How to Select the Right Carriage Bolt for Your Project
By Mutual Screw & Supply | Published: 2026-10-21
Category: Buying Guides
Learn how to choose carriage bolts by size, thread, grade, and finish. Compare A307 Grade A zinc plated vs plain steel for your wood and metal projects.
Quick Answer
For most wood and metal connections, choose a carriage bolt with the right diameter and length for your material thickness, and pick a zinc plated finish for outdoor or damp conditions, or plain steel for indoor dry applications.
Why Carriage Bolt Size Matters
Carriage bolts are sized by diameter and length. The diameter determines the hole size needed and the load capacity, while the length must be long enough to pass through the materials and still allow for a nut and washer. A bolt that is too short won't engage properly, and one that is too long may protrude excessively. The square neck under the head pulls into the material to prevent turning during installation, so the hole must be sized to allow the square neck to seat properly.
Fully Threaded vs. Partially Threaded
Fully threaded carriage bolts provide grip along the entire shank, which can be useful when clamping thick materials or when you need threads for most of the connection. Partially threaded bolts offer a smooth shank that can reduce stress concentrations in shear applications, but they are less common for general projects. For most wood-to-wood and wood-to-metal connections, fully threaded bolts are easier to work with because you don't have to worry about thread engagement length.
Material and Finish Trade-offs
A307 Grade A is a common specification for carbon steel bolts used in general applications. Zinc plated steel offers corrosion resistance suitable for outdoor or humid environments, but it is not as corrosion-resistant as stainless steel. Plain steel is cheaper but will rust if exposed to moisture. For indoor dry projects, plain steel is fine; for outdoor or damp conditions, choose zinc plated. If you need maximum corrosion resistance, consider stainless steel instead.
Decision Rules for Choosing a Carriage Bolt
- Measure your material thickness: Add the thickness of all materials you are fastening, then add at least 1/2 inch for the nut and washer. Choose a bolt length that matches this total. Always measure twice and account for washers and nuts.
- Pick the diameter based on load: For light-duty projects like small furniture, 1/4-inch diameter may suffice. For medium to heavy loads, use 1/2-inch or 5/8-inch diameter bolts. Larger diameters provide higher shear strength. When in doubt, choose a larger diameter for safety.
- Decide on finish: zinc plated or plain steel: Use zinc plated for outdoor, humid, or corrosive environments. Use plain steel for indoor, dry applications where cost is a priority. If there is any chance of moisture, choose zinc plated.
- Check thread type and grade: A307 Grade A is suitable for general-purpose connections. Coarse threads (like 1/2-13 or 5/8-11) work well in wood and with nuts. For higher strength requirements, consider Grade 5 or stainless steel. Verify the grade matches your project's strength needs.
Carriage Bolt Options at Mutual Screw & Supply
- 1/2-13 X 1" Carriage Bolts | Fully Threaded | A307 Grade A Zinc Plated Steel (100 / Pkg.): 1/2-13 x 1" carriage bolts are fully threaded in A307 Grade A zinc plated steel. This size is ideal for medium-duty connections where a zinc finish adds corrosion protection for outdoor or damp environments.
1/2-13 X 1" Carriage Bolts | Fully Threaded | A307 Grade A Zinc Plated Steel (100 / Pkg.) - 5/8-11 X 2" Carriage Bolts | Fully Threaded | A307 Grade A Zinc Plated Steel (25 / Pkg.): 5/8-11 x 2" carriage bolts are fully threaded in A307 Grade A zinc plated steel. The larger diameter and longer length suit heavier wood-to-wood or wood-to-metal joints that need extra strength and reach.
5/8-11 X 2" Carriage Bolts | Fully Threaded | A307 Grade A Zinc Plated Steel (25 / Pkg.)
Who Should Use These Carriage Bolts?
Best for
- DIYers and woodworkers building furniture, decks, or play structures
- Contractors needing reliable wood-to-wood or wood-to-metal fasteners
Not for
- High-strength structural connections requiring Grade 5 or higher
- Applications demanding maximum corrosion resistance, where stainless steel is preferred
