Lag Bolt Size Chart: Diameter, Length & Pilot Hole Guide
You're bolting a ledger board to a house rim joist, or attaching a fence post to a 4×4 rail — and suddenly you're staring at a wall of hex-head lag screws in 10 different sizes with zero guidance. This chart covers every common structural application with the exact sizes, pilot hole specs, and material requirements so you don't make the kind of mistake that causes a deck to pull away from a house.
What Is a Lag Bolt (Lag Screw)?
A lag bolt — also called a lag screw — is a heavy-duty fastener with a hex (six-sided) head, a partially threaded shank, and wood-screw-style coarse threads with a pointed tip. Unlike machine bolts that require a through-hole and a nut, lag screws thread directly into wood. You drive them with a wrench or impact socket, not a screwdriver.
Lag bolt anatomy — hex head (driven with socket wrench), unthreaded shank section, coarse wood-screw threads, pointed self-starting tip. Diameter (Ø) measured at the shank.
The term "bolt" is technically a misnomer — lag screws don't use a nut. The name stuck historically, and you'll see both terms used interchangeably on hardware store shelves.
Lag Bolt Size Chart: Common Applications
The right lag bolt diameter is determined by the structural load. Length is determined by how deep you need to embed into the receiving member — the minimum penetration is 10× the diameter, but structural applications like ledger boards have specific code requirements.
| Application | Diameter | Length | Spacing / Pattern | Notes |
|---|---|---|---|---|
| Deck ledger to rim joist | 1/2" | 3" – 4" | 16" o.c., staggered 2 rows | IRC code minimum; HDG or SS required |
| Deck post to beam | 1/2" | 4" – 5" | 2 per connection | Through-bolt preferred over lag |
| Fence post to rail | 3/8" | 3" – 4" | 2 per connection | Pilot hole critical in cedar/redwood |
| Pergola/arbor beam to post | 1/2" | 4" – 6" | 2 per connection, offset | Use washer always; torque to snug |
| Framing: stud to sill plate | 1/4" – 5/16" | 2" – 3" | Per engineer spec | Often replaced by structural screws today |
| Outdoor furniture joints | 1/4" – 5/16" | 1-1/2" – 2-1/2" | 1 per joint | Countersink head for clean look |
| Shed wall to floor plate | 3/8" | 3" – 4" | 24" o.c. | Anchors to concrete anchor bolts below |
| TV wall mount into stud | 5/16" – 3/8" | 2-1/2" – 3" | Per mount spec | Must hit stud, not just drywall |
Pilot Hole Sizes: The Two-Step Method
Skipping the pilot hole on a lag bolt is how decks split and joints fail. You need two holes, not one:
- Clearance hole through the first board — drill the full shank diameter so the lag slides through freely without gripping. This prevents the first board from riding up on the threads.
- Lead (pilot) hole into the receiving member — drill 65–75% of the shank diameter. This gives the threads something to bite into while reducing splitting pressure.
Two-step pilot hole method: clearance hole (full diameter) through the first board, smaller lead hole (65–75% diameter) into the receiving member. This prevents splitting and pulls boards tight.
Pilot Hole Size Table by Bolt Diameter
| Lag Bolt Ø | Clearance Hole (Board 1) | Lead Hole Softwood | Lead Hole Hardwood |
|---|---|---|---|
| 1/4" (6.35mm) | 1/4" | 5/32" | 3/16" |
| 5/16" (7.94mm) | 5/16" | 3/16" | 7/32" |
| 3/8" (9.53mm) | 3/8" | 1/4" | 9/32" |
| 1/2" (12.7mm) | 1/2" | 5/16" | 3/8" |
| 5/8" (15.9mm) | 5/8" | 3/8" | 7/16" |
Deck Ledger Connections: The Code Requirement
The ledger-to-rim-joist connection is the most structurally critical lag bolt application in residential construction. The IRC (International Residential Code) specifies this connection because ledger failures are the leading cause of deck collapses.
The rules as of the 2021 IRC:
- Minimum diameter: 1/2" lag screws or 1/2" through-bolts
- Spacing: IRC Table R507.9.1.3(1) — for a 12-ft joist span, 16" maximum spacing
- Pattern: Staggered in two rows, 1-1/2" from top edge, 3/4" from bottom edge
- Embedment: Minimum 1-1/2" into the rim joist (2-1/2" preferred for 4" lags)
- Corrosion protection: Hot-dip galvanized (ASTM A153) or Type 304/316 stainless steel
Identify Your Fastener Now
Use our free identifier tool — pick the head type, drive type, enter measurements, get the designation in seconds.
Open Identifier →Lag Bolt Head Types and Drive Methods
Standard lag screws have a hex head driven with a socket wrench or impact driver with a socket. This is the only correct way to install structural lag bolts — never use a crescent wrench on large lags, which rounds the head and makes tightening and removal difficult.
Some manufacturers now offer structural screws (like GRK RSS, Simpson SDS, FastenMaster Headlok) that use a T25 or T40 star drive and can be driven flush with a standard bit. These are code-approved substitutes in many applications and dramatically faster to install. They are not traditional lag screws, but achieve similar or superior holding values.
Structural Screws vs. Lag Screws
| Feature | Traditional Lag Screw | Structural Screw (GRK/Simpson) |
|---|---|---|
| Drive method | Socket wrench / impact + socket | T25/T40 bit on impact driver |
| Pilot hole required? | Always — splits wood without | Usually not (self-tapping tip) |
| Withdrawal strength | Good — depends on embedment | Equal or superior for same size |
| Speed of installation | Slow (socket change, tapping) | Very fast (one-step, one tool) |
| Price per fastener | $0.30–$0.80 | $1.00–$2.50 |
| ICC code approval | Per NDS standard | ICC-ESR number on packaging |
For decks and high-visibility structural work, structural screws are often the professional choice. For occasional repairs or non-structural connections, traditional lag screws are perfectly fine.
Material and Coating: What Works Outdoors
Modern pressure-treated lumber uses ACQ (alkaline copper quaternary) or CA (copper azole) preservatives — both are highly corrosive to ordinary zinc-plated fasteners. Electroplated zinc coatings (the bright shiny look of standard hardware store lags) will corrode within 2–5 years in PT lumber outdoors.
For outdoor and PT lumber applications, use one of these:
- Hot-dip galvanized (HDG): ASTM A153, Class D — thick zinc coating applied in a molten bath. Dull gray appearance. The ICC minimum for most outdoor PT lumber contact.
- Type 304 stainless steel: Excellent for coastal environments. Higher cost. Required within a few miles of saltwater or where chloride exposure is high.
- Type 316 stainless steel: Marine-grade. For direct saltwater exposure (boat docks, ocean-front structures).
- G185 galvanized: Simpson Strong-Tie's designation for their 1.85 oz/ft² coating — meets ACQ contact requirement per their testing.
How to Install a Lag Bolt: Step by Step
- Mark position — lay out hole locations per your spacing plan. For ledgers, mark both rows offset (staggered) to avoid splitting the board along a single grain line.
- Drill clearance hole — through the first board (ledger, bracket, or connecting piece) at full shank diameter.
- Drill lead hole — into the receiving member at 65–75% of shank diameter, to the full thread engagement depth you want.
- Place washer — always use a flat washer under the head to distribute load and prevent head crushing.
- Drive the lag — start by hand (to confirm threading, not cross-threading), then drive with an impact driver in low-gear or a socket wrench. Stop when snug — over-torquing crushes wood fibers and reduces holding power.
- Check angle — lag should be perpendicular to the joint face for maximum withdrawal strength. Angled driving (toe-nailing style) reduces withdrawal capacity significantly.
Recommended Tool
TEKTON 3/8 in. Drive Socket Set
A 3/8" drive socket set with deep sockets in 1/2", 9/16", 3/4", and SAE/metric coverage handles every standard lag bolt head size. TEKTON sets include a ratchet, extension, and sockets in a tray — everything you need for structural lag work without overpaying.
View on Amazon →Frequently Asked Questions
- What size lag bolt for a deck ledger?
- IRC code requires 1/2" diameter lag screws (or through-bolts) for attaching a deck ledger board to a house rim joist. Spacing is typically 16" on center in a staggered two-row pattern. Use hot-dip galvanized or stainless steel for outdoor applications with pressure-treated lumber.
- What size pilot hole for a lag bolt?
- Two holes are required: (1) a clearance hole through the first board equal to the full shank diameter, and (2) a lead hole into the receiving member at 65–75% of the shank diameter. For a 1/2" lag: 1/2" clearance hole through the ledger, 5/16" lead hole into the rim joist.
- What is the difference between a lag bolt and a lag screw?
- They are the same fastener. "Lag bolt" and "lag screw" are interchangeable names — both refer to a hex-head fastener that threads directly into wood without a nut. "Bolt" is technically a misnomer, but the name is widely used by both consumers and pros.
- How long should a lag bolt be?
- Long enough to penetrate the receiving member by at least 10× the bolt diameter, with a practical minimum of 1-1/2" of thread embedment. For a 1/2" lag connecting a 1.5" ledger to a rim joist, a 3" lag is the minimum; 4" provides a full 2.5" embedment and is the field-preferred length.
- Do I need a washer with a lag bolt?
- Yes — always. Without a flat washer under the head, the hex head will crush wood fibers as you torque it down, reducing clamping force over time. Use SAE flat washers matched to the bolt diameter, and for outdoor/PT lumber use stainless or hot-dip galvanized washers.