The pilot hole required depends on the density of wood being used. For a 1/4-inch lag bolt attached to softwood, the appropriate pilot hole size is 3/32 inch. Medium-density wood requires a 5/32-inch hole, and hardwood requires a 3/16-inch hole.
Lag bolts are one of the few bolts that typically use a pilot hole during the installation process. Other bolts use a hole and nut to fasten two or more pieces of wood together. For these bolts, the correct size of the hole is 1/16 inch larger than the diameter of the bolt. For example, a 1/4-inch bolt needs a 5/16-inch hole.