This chart gives the standard dimensional ranges for inch-series carriage bolts. Every value is a minimum to maximum range, because real bolts vary within tolerance.

How to read the chart

• Nominal size is the shank and thread size, not the head size.
• Head diameter, head height, square width and square depth are separate dimensions.
• Dimensions are for ASME B18.5 (1990, R1998 as reproduced by Portland Bolt), which covers diameters up to 1 in.; larger bolts use custom heads.
• 1 in. = 25.4 mm, so a 1/2 in. bolt is 12.7 mm.

Symbols follow the Portland Bolt table (C head diameter, P square width, F square depth). ASME B18.5 itself uses A, O and P, so P means width here and depth in the ASME table. These are dimensional ranges, not load capacities.

A carriage bolt has a smooth round head, a square neck directly beneath it, a cylindrical shank and a threaded end. It is tightened from the nut side.

As the nut tightens, the square neck embeds into the wood and keeps the bolt from turning, so no wrench is needed on the head.

Because the head has no wrench flats, the bolt is common in wood-to-wood and wood-to-metal connections. For other bolt types see the Hex Bolt Size Chart.

Define these before reading the charts. The letters match the Portland Bolt table; the thread pitch is written out as pitch to avoid confusion with square width P.

For general bolt diameters across bolt types, see the Bolt Diameter Chart and the U.S. Bolt Sizes Chart.

Nominal diameter is a label. The actual shank falls between a maximum and a minimum.

Nominal diameter does not equal the exact maximum or minimum manufactured body diameter. Body diameters for 1/2 to 1 in. match the Portland Bolt table; 1/4 to 7/16 in. match published ASME B18.5 reproductions.

Common inch carriage bolts use the Unified National Coarse (UNC) series defined by ASME B1.1.

For other thread series see the UNC Thread Chart and the Thread Size Chart, or use the Thread Calculator.

A carriage bolt call-out looks like 1/2-13 x 4.

• 1/2 is the nominal diameter in inches.
• 13 is threads per inch.
• 4 is the nominal length in inches, measured under the head.

If UNC is stated it reads 1/2-13 UNC x 4. The 13 is not a grade, a length or a wrench size.

Material, grade and finish are separate. A purchasing description may add the grade (for example ASTM A307 Grade A), a finish (hot-dip galvanized) and the nut requirement.

The round head is much wider than the shank, which spreads the bearing load on the wood.

• 1/4 in. bolt: head diameter 0.563 to 0.594 in.
• 3/8 in. bolt: 0.782 to 0.844 in.
• 1/2 in. bolt: 1.032 to 1.094 in.
• 3/4 in. bolt: 1.469 to 1.594 in.

Allow clearance around the head if it sits inside a recess or against a fixture. Do not drill for the head diameter; the head sits on the surface.

Square width is roughly the shank size. Square depth is how far the neck extends under the head. Each has its own tolerance.

• 1/4 in.: width 0.245 to 0.260 in.
• 3/8 in.: 0.368 to 0.388 in.
• 1/2 in.: 0.492 to 0.515 in.
• 3/4 in.: 0.741 to 0.768 in.

Square width versus square depth

Width is the size across the square. Depth is the axial thickness of the neck. Both matter in thin plate, brackets and assemblies where the neck affects seating.

The neck is designed to bite into suitable material such as wood so the bolt does not spin.

A conventional carriage bolt is measured from the bearing surface under the head to the end of the bolt. The domed head is not counted.

A 4 in. bolt therefore has 4 in. below the head. Countersunk fasteners are often measured to the top of the head, so do not apply this rule to them.

There is no universal stock-length table. Available lengths depend on diameter, material, grade, manufacturer, finish and thread style.

Example only: these are galvanized lengths Portland Bolt lists as stock; it lists stock up to 18 in. and makes other sizes to order. Do not assume every diameter is made in every length.

• Grip thickness: measure the connected members, not their nominal size.
• Nut and washer allowance: include every washer.
• Thread projection: leave thread visible past the nut.

Portland Bolt notes that many imported stock carriage bolts have a 6 in. thread and are fully threaded when 6 in. or shorter.

• Fully threaded: thread runs the whole length, common in short imported stock.
• Partially threaded: an unthreaded shank sits under the head and the thread is at the end.

The connection thickness must fit the thread arrangement. Do not assume a short bolt is always fully threaded, and confirm the thread length of the exact product.

The hole is for the round shank. Choose the hole by the type of connection.

Calculated example of +1/32 to +1/16 in. clearance, not a universal carriage-bolt hole requirement.

Ordinary non-structural wood assemblies often use a close-fitting hole so the square neck seats. For drill sizes see the Drill Bit Size Chart, the Bolt Hole Size Chart and the Clearance Hole Size Chart.

The drilled hole is generally intended for the round shank. The square neck then seats into suitable material to resist rotation.

• Do not drill a square hole in ordinary wood.
• Do not drill for the head diameter.
• In steel, hole geometry and neck engagement must match the actual connection detail.

Zinc, Galvanized and Stainless Carriage Bolts

Finish and corrosion exposure.

Finish decides how long a bolt lasts outdoors.

Carriage bolt finishes compared

Finish	Typical use	Notes
Plain steel	Indoor dry	No corrosion protection
Zinc-plated	Mostly indoor or dry	Thin coating
Hot-dip galvanized	Exterior and treated lumber	Thick zinc coating per ASTM A153
Stainless steel	Wet or corrosive	Match nuts and washers

🔗 Embed This Table

• Ordinary zinc plating and hot-dip galvanizing are not equivalent. ASTM A153 Class C, for fasteners over 3/8 in. diameter, requires a minimum average zinc coating of about 2.1 mils.
• The 2021 IRC (R317.3) requires fasteners, nuts and washers in contact with preservative-treated wood to be hot-dipped zinc-coated galvanized steel, stainless steel, silicon bronze or copper.
• Use matching bolts, nuts and washers so finishes are compatible.

Carriage Bolts for Wood

Square-neck seating in a typical joint.

Carriage bolts are common in wood-to-wood and wood-to-metal connections, timber framing, fences, playground and agricultural equipment, picnic tables and furniture.

• The square neck seats in the wood to stop rotation.
• A washer goes under the nut.
• Leave a little thread past the nut.

Figure 4. Generic carriage-bolt assembly through two wood members.

This is not a structural connection table.

Carriage Bolts for Metal

Neck engagement in steel.

The square neck helps most when it seats in a matching feature. In thin plate that could be a square-punched hole or a deformable bearing surface.

It will not automatically lock in a round metal hole. Follow the manufacturer or project-specific detail.

Carriage Bolts for Deck Construction

Cautious guidance.

Carriage bolts appear in some deck and fence connections, but structural deck details may specify bolts, structural screws, lag screws or other fasteners.

• Use hot-dip galvanized or stainless hardware with treated lumber.
• Use washers and nuts compatible with the bolt.
• Follow the connection design and the local building code.

Do not assume carriage bolts are right for all structural deck connections.

Carriage Bolt Spacing and Edge Distance

Why there is no generic table.

Wood connections need spacing, end distance and edge distance. The AWC defines these for loaded and unloaded edges and rows of fasteners.

• Bolt spacing: distance between bolts.
• End distance: from the bolt to the end of the member.
• Edge distance: from the bolt to the side of the member.

Required values depend on bolt diameter, load direction, grain direction, member geometry and connection design. There is no table that says a 1/2 in. bolt must always be a fixed distance apart.

How Much Weight Can a Carriage Bolt Hold?

Why diameter alone cannot say.

There is no valid diameter-to-load chart. Capacity depends on:

• Material and grade, diameter and tensile area.
• Threaded or unthreaded shear plane and number of shear planes.
• Wood species or steel plate thickness.
• Edge distance, end distance and spacing.
• Load direction, washers and bearing, and connection failure mode.

ASTM A307 Grade A requires 60 ksi minimum tensile strength, which is a material property and not the allowable load of a connection.

Carriage Bolt vs. Hex Bolt and Lag Screw

Three comparisons.

Carriage bolt vs. hex bolt

Carriage bolt versus hex bolt

Feature	Carriage bolt	Hex bolt
Head	Smooth round	Hexagonal
Anti-rotation	Square neck	Wrench on head
Typical dimensional standard	ASME B18.5	ASME B18.2.1
Head access	Usually not needed for tightening	Usually required
Common applications	Wood and wood-to-metal	Broad mechanical and structural

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Neither is universally stronger; strength depends on the material specification and grade. See the Hex Bolt Size Chart.

Carriage bolt vs. lag screw

A carriage bolt passes through the assembly and uses a nut with a machine thread. A lag screw threads directly into the material and normally needs no nut. Their installation and design requirements differ. See the Lag Bolt Sizes Chart.

Carriage bolt vs. elevator bolt

An elevator bolt has a flat countersunk head instead of the round domed carriage head. Dimensional details differ.

Do not confuse carriage bolts with concrete anchors; see the Anchor Bolt Size Chart.

Common Carriage Bolt Sizing Mistakes

Errors that cost a trip back to the store.

📏

Confusing nominal diameter with head diameter

The head is much wider.

🔢

Reading 1/2-13 as a length

13 is threads per inch.

🔄

Confusing TPI with grade

TPI is not strength.

📑

Including the dome in bolt length

Measure from under the head.

📐

Measuring only the threaded length

Use L and T separately.

🔧

Ignoring square-neck dimensions

Check width and depth.

💧

Drilling for the head diameter

Drill for the shank.

▭

Drilling a square hole

The neck seats by itself.

🔩

Assuming the neck fits every metal hole

Match the detail.

📥

Assuming all carriage bolts are fully threaded

Check the product.

📏

Ignoring thread length

Grip must fit the unthreaded shank.

🧱

Choosing length without nut and washer

Add them.

📖

Assuming all are ASTM A307

Check the grade.

📑

Treating A307 as a dimensional standard

B18.5 sets dimensions.

📝

Treating B18.5 as a strength grade

It is dimensions only.

🏗️

Using zinc plating where HDG is needed

Finish durability differs.

🔀

Mixing incompatible finishes

Match bolt, nut and washer.

⚖️

Using a load rating from diameter alone

Capacity depends on the connection.

📐

Ignoring edge and end distances

Structural wood needs them.

📦

Assuming every diameter comes in every length

Check availability.

More references are in the Construction Charts hub.

Carriage Bolt Size FAQs

Short answers to common carriage bolt questions.

What are standard carriage bolt sizes?

What does 1/4-20 mean on a carriage bolt?

What thread is a 3/8-inch carriage bolt?

What thread is a 1/2-inch carriage bolt?

What is the head diameter of a 1/2-inch carriage bolt?

What size is the square neck on a 1/2-inch carriage bolt?

How is carriage bolt length measured?

Does carriage bolt length include the head?

What size hole should I drill for a carriage bolt?

Should a carriage bolt hole be tight?

Does the square neck go into the wood?

What nut fits a 3/8-16 carriage bolt?

What washer fits a 1/2-inch carriage bolt?

How long should a carriage bolt be?

How much thread should extend past the nut?

Are carriage bolts fully threaded?

What is the standard thread length?

What grade are carriage bolts?

What is an ASTM A307 carriage bolt?

Are galvanized carriage bolts suitable outdoors?

Can carriage bolts be used with pressure-treated lumber?

Are stainless carriage bolts stronger than steel carriage bolts?

Can carriage bolts be used in metal?

Can carriage bolts be used on decks?

How much weight can a carriage bolt hold?

What is the difference between a carriage bolt and hex bolt?

What is the difference between a carriage bolt and lag screw?

Related Charts and Calculators

Continue with these ConcreteCalculate references.

📋Bolt Diameter Chart📋U.S. Bolt Sizes Chart📋Hex Bolt Size Chart📋Lag Bolt Sizes Chart📋Anchor Bolt Size Chart📋Nut Size Chart📋Washer Size Chart📋Drill Bit Size Chart📋Bolt Hole Size Chart📋Clearance Hole Size Chart📋UNC Thread Chart📋Thread Size Chart📋Bolt Grade Chart📋Socket Size Chart📋All Construction Charts🧮Thread Calculator🧮Bolt Torque Calculator

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ASME B18.5 head and neck rangesUNC thread and pitchThread length and hole examplesWorked length examples

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