Lag screw pilot hole chart
A lag screw needs two lead holes: one the full size of the smooth shank, as deep as the shank goes, and a smaller one for the threads. The USDA Wood Handbook sizes the thread hole as a share of the shank diameter that rises with the wood’s density, the smaller share for the smaller lag screws. The chart works that rule out for 1/4 to 1/2 inch lags.
One source: USDA Forest Products Laboratory, the federal wood research laboratory
| Lag screw | Shank hole | Soft, light woods (40%–70%) | Douglas-fir, Southern Pine (60%–75%) | Dense hardwoods (65%–85%) |
|---|---|---|---|---|
| 1/4 in | 1/4 in | 7/64 to 11/64 in0.1–0.175 in | 5/32 to 3/16 in0.15–0.188 in | 11/64 to 13/64 in0.163–0.213 in |
| 5/16 in | 5/16 in | 1/8 to 7/32 in0.125–0.219 in | 3/16 to 15/64 in0.188–0.234 in | 13/64 to 17/64 in0.203–0.266 in |
| 3/8 in | 3/8 in | 5/32 to 1/4 in0.15–0.262 in | 15/64 to 9/32 in0.225–0.281 in | 1/4 to 5/16 in0.244–0.319 in |
| 7/16 in | 7/16 in | 3/16 to 19/64 in0.175–0.306 in | 17/64 to 21/64 in0.263–0.328 in | 19/64 to 23/64 in0.284–0.372 in |
| 1/2 in | 1/2 in | 13/64 to 11/32 in0.2–0.35 in | 5/16 to 3/8 in0.3–0.375 in | 21/64 to 27/64 in0.325–0.425 in |
Which column is your wood?
- Soft, light woods: the Handbook’s examples are low-density softwoods such as the cedars and white pines, 40% to 70% of the shank.
- Douglas-fir, Southern Pine: the Handbook’s examples are Douglas-fir and Southern Pine, 60% to 75% of the shank.
- Dense hardwoods: the Handbook’s examples are dense hardwoods such as the oaks, 65% to 85% of the shank.
Within each range, use the smaller end for the smaller lag screws and the larger end for the bigger ones. The Handbook also advises soap or a similar lubricant on the threads, and a slightly bigger lead hole for long lags. The drill sizes listed are the fractional drills (1/64 in steps) that fall inside the range.
This is a guide to drilling, not a load rating: for a structural connection, the National Design Specification and an engineer set the lag size and spacing. Wood screws are on the pilot hole chart.