Lag Bolt Capacity Calculator

About the Lag Bolt Capacity Calculator

Lag screws (lag bolts) are large-diameter screws used for heavy-duty wood-to-wood and wood-to-steel connections. They provide both shear resistance (perpendicular to the shank) and withdrawal resistance (along the shank axis). Common applications include deck ledgers, beam-to-post connections, and structural hardware mounting.

This calculator estimates the reference design capacities for lag screws in wood based on the NDS (National Design Specification) methodology. The capacity depends on the lag screw diameter, the depth of thread embedment in the main member, the wood species (specific gravity), and the direction of loading relative to grain.

Lag screw design values have been significantly updated in recent NDS editions. The yield-limit equations now govern, and design values may differ from older tables. Always verify critical connections with current NDS values or an engineer.

When This Page Helps

Knowing lag bolt capacity is essential for designing connections that meet code. It gives quick reference values to help you select the right diameter and quantity for your connection force.

How to Use the Inputs

1. Select the lag screw diameter.
2. Enter the thread embedment depth in the main member (wood).
3. Select the wood species group.
4. Read the estimated shear and withdrawal capacities per bolt.
5. Divide the total required force by the per-bolt capacity to find the number of bolts needed.

Example Calculation

Tips & Best Practices

• Always pre-drill a lead hole—the shank hole equals the shank diameter, the thread hole is about 60–75% of the shank diameter.
• Do not over-tighten lag screws—stop when the head is snug against the washer and wood surface.
• Use a washer under the lag screw head to distribute the bearing force and prevent wood crushing.
• Lag screws in end grain have 75% of the reference withdrawal value—avoid end-grain connections when possible.
• Minimum embedment depth is 4 times the shank diameter for withdrawal and 4×D for shear (NDS).
• Group A species (DF-L, Southern Pine) have higher design values than Group D species (SPF, cedars).

NDS Lag Screw Design Values

Lag screw shear (Z) values are determined by the yield-limit equations in NDS Chapter 12. These equations consider four possible failure modes: bearing in the side member, bearing in the main member, fastener bending at the shear plane, and a combination. The minimum value from all modes governs.

Reduction Factors

Several factors reduce lag screw capacity: end-grain factor (Ceg = 0.75 for withdrawal), group action factor (Cg) for rows of 4+ fasteners, geometry factor (CΔ) for edge/end distance, and wet-service factor (CM). All apply to the reference design value.

Lag Screws vs. Structural Screws

Modern structural screws (Simpson SDS, GRK RSS) are replacing traditional lag screws in many applications. They install faster (no pre-drilling in most softwoods), have published design values, and are recognized by the IRC for specific connections like deck ledgers.

Frequently Asked Questions

• What size lag bolt for a deck ledger?

  IRC Table R507.9.2 specifies 1/2″ diameter lag screws with a minimum 2-3/4″ embedment into the band joist, or 3/8″ through-bolts with nuts and washers. The number and spacing depend on the joist span.

• Do I need washers with lag screws?

  Yes. Washers are required to prevent the lag screw head from pulling into the wood. Use a flat washer sized for the bolt diameter (e.g., 3/4″ washer for a 3/8″ lag).

• Can I use lag screws in pressure-treated wood?

  Yes, but use hot-dipped galvanized or stainless steel lag screws. The ACQ and CA-B preservatives in modern pressure-treated wood corrode plain steel. Check the preservative label for fastener requirements.

• What is the difference between lag screws and bolts?

  Lag screws thread into wood—no nut is needed on the back side. Through-bolts pass completely through both members with a nut and washer on the back. Through-bolts generally have higher shear capacity but require access to both sides.

• How deep should the lag screw penetrate?

  Minimum thread penetration is 4D (4 times diameter) for shear and withdrawal. For a 1/2″ lag: 4 × 0.5 = 2″ minimum. For full design value, 7–10D penetration is ideal.

• Can I use impact drivers for lag screws?

  Impact drivers work but make it harder to control torque. Over-driving strips the hole and reduces capacity. Use a socket on a slow-speed impact or a ratchet wrench for best results.