Pilot Hole Sizes for Wood Screws and Lag Bolts

For a lag bolt, drill two holes. The threaded part gets a lead hole sized as a percentage of the bolt's diameter: 40% to 70% in lighter woods like spruce-pine-fir, hem-fir and Douglas fir-larch, 60% to 75% in Southern pine and maple, and 65% to 85% in oak and other dense hardwoods. The smooth shank under the head gets a clearance hole the same diameter as the shank, as deep as the shank is long. For a 3/8" lag bolt that works out to a lead hole between 5/32" and 1/4" in spruce-pine-fir, 15/64" to 9/32" in Southern pine, and 1/4" to 5/16" in oak.

Wood screws are sized off the root diameter (the core of the screw inside the threads), not the outside diameter. In dense hardwood, a screw loaded in withdrawal gets a lead hole about 90% of its root diameter; in mid-density wood, about 70%. In light softwoods the rules don't require one at all. These figures come from the American Wood Council's National Design Specification for Wood Construction (NDS, 2018 edition), sections 12.1.4 and 12.1.5, and the fastener dimensions from its Appendix L, which follows ASME B18.2.1 for lag screws and ASME B18.6.1 for wood screws. The charts below do the arithmetic for you.

Why does wood density matter more than "softwood or hardwood"?

The NDS sorts wood by specific gravity (G), its oven-dry density relative to water, not by whether it comes from a conifer. That matters because the labels mislead: Southern pine is a softwood, but at G = 0.55 it falls in the middle band with mixed maple, not with spruce.

Assigned specific gravities from NDS Table 12.3.3A for common lumber:

If you don't know the species, ask the lumber supplier before you pick a column. When in doubt between two bands, the hole sizes overlap, so a size inside both ranges is a reasonable compromise.

What size pilot hole does a lag bolt need?

Two holes, drilled in this order:

1. Clearance hole for the shank. Same diameter as the shank, drilled as deep as the unthreaded shank is long (NDS 12.1.4.2(a)). A 1/2" lag gets a 1/2" clearance hole. Some lag screws are made with a reduced body, where the shank is close to the root diameter (NDS Table L2, footnote 1), so measure the shank rather than assuming it matches the nominal size.
2. Lead hole for the threads. Sized from the shank diameter by wood density, and at least as long as the threaded portion (NDS 12.1.4.2(b)). The NDS says to use the larger end of each range for larger diameters, so a 1/4" lag sits near the low end and a 3/4" lag near the high end.

Lead hole for the threaded portion, in inches. The fractional bits shown are the 1/64" sizes that fall inside the range.

Basis: AWC NDS 2018, section 12.1.4.2(b), applied to the nominal shank diameter. Ranges calculated, not rounded to a single bit.

Drive the threads with a wrench or socket, not a hammer (NDS 12.1.4.4). The NDS also notes that soap or another lubricant in the hole or on the threads doesn't reduce the connection's design values (12.1.4.5), which helps on long lags in dense wood.

For head sizes and the wrench or socket each lag bolt takes, see the bolt size chart. If you're still deciding between a lag and a through-bolt, the hex bolt vs. other bolt types guide compares them.

Can you skip the pilot hole on a lag bolt?

Sometimes. NDS 12.1.4.3 allows lag screws 3/8" and smaller to go in without a lead or clearance hole when three conditions all hold: the wood has G ≤ 0.5, the screw is loaded mainly in withdrawal (pulling straight out) rather than sideways, and the edge distance, end distance and spacing are enough to keep the wood from splitting.

A deck ledger lag carries the deck's weight sideways across the screw, so it doesn't qualify. It also rules out lags close to the end of a board. If any one of those conditions isn't met, drill.

How deep should a lag bolt go?

The NDS sets a minimum penetration of four times the lag diameter (4D) into the main member, not counting the tapered tip (NDS 12.1.4.6). For a 3/8" lag that is 1-1/2" of thread and shank in the member you're fastening into, plus the tip. Edge distance, end distance and spacing for design loads are set by NDS Tables 12.5.1A through 12.5.1E; check those, or the project's engineered drawings, for anything structural.

If you need to recess the head and a washer below the surface, drill the counterbore first with a bit larger than the washer's outside diameter, then the clearance hole, then the lead hole. Each later hole stays centered that way.

What size pilot hole does a wood screw need?

Start with the screw's root diameter. NDS Appendix Table L3 lists it for standard ASME B18.6.1 wood screws. Then pick the column that matches how the screw is loaded.

Screws loaded in withdrawal (the load tries to pull the screw straight out, such as a screw holding a hanging shelf cleat to a ceiling joist). NDS 12.1.5.2: about 90% of the root diameter for G > 0.6, about 70% for 0.5 < G ≤ 0.6, and no lead hole required for G ≤ 0.5.

Basis: diameters from AWC NDS 2018 Appendix Table L3 (ASME B18.6.1 tolerances); lead holes per NDS 12.1.5.2. Fraction in brackets is the closest 1/64" bit.

Screws loaded laterally (the load pushes sideways across the screw, as in most two-board joints). NDS 12.1.5.3 sizes both parts of the hole: in wood with G > 0.6, the shank hole is about the shank diameter and the thread hole about the root diameter; in wood with G ≤ 0.6, both are about 7/8 of those sizes.

Basis: AWC NDS 2018, section 12.1.5.3 and Appendix Table L3. Fraction in brackets is the closest 1/64" bit.

Like lag screws, wood screws should be turned in, not hammered (12.1.5.4), and the NDS minimum penetration into the main member is six times the screw diameter (6D), tip included (12.1.5.6).

Do deck screws follow the same chart?

Not exactly. The NDS chart is for ASME B18.6.1 wood screws. Many deck screws use their own thread forms and points, such as a gimlet or cutting tip, so their root diameter may not match Table L3. Measure the root diameter of the screw you have with calipers (our guide to measuring bolts and screws shows how) and apply the same percentages. Where the screw maker publishes its own pilot hole size, use theirs.

Does a stainless steel lag bolt need anything different?

The hole sizes are the same. The NDS rules are based on the wood and the fastener's dimensions, not its material.

One thing to keep apart: galling, where stainless threads seize, happens when stainless threads slide against other stainless threads, as with a stainless bolt and nut. A lag bolt threads into wood, so it isn't the same problem. If your joint does use stainless bolts and nuts, our post on preventing stainless steel galling covers lubrication and driving speed.

Bottom line

Size a lag bolt's lead hole from its shank diameter and the wood's specific gravity, drill a full-diameter clearance hole for the shank, and turn it in with a wrench. Size a wood screw's hole from its root diameter, and check whether it is loaded in withdrawal or sideways before you pick a column.

Fastenio's lag bolts come in 18-8 stainless steel, 316 stainless steel and silicon bronze, from 1/4" to 3/4". The wood screws cover brass, silicon bronze and 18-8 stainless steel, and the deck screws include 18-8 and 316 stainless steel and silicon bronze in #6 through #14.