), which helps explain why its values sit above the Handbook's two-thirds rule of thumb.

Pilot holes and shank holes

A screw joint usually needs two holes. The shank hole (often called the clearance hole) goes through the piece being attached, so the screw pulls that piece tight instead of threading into it. The pilot hole (lead hole) goes into the piece receiving the point, so the threads cut clean grooves without splitting the wood.

What the sources say

The Wood Handbook's withdrawal equation for wood screws applies when lead holes are about 70% of the thread root diameter in softwoods and about 90% in hardwoods. For lateral (shear) loads it recommends a shank lead hole the same diameter as the shank or slightly smaller, and a threaded lead hole about the root diameter in dense species or slightly smaller in low-density species.

The NDS (Section 12.1.5) classifies by specific gravity, G, instead of "softwood" and "hardwood". For withdrawal it calls for about 90% of root diameter where G is above 0.6, about 70% where G is above 0.5 and up to 0.6, and no lead hole at all where G is 0.5 or less. For lateral loads it uses the full shank and root diameters above G 0.6, and about 7/8 of each at or below G 0.6.

The two sources are close but not identical, so the chart uses the Wood Handbook's simpler softwood and hardwood split. Where your species sits matters more than the label: many construction softwoods fall below G 0.5, while some "softwoods" and most oaks do not.

The Handbook also says screws should always be turned in, never started or driven with a hammer, and that soap or a similar lubricant eases insertion in dense woods with little effect on withdrawal strength.

Choosing screw length

Length is set by how far the screw needs to reach into the piece holding the point. The Wood Handbook's lateral-load equation applies when that penetration is at least 7 shank diameters, and it recommends a side member (the piece being attached) about half as thick as that penetration. The NDS minimum is 6 diameters, including the tapered tip.

length ≥ side-member thickness + 7 × shank diameter

Wood Handbook Table 8–8 lists the gauges its withdrawal equation covers at each length, which is a useful check that a gauge and length are a sensible pairing:

The Handbook adds that in dense hardwoods the longer standard lengths are superfluous: the wood holds the threads so well that the screw fails in tension before it pulls out.

Metric screw equivalents (approximate)

For a screw specified by diameter in millimetres, there is no exact gauge for most sizes. The table compares each common metric diameter with the gauges whose Table 8–9 shank diameter sits just below and just above it.

Worked examples

1. Fixing a 19 mm board to the face of a 2×4

A 2×4 is 38.1 mm (1-1/2 in.) thick (PS 20 Table 3). With a #8 screw (4.17 mm shank), seven diameters of penetration is 29.2 mm, so the screw needs to be at least 48.2 mm long.

• A 2 in. (50.8 mm) #8 penetrates 31.8 mm, or 7.6 shank diameters. That meets the Handbook's seven diameters and the NDS minimum of 25.0 mm, stops 6.3 mm short of the back face, and Table 8–8 lists #8 within the 2 in. range.
• Drill a 11/64 in. (4.4 mm) shank hole through the 19 mm board.
• In Spruce-Pine-Fir (G 0.42), the NDS does not require a lead hole for withdrawal. If you drill one, the Wood Handbook's 70% rule gives 0.092 in., or a 3/32 in. (2.4 mm) bit.

2. A #10 screw into white oak

White oak has an assigned G of 0.73, above the 0.6 threshold, so both sources call for the larger pilot. The NDS root diameter of a #10 is 0.152 in.; 90% of that is 0.137 in. (3.5 mm), which rounds to a 9/64 in. bit. The 70% softwood figure (7/64 in.) is meant for lower-density wood. Seven shank diameters is 1.330 in. (33.8 mm) of penetration, and the shank hole is 13/64 in.

Screws or nails?

The Wood Handbook's withdrawal equations show why screws are chosen where pull-out matters. For the same diameter and penetration, its wood screw equation (8–10) divided by its smooth common nail equation (8–1) gives a ratio of 2 ÷ √G. In Spruce-Pine-Fir at G 0.42 that is about 3.1 times the withdrawal resistance; in white oak at G 0.73, about 2.3 times. This is a comparison of the two equations, not a design value.

Nails are quicker to drive, and the Handbook notes that variations in the wood, the nail and the conditions of use affect withdrawal resistance much more than lateral (sideways) resistance. Screws suit joints loaded in withdrawal and parts that may need to come apart again. The Handbook notes that screws driven into end grain average 75% of the side-grain withdrawal load. For structural connections, follow the fastener schedule your building code or engineer specifies. See the nail size chart for penny sizes and nail diameters, and the fastener spacing calculator for minimum screw spacing and edge and end distances by diameter.

Common mistakes

• Sizing the pilot from the shank. Pilot holes are a share of the root diameter. A pilot near the shank diameter leaves the threads almost nothing to grip.
• Skipping the shank hole. If the threads engage both pieces, the screw can hold them apart instead of drawing them together.
• Using the softwood pilot in hardwood. The 70% figure is for softwoods; the Wood Handbook and the NDS both call for about 90% of root diameter in dense wood.
• Too little penetration. Below seven diameters the Wood Handbook says ultimate lateral load falls roughly in proportion to the shortfall.
• Treating metric and gauge sizes as interchangeable. A 4.0 mm screw is neither a #7 nor a #8; check the maker's data before reusing a pilot size.
• Hammering screws to start them. The Handbook warns this tears the wood fibres and damages the threads, seriously reducing holding capacity.

Common questions