Embedment — in | — load | safety factor —
Selection Summary
- Lag bolt—
- Embedment—
- Pilot hole—
- Estimated capacity—
- Status—
Lag Bolt Size and Pilot Hole Chart
| Diameter | Pilot (softwood) | Pilot (hardwood) |
|---|---|---|
| 1/4 in | 11/64 in | 3/16 in |
| 5/16 in | 3/16 in | 1/4 in |
| 3/8 in | 1/4 in | 5/16 in |
| 1/2 in | 5/16 in | 3/8 in |
| 5/8 in | 13/32 in | 7/16 in |
| 3/4 in | 1/2 in | 9/16 in |
Pilot the shank full diameter, the threaded part smaller; harder wood needs a larger thread pilot to avoid splitting.
Wood Species and Holding Power
| Species | Specific gravity | Holding |
|---|---|---|
| SPF / pine | ~0.42 | Lower |
| Douglas fir | ~0.50 | Medium |
| LVL | ~0.50 – 0.55 | Medium-high |
| Oak / maple | ~0.65 – 0.70 | High |
What Size Lag Bolt Do I Need?
Picking a lag bolt means matching the diameter and length to the load, the base material, and how deep the threads can bite. Bigger diameter and deeper thread embedment both raise holding power, and denser wood holds far more than soft framing lumber. This calculator takes the material, diameter, length, embedment, and load direction, then estimates a capacity, recommends a pilot hole, and flags that you should confirm against code tables, because lag bolts are easy to under-size and hard to inspect once they are buried in wood.
Pilot Holes for Lag Bolts
Lag bolts need two-part pilot holes: a clearance hole the full shank diameter for the smooth upper portion, and a smaller lead hole for the threads, roughly 60 to 75 percent of the shank in softwood and a little larger in hardwood. Skipping the pilot, or drilling it too small, splits the wood and can shear the bolt while driving. Always lubricate the threads with wax or soap, never overdrive, and let the threads pull the parts together rather than the head crushing the wood.
Shear vs Withdrawal Loading
Lag bolts are happiest in shear, loaded sideways like a ledger board bolted to a rim joist. Withdrawal, a straight pull along the bolt, is weaker and depends almost entirely on thread embedment and wood density, so it is the wrong way to hang a heavy load if you can avoid it. For ceilings, overhead racks, or anything where failure means something falls, prefer a through-bolt with a backing plate, or a rated structural connector, over a lag in withdrawal.
Frequently Asked Questions
What size lag bolt do I need? Size it to the load and material; common structural lags are 3/8 to 1/2 inch with several inches of thread embedment.
Do I need a pilot hole? Yes, always, sized to the shank for the clearance hole and smaller for the threads.
How deep should a lag bolt go? As much thread embedment as possible; capacity grows with embedment up to the bolt length.
Shear or withdrawal? Lags are strong in shear and weaker in withdrawal; avoid hanging heavy loads in pure withdrawal.