Lag Bolt Calculator

Find the right lag bolt size for your shear load, with pilot hole diameter, embedment depth, and a full comparison of every viable diameter for your wood species.

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RECOMMENDED LAG BOLT
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diameter × length

Usage Tip

Always pre-drill at 85% of bolt diameter — lag bolts split wood when driven without pilot holes. Use hot-dipped galvanized or stainless outdoors.

THE MATH
Bolts = ⌈ (Load × SF) ÷ Capacity ⌉
Capacity varies by wood species and bolt diameter
• Pre-drill the pilot hole to 85% of shank to prevent splitting
• Embedment should be at least 8× the bolt diameter for full capacity
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The calculators and tools on Formula Factory are provided for general guidance and informational purposes only. Results are estimates based on standard formulas and the values you enter — they do not constitute professional engineering, electrical, or architectural advice. Always verify calculations with a qualified professional before making decisions for any safety-critical, code-compliance, or commercial application. Formula Factory makes no representations or warranties as to the accuracy or completeness of any result, and accepts no liability for errors, omissions, or any outcomes arising from reliance on this information.