Carriage Bolts 304 Stainless Steel — M4 M5 M6 M8 M10 M12 Cup Square Dome Head

Measure the total thickness of all materials being joined, then add typically 5–10mm (about 3/16"–3/8") for the nut and washer stack. For example, joining two 20mm (≈3/4") boards requires a bolt of at least 45mm (≈1-3/4"). When in doubt, size up — a slightly longer bolt is easier to work with than one that's too short. Use the variant selector above to pick your exact length in mm; inch equivalents are listed in the specs table.

For light-duty wood joinery and furniture, M4 or M5 is generally sufficient. M6 and M8 are the most common choices for general construction, decking, and framing. M10 and M12 are typically used for heavy structural connections, timber framing, and load-bearing applications. If you're unsure, M8 is a solid all-around starting point for most outdoor deck and fence projects.

These bolts are made from 304 (A2) stainless steel, which generally provides a tensile strength of around 500–700 MPa depending on diameter and manufacturing tolerances. They are suitable for most structural wood-to-wood and wood-to-metal connections. For critical load-bearing applications such as structural beams or safety-rated equipment, consult an engineer for exact load ratings specific to your configuration.

Yes — 304 stainless steel offers excellent resistance to rust and atmospheric corrosion, making these bolts well-suited for outdoor decking, fencing, pergolas, and garden structures. For highly aggressive marine environments with constant saltwater spray or immersion, 316 stainless steel is generally recommended instead. In most coastal and outdoor residential applications, 304 performs reliably for many years.

These bolts use standard ISO metric coarse threads (DIN 603 / ISO 8677 equivalent). Each diameter follows standard metric pitch — M6 is typically 1.0mm pitch, M8 is typically 1.25mm pitch, M10 is typically 1.5mm pitch, and M12 is typically 1.75mm pitch. They are fully compatible with standard metric hex nuts, flange nuts, and flat washers available at any hardware store.

Using a flat washer under the nut is generally recommended. It distributes the clamping load over a larger surface area, reduces the risk of the nut pulling through softer materials like wood, and helps prevent surface damage when tightening. Use a stainless steel washer to match the bolt material and avoid galvanic corrosion. Washers are sold separately.

Drill a clearance hole matching the bolt's thread diameter through all materials to be joined. Insert the bolt from the front (dome head side). Tap the head firmly with a mallet so the square neck section embeds into the wood — this is what prevents the bolt from spinning during tightening. Place a flat washer and hex nut on the threaded end and tighten with a wrench. No tool is needed on the head side once the square neck is seated.