A carriage bolt has a rounded head and a square neck directly beneath it. In wood, the neck bites into the material to resist turning while you tighten a nut from the other side. That makes carriage bolts useful for many wood-to-wood and wood-to-metal joints—but it does not make them interchangeable with ordinary bolts in engineered connections.
What is a carriage bolt?
A carriage bolt is a bolt with a smooth, rounded head and a square neck under the head. The head has no wrench flats, so installation and removal generally require access to the nut side. Inch-series round-head bolts, including round-head square-neck types, are covered dimensionally by ASME B18.5, listed as the 2012 edition reaffirmed in 2023. That standard does not, by itself, set every requirement for a bolt: thread dimensions, material, mechanical properties, and finishes rely on additional standards.
What is the square neck for?
As the nut is tightened, the square neck seats into the wood and resists the bolt turning. This lets an installer tighten the nut without gripping the smooth head. Portland Bolt describes the action this way: “The square neck sits directly under the rounded head and embeds into the wood as the nut is tightened.” The fit depends on the substrate and hole; do not assume the neck will lock reliably in every material or an oversized hole.
Where are carriage bolts used?
They are common in wood-to-wood and wood-to-metal joints when a rounded exposed head is wanted and the fastener can be installed with access to the far side. Examples include decks, fences, timber assemblies, playground and agricultural equipment, picnic tables, and general wood assemblies. These are examples of use, not a blanket structural approval for any particular joint.
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#1 Best Overall
- Diameter: 1/2" | Length: 6" | Head diameter: 1.062" | Nut diameter: 1/2" | Nut wrench size: 3/4" | Washer outside diameter: 1-3/8"
- Kit includes 25 Grade 307A carriage bolts, 25 USS thick washers, 25 Grade 2/A hex nuts
- Hot dip galvanized coating for superior corrosion resistance, Suitable for use in ACQ treaded lumber
- The flat head provides a semi-flush finish and is perfect for decks, gazebos, pergolas, and other heavy duty applications
- Meets or Exceeds: ASME B18.5, ASTM A307, ASME B18.2.2, ASTM A563, ASME B18.21.1, ASTM F2329
Can carriage bolts be designed under the NDS?
Yes, but not by simply applying the National Design Specification (NDS) tabulated values for ordinary hex or square bolts. The American Wood Council explains that carriage bolts do not conform to the ANSI/ASME B18.2.1 dimensions for those bolts, so those tabulated values do not apply to carriage-bolt dimensions. Its September 27, 2021 FAQ says carriage bolts may be used when their actual characteristics are accounted for in connection design and installation.
For an engineered wood connection, the design must address the bolt’s material, bending yield strength, diameter, and the potential effect of the square neck on lateral calculations. Installation instructions also matter. If the connection is structural or safety-critical, have the design checked by a qualified professional using applicable standards and verified product data rather than substituting a carriage bolt based on nominal diameter alone.
Rank #2
- Diameter: 1/2" | Length: 8" | Head diameter: 1.062" | Nut diameter: 1/2" | Nut wrench size: 3/4" | Washer outside diameter: 1-3/8"
- Kit includes 25 Grade 307A carriage bolts, 25 USS thick washers, 25 Grade 2/A hex nuts
- Hot dip galvanized coating for superior corrosion resistance, Suitable for use in ACQ treaded lumber
- The flat head provides a semi-flush finish and is perfect for decks, gazebos, pergolas, and other heavy duty applications
- Meets or Exceeds: ASME B18.5, ASTM A307, ASME B18.2.2, ASTM A563, ASME B18.21.1, ASTM F2329
What should you verify before buying or specifying one?
- Diameter and length: Confirm the required dimensions for the joint, including material thickness, washers, and nut engagement.
- Thread length: Do not assume every carriage bolt has the same thread coverage. Portland Bolt describes a common convention of 2 × diameter + 1/4 inch for bolts 6 inches and shorter, and 2 × diameter + 1/2 inch for bolts longer than 6 inches. It also notes that many imported stock bolts 6 inches or less are fully threaded. These are supplier observations, not universal rules; check the actual drawing or product specification.
- Material, grade, and strength: Match the required mechanical properties to the connection. Portland Bolt says its experience is that carriage bolts are most often supplied to ASTM A307, and it lists other grades; availability varies by supplier and product.
- Finish and exposure: For outdoor or exposed use, account for corrosion protection and compatibility with the surrounding materials. Hot-dip galvanizing is one available finish described by Portland Bolt. A Seattle City Light standard specifies a zinc-coated, high-strength carriage bolt measuring 7/16-inch nominal diameter by 8-1/2 inches long for a particular overhead-conductor application; that is a specialized utility specification, not a general-purpose recommendation.
- Nut compatibility: Use a nut with matching thread dimensions and suitable material and finish.
- Substrate and installation: Check hole size, material, access to both sides, and whether the square neck can seat and resist rotation as intended.
- Equipment-specific replacement: For machinery or equipment repairs, verify the manufacturer’s specified part or its full specification. Cat’s listing for part 1D-5166 identifies a 1/2-13 × 2-inch carriage bolt and notes that its fasteners are selected for application-specific size, strength, clamp load, and past performance.
How to compare a carriage bolt with another fastener
Choose by the connection’s requirements rather than head shape alone. Compare the exposed head’s appearance and protrusion, whether both sides are accessible, how the fastener resists rotation in the joined material, the substrate and hole geometry, the required strength and design method, and the material or finish needed for the environment. A smooth head can be difficult to grip, but that alone does not establish tamper resistance or security performance.
For a product listing, compare the exact diameter, length, thread, material or grade, finish, and intended joint. Supplier stock and conventions differ, so rely on the product drawing or specification for the bolt you will use.
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Best Value
- Strong Carriage Bolts: Carriage bolt is dome head bolt fastener with a locking neck style, which prevents turning when the nut it tightened or removed. Can be used in wood, metal applications
- Stainless Carriage Bolts: Our 304 stainless steel carriage bolts 1/4 are of excellent corrosion resistance and durability. These carriage bolts 1/4 x 1-1/4 are ideal for both indoor and outdoor use
- Well Made Carriage Bolts 1-1/4 Inch: All of the stainless steel bolts and hex nuts and flat washers are well manufactured with no burs, smooth-symmetrical head, precise and clean threads
- Complete Bolts Nuts Washers Kit: You will get 24pcs 1/4-20 x 1-1/4" carriage bolts, 24pcs 1/4-20 hex nuts, 24pcs 1/4" ID flat washers in one package. Standard size, the bolt conforms to ASME B 18.5
- Versatile: Carriage bolts aren't just for carriages, these bolts can be used on wood projects, timber frame shed build, trailer deck boards, deck project, outdoor bench seat rebuild, etc.
Rank #4
- Diameter: 3/8" | Length: 8" | Head diameter: 0.813" | Nut diameter: 3/8" | Nut wrench size: 9/16" | Washer outside diameter: 1"
- Kit includes 50 carriage bolts, 50 USS thick washers, 50 hex nuts
- Hot dip galvanized coating for superior corrosion resistance
- Grade 307A carriage bolts, grade 2 hex nuts, and USS flat washers
- The flat head provides a semi-flush finish and is perfect for decks, gazebos, pergolas, and other heavy duty applications
Rank #3
- Diameter: 3/8" | Length: 3-1/2" | Head diameter: 0.813" | Nut diameter: 3/8" | Nut wrench size: 9/16" | Washer outside diameter: 1"
- Kit includes 50 carriage bolts, 50 USS thick washers, 50 hex nuts
- Hot dip galvanized coating for superior corrosion resistance
- Grade 307A carriage bolts, grade 2 hex nuts, and USS flat washers
- The flat head provides a semi-flush finish and is perfect for decks, gazebos, pergolas, and other heavy duty applications
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.
