A lag screw goes into two holes, not one. The top piece gets a clearance hole the full size of the shank, so the lag pulls the joint together instead of jacking it apart. The piece underneath gets a smaller pilot (lead) hole for the threads to bite into. How big that pilot is depends on the wood: softwoods (pine, fir, cedar) take a smaller hole, hardwoods (oak, maple, hickory) take a slightly larger one so you’re not fighting the screw or splitting the grain.
Sized to the shank so the lag slides through and clamps down. Equal to the lag’s nominal diameter.
Smaller hole for the threads to grip. Smaller in softwood, a touch larger in hardwood.
| Lag size | Clearance hole (shank) | Pilot — softwood | Pilot — hardwood |
|---|---|---|---|
| 1/4″ | 1/4″ | 9/64″ | 11/64″ |
| 5/16″ | 5/16″ | 13/64″ | 15/64″ |
| 3/8″ | 3/8″ | 15/64″ | 17/64″ |
| 7/16″ | 7/16″ | 19/64″ | 21/64″ |
| 1/2″ | 1/2″ | 11/32″ | 3/8″ |
| 5/8″ | 5/8″ | 7/16″ | 15/32″ |
| 3/4″ | 3/4″ | 35/64″ | 37/64″ |
| 7/8″ | 7/8″ | 21/32″ | 11/16″ |
| 1″ | 1″ | 3/4″ | 25/32″ |
Always test in scrap first. Wood density swings a lot — dense oak isn’t soft poplar, and pressure-treated pine drills differently when wet. Bore a test hole in an offcut of the same material and check that the lag draws tight without crushing or splitting before you commit to the real piece.
Lubricate the threads. A little wax or soap on the lag cuts the driving torque and saves the head — especially in hardwood and in the larger diameters.
Don’t over-drill. Too large a pilot and the threads have nothing to hold; the lag spins without drawing tight. When in doubt, start on the smaller side and step up only if it’s fighting you.
Drive it, don’t hammer it. Lag screws are turned in with a wrench or socket, never driven like a nail — pounding splits the wood and ruins the threads.
