Lag Screw Pilot Hole Chart by Wood Type

A lag screw (often called a lag bolt) needs two holes: a clearance hole the full shank diameter for the smooth part, and a smaller lead hole for the threads. Too small a lead hole and the screw is hard to turn, may snap, or splits the wood; too large and it strips. The USDA Wood Handbook gives the lead hole for the threaded part as a percentage of the shank diameter that depends on the wood. This chart turns those percentages into drill sizes for lag screws from 1/4 to 1 inch.

Lag screw pilot hole chart

How the chart is worked out

The Handbook gives a range for each wood group and says the low end is for small lag screws and the high end for large ones, but does not say exactly where each size falls. We spread each range evenly over the common sizes, from 1/4 in at the bottom of the range to 1 in at the top. For a 3/8 in lag in Douglas-fir that gives 60% + (75% − 60%) x (0.375 − 0.25) / 0.75 = 62.5%, and 62.5% of 0.375 in is 0.234 in, which rounds to a 15/64 in drill. The shank hole is the same as the shank, 3/8″, and is drilled only as deep as the smooth part of the shank goes.

If your drill set has only 1/16 in steps, or the screw is long, go one size up rather than down. The Handbook itself recommends lead holes slightly larger than the ideal for long lag screws, and soap or wax on the threads so they turn more easily.

Thread length and how deep to go

• The threaded part of a lag screw is about 3/4 in long on 1 in and 1-1/4 in screws, and half the length on screws longer than 10 in (Wood Handbook). Check your screws: the lead hole only needs to be as deep as the thread will reach.
• Penetration of the thread into the main member of about 7 times the shank diameter in dense wood (specific gravity above 0.61) and 10 to 12 times in light wood (below 0.42) develops roughly the full tensile strength of the screw; between those, interpolate in a straight line. The last column of the next table does this for each species.
• Withdrawal from end grain is about three quarters of withdrawal from side grain.
• Always turn a lag screw in with a wrench or socket. Do not hammer it, and do not overtighten until the head crushes the wood.

Withdrawal by wood species

The Handbook’s equation for the maximum withdrawal load of a lag screw from the side grain of dry wood is p = 8,100 G^3/2 D^3/4 L, in pounds and inches, with L the length of thread in the main member. The table gives pounds per inch of thread for six common diameters, and the penetration that develops the screw’s full strength.

Worked example: a 3/8 in lag into Douglas-fir

For a 3/8 in lag screw with 2-1/2 in of thread in coast Douglas-fir (G = 0.48): p = 8,100 x 0.48^1.5 x 0.375^0.75 x 2.5 = 3,227 lb at failure in a test. Full-strength penetration at that density is about 9.7 x 0.375 = 3.7 in, so with 2-1/2 in of thread the wood is still the weak link: it would take about 3.7 in of thread before the screw’s own strength became the limit. The lead hole for the thread is 15/64 in and the shank hole 3/8 in.

Safety and structural use

Lag screws are often used where failure is dangerous: deck ledgers, railing posts, swing and hammock hangers, and heavy shelving. The loads on this page are laboratory averages at failure and are not design values. Building codes set the fasteners, spacing and edge distances for deck ledgers and guards, and many jurisdictions require through-bolts or listed structural screws in those places. Check your local code, follow the hardware maker’s instructions, and use a licensed engineer for anything that carries people.

Frequently asked questions

What size pilot hole for a 3/8 lag screw?

A 3/8 in hole for the smooth shank, and for the threads about 11/64 in in cedar or white pine, 15/64 in in Douglas-fir or southern pine and 1/4 in in oak (Wood Handbook percentages).

What size pilot hole for a 1/4 lag screw?

A 1/4 in shank hole and a thread hole of about 3/32 in in cedar or pine, 5/32 in in Douglas-fir or southern pine and 5/32 in in oak.

What size pilot hole for a 1/2 lag screw?

A 1/2 in shank hole and a thread hole of about 1/4 in in cedar or pine, 21/64 in in Douglas-fir or southern pine and 23/64 in in oak.

Do lag screws need a pilot hole?

Yes. The Wood Handbook says lag screws, like wood screws, require prebored holes: one the size of the shank for the smooth part, and a smaller one for the threads.

How deep should a lag screw go?

For full strength, about 7 times the shank diameter of thread in dense hardwoods and 10 to 12 times in light softwoods such as cedar. For a 3/8 in lag that is about 2-5/8 in in oak and up to 4-1/2 in in cedar.

Sources and method

The lead-hole percentages and the equation are from the Handbook. Where it gives a range, the spread by diameter, the rounding to 1/64 in and every load in the tables are our computation, shown under each table.

1. USDA Forest Service, Forest Products Laboratory. Wood Handbook: Wood as an Engineering Material, General Technical Report FPL-GTR-190 (2010), chapter 8, Fastenings (D. R. Rammer). A US Government publication in the public domain. Equation 8-14 and the text on lag screw lead holes, threaded length, penetration and end-grain withdrawal (pp. 8-12 and 8-13).
2. Specific gravity of each species: Wood Handbook FPL-GTR-190, Tables 5-3b and 5-5b (ovendry weight, volume at 12% moisture content), the same data as our wood species database.

More thread and fastener tools

• All thread and fastener charts
• UNC and UNF thread chart
• Metric thread pitch chart
• Tap drill chart (inch and metric)
• Clearance hole chart
• Nail size chart
• Wood screw size chart
• Lag screw pilot hole chart
• Bolt torque chart
• Wrench size chart
• Drill bit size chart
• Thread engagement chart
• Nail gun nail sizes by gauge
• Screw and nail size calculator
• Tap drill size calculator
• Clearance hole size calculator
• Tapped hole depth calculator

Last reviewed: 24 October 2026. How we build and check our data: methodology and corrections. Spotted an error? Please let us know.