Lag Bolt Pilot Hole Size Guide (Charts for Softwood, Hardwood & More)
Yes—lag bolts need a pilot hole. For a quick rule of thumb in typical studs: 3/8″ lag → 15/64″ pilot, 1/2″ lag → 5/16″ pilot. For best results, match the pilot to your wood group (soft/medium/hard) using the charts below.
Shop Lag Screws and install with the correct pilot to avoid splitting and maximize holding power.
Quick Pilot Hole Chart (Most Situations)
Before using a lag bolt in a wood stud, pre-drill a pilot hole to prevent splitting and avoid snapping the bolt head.
| Lag Bolt Size | Pilot Hole Size |
|---|---|
| 1/4" | 5/32" |
| 5/16" | 3/16" |
| 3/8" | 15/64" |
| 1/2" | 5/16" |
| 5/8" | 13/32" |
| 3/4" | 1/2" |
Pilot Hole Sizes by Wood Group
Pilot sizes vary with wood type, bolt length, and diameter. Wood groups (ALSC) commonly map as: Soft Wood (Groups 3–4), Medium Wood (Group 2), Hard Wood (Group 1). Many newer U.S. homes use studs of spruce (Group 3–4) or Douglas fir (Group 2–3).
| Lag Bolt Size | Pilot Hole Size | ||
|---|---|---|---|
| Soft Wood* | Medium Wood* | Hard Wood* | |
| 1/4" | 3/32" | 5/32" | 3/16" |
| 5/16" | 9/64" | 3/16" | 13/64" |
| 3/8" | 11/64" | 15/64" | 1/4" |
| 1/2" | 15/64" | 5/16" | 11/32" |
| 5/8" | 5/16" | 13/32" | 29/64" |
| 3/4" | 13/32" | 1/2" | 9/16" |
* Soft Wood (Group 3–4), Medium Wood (Group 2), Hard Wood (Group 1). Consult an engineer for specific applications.
Installation Notes & Tips
- Depth: Drill the pilot to at least the intended threaded embedment; clear chips.
- Location: Center in the stud’s middle third; avoid end grain for maximum holding power.
- Driving: Use an impact driver or wrench, then finish by hand to avoid over-torquing and head snap.
- Exterior: Choose hot-dipped galvanized or stainless lag screws outdoors. Consider a washer to spread load.
Shop Lag Screws
Need help choosing? Call (850) 764-2658 or email support@MonsterBolts.com — we’re happy to help.
Frequently Asked Questions
Do lag bolts need pilot holes?
Yes. Pilots prevent splitting and allow the threads to bite correctly, especially in dense or dry lumber.
What pilot for a 3/8″ lag bolt?
Most situations: 15/64″. For hard woods, see the wood-group chart above.
What pilot for a 1/2″ lag bolt?
Most situations: 5/16″. Adjust for soft/medium/hard woods per the chart.
Can I drive lag bolts directly into studs?
Not without a pilot. Pre-drill using the chart that matches your bolt size and wood group.
4 comments
Bob Forrest
Is 23 year old Doug fir on my walls considered hard wood because it is so dry and aged? Re lag bolts for a 20’ awning weighs 220 lbs ? Going on outside of house wall. There are six steel brackets w two lag bolts each
Daniel Collins
Thank you
Matt Szczepkowski
Great assortment
Lynn C.
Today I need to install a couple of Lag Bolts for my garage door motor. (Of course I’m in a hurry but need to be safe too.) Thank you for providing such an easy to access resource!
I’ll be back to look further into your site.
Thanks again and
Have a Great Day!
Lynn C.