304 Stainless Steel Carriage Bolts M3 M4 M5 M6 — Round Head Square Neck, Multiple Lengths

A general rule is to select a bolt length equal to the total thickness of the materials being joined, plus enough thread engagement for the nut — typically an additional 1–2× the bolt diameter. For example, joining two 10mm (≈3/8") boards with M5 bolts would generally call for a length of 25–30mm (≈1"–1-3/16"). When in doubt, size up rather than down to ensure full thread engagement.

These bolts follow the metric coarse thread standard per DIN 603 / ISO 8677, which is the most widely used metric thread pitch in hardware and construction. M3, M4, M5, and M6 sizes are fully compatible with standard metric hex nuts and flat washers of the same designation — no specialty hardware required.

Yes. Made from 304 stainless steel (also known as A2 stainless), these bolts offer strong resistance to rust, oxidation, and general atmospheric corrosion, making them well-suited for outdoor furniture, garden structures, fencing, and light marine environments. For highly aggressive saltwater immersion or chemical exposure, 316 stainless steel is generally recommended instead.

304 stainless steel carriage bolts typically have a tensile strength of around 500–700 MPa, depending on the manufacturing process. They are suitable for general structural fastening in wood, light metal, and composite materials. For heavy structural or load-bearing applications — such as deck ledger boards or structural timber frames — consult a licensed engineer to confirm proper bolt sizing and spacing.

The square neck sits directly beneath the round dome head. When the bolt is driven into a pre-drilled hole in wood or a soft material, the square shoulder bites into the surrounding material and prevents the bolt from spinning as you tighten the nut from the other side. This means you only need one wrench — on the nut — making installation faster and easier, especially in tight spaces.

These carriage bolts are most commonly used with wood and wood composites, where the square neck can grip the material naturally. They can also be used with thin metal panels or plastic when a square-hole punch or appropriate insert is used to seat the neck. In metal-to-metal applications, the anti-rotation feature may not engage — in that case, a standard hex bolt with a wrench on both ends is generally a better choice.

These listings are for carriage bolts only — nuts and washers are not included. We recommend pairing with standard metric hex nuts and flat washers of the same size designation (M3, M4, M5, or M6) for a complete fastening assembly. These are widely available at any hardware store or online.

A quick reference for the lengths available in this listing:

• 8mm ≈ 5/16"
• 10mm ≈ 3/8"
• 12mm ≈ 15/32"
• 14mm ≈ 9/16"
• 16mm ≈ 5/8"
• 18mm ≈ 11/16"
• 20mm ≈ 25/32"
• 25mm ≈ 1"
• 30mm ≈ 1-3/16"
• 35mm ≈ 1-3/8"
• 40mm ≈ 1-9/16"
• 45mm ≈ 1-3/4"
• 50mm ≈ 2"
• 70mm ≈ 2-3/4"

To convert any mm value to inches, divide by 25.4.