Lag Screw Pilot Hole Calculator
Enter the lag screw size, wood specific gravity, screw length, shank length, and loading direction. The calculator returns the NDS threaded lead-hole range, common fractional bits inside that range, and the two drilling depths.
Common fractional bits within range15/64 in (0.234 in) to 9/32 in (0.281 in)
- NDS threaded lead-hole range
- 0.225 in to 0.281 in (0.6D to 0.75D, 0.50 less than G up to 0.60)
- Shank clearance bit
- 3/8 in (0.375 in)
- Small-bit total drilling depth
- 3.5
- Shank clearance depth
- 1.5
- Threaded-section length
- 2
- Preboring requirement
- Prebore the threaded lead hole for this diameter, density, or loading condition.
- Drilling sequence
- Drill the threaded lead hole within 15/64 in (0.234 in) to 9/32 in (0.281 in) to 3.5 in, then enlarge the entry 1.5 in to 3/8 in (0.375 in) for the smooth shank.
Depth values are inches measured from the entry surface.
This assumes a conventional lag screw driven into wood; proprietary structural screws require the manufacturer installation instructions.
How to use this calculator
- Choose the nominal lag screw diameter marked for the fastener.
- Enter the wood specific gravity on the NDS oven-dry and 12% moisture-volume basis.
- Measure the screw length under the head and the smooth unthreaded shank length.
- Choose the primary loading direction before reading the bit range and drilling sequence.
How the pilot hole is sized
Lag screws need a smaller lead hole through the threaded portion and, when the screw has a smooth shank, a larger clearance hole through the upper member. The threaded hole lets the threads cut and hold without forcing the core of the screw to wedge the wood apart. The clearance hole lets the smooth shank pass without jacking the outer piece away from the receiving member.
The NDS procedure uses wood specific gravity instead of a broad hardwood or softwood label. Let D be the nominal lag screw shank diameter and G be the wood specific gravity. The permitted threaded lead-hole interval is:
0.65D to 0.85DwhenG > 0.60.0.60D to 0.75Dwhen0.50 < G <= 0.60.0.40D to 0.70DwhenG <= 0.50.
The calculator multiplies those factors by the selected diameter, then rounds inward to common 1/64-inch fractional bits: ceil(64 x dMin) / 64 for the smallest displayed bit and floor(64 x dMax) / 64 for the largest. Rounding inward matters because a bit just outside the range is no longer an NDS lead-hole size. If no 1/64-inch bit fits wholly inside the decimal interval, use a numbered, lettered, or metric bit that does.
Depths and the two-bit sequence
The small threaded lead hole is drilled to the screw length measured under the head. The shank clearance hole is then drilled only as deep as the smooth shank length, using a bit equal to the nominal shank diameter. The threaded section length is simply L - S, where L is screw length and S is unthreaded shank length. If S is zero, the calculator omits the separate clearance-hole step.
What changes the result
Diameter controls the absolute hole sizes. Specific gravity controls the percentage band: denser stock uses a larger lead hole because splitting and installation torque increase. Loading direction affects only the narrow no-prebore exception, not the computed NDS diameter range.
What this calculator leaves out
This is installation guidance, not a connection-capacity design. It does not check withdrawal capacity, lateral capacity, edge distance, end distance, fastener spacing, moisture effects, treatment chemistry, or local code requirements. Proprietary structural screws, engineered fasteners, lag shields, masonry anchors, metal anchors, and plastic anchors can have different manufacturer instructions that supersede this conventional lag screw guidance.
Worked example
Suppose a 3/8 in conventional lag screw is going into wood with specific gravity 0.55. That falls in the middle NDS band, so the threaded lead-hole range is 0.60 x 0.375 = 0.225 in through 0.75 x 0.375 = 0.28125 in. Rounding inward to 1/64-inch bits gives 15/64 in through 9/32 in.
For a 3.5 in screw with a 1.5 in unthreaded shank, drill the smaller lead hole to 3.5 in total depth. Then enlarge the entry 1.5 in to 3/8 in for the smooth shank. The threaded section length is 3.5 - 1.5 = 2.0 in.
Common questions
Why use specific gravity instead of hardwood versus softwood?
Specific gravity tracks wood density more directly than a species label. Some softwoods are dense and some hardwoods are relatively light, so the NDS bands use specific gravity to choose the lead-hole percentage range.
Why are two different hole diameters needed?
The threaded part needs a smaller lead hole so the threads can bite. The smooth shank needs a clearance hole about equal to the nominal shank diameter so it can pass through the upper piece without splitting it or holding it off the joint.
Which bit should I choose within the NDS range?
Choose a bit inside the displayed range, then test in representative scrap when appearance or splitting risk matters. Denser, drier, or more brittle stock often benefits from a bit near the larger end of the range.
When does the NDS allow a lag screw without a lead hole?
The exception is narrow: diameter 3/8 in or smaller, specific gravity 0.50 or lower, and loading primarily in withdrawal. Adequate edge distance, end distance, and spacing still have to prevent unusual splitting.
Does this apply to structural screws or lag shields?
No. Proprietary structural screws, masonry lag shields, concrete anchors, metal anchors, and plastic anchors have separate installation instructions. Use the manufacturer instructions for those products.
Should a lag screw be hammered or turned with a wrench?
Turn a lag screw with a wrench, ratchet, or driver suited to the head. Hammering damages threads, crushes fibers unpredictably, and is not the installation method assumed by lag screw design guidance.