How to Read Bolt Specifications

Bolt Notation Basics

Bolts are specified using a simple two-part notation that tells you everything you need to know about the bolt's thread:

What Each Number Means

Part 1: Diameter (The First Number)

This is the outer diameter of the bolt shaft in inches (or fractions of inches). Common sizes include:

• 1/4" — Light machinery, small structural connections
• 3/8" — Medium machinery, common structural bolts
• 1/2" — Heavy structural work, beams, major fastening
• 5/8" — Extra-heavy structural, bridges, critical applications

Important: This is the bolt's outer diameter, not the hole size you need. The hole is typically â…›" larger than the bolt diameter.

Part 2: Thread Pitch (The Number After the Dash)

This is the number of threads per inch (TPI). It tells you how tightly the threads are spaced:

• Coarse threads: 20, 18, 16 TPI (fewer, deeper threads)
• Fine threads: 28, 24 TPI (more, shallower threads)

Coarse threads are standard and easier to work with. Fine threads provide a tighter grip but are harder to install and remove.

Complete Bolt Specification Table

Why Thread Pitch Matters

Thread pitch affects how the bolt behaves:

Coarse Threads (Larger Number = Fewer Threads)

• Easier to assemble and remove
• Better for materials that are soft or tend to strip
• Faster to install/remove (fewer rotations needed)
• Most common choice for construction and machinery

Fine Threads (Smaller Number = More Threads)

• Tighter, more secure grip
• Better for vibration-prone applications (automotive)
• More turns needed to install/remove
• Can strip if over-tightened

Identifying a Bolt You Already Have

If you have a bolt and need to match it:

1. Measure the diameter with calipers or a ruler (should be in fractions)
2. Count the threads per inch: Look at the threads, count how many complete threads fit in one inch
3. Match to standard specs (1/4-20, 3/8-16, etc.)

Most bolts follow standard specifications, so you should find a match.

Bolt Length Specification

Bolts also need a length, which is specified separately. For example:

Always verify the length you need for your application — too short won't hold, too long wastes money.

Common Mistakes

• Confusing thread pitch with thread direction: Almost all bolts are right-handed (turn clockwise to tighten). Left-handed bolts are extremely rare.
• Using the wrong grade: Bolts come in different grades (strength ratings). Structural work requires Grade 5 or Grade 8 bolts, not standard hardware store bolts.
• Measuring diameter wrong: Measure the bolt shaft, not the thread peaks.
• Forgetting to account for bolt length: The length should accommodate the thickness of materials being bolted plus room for the nut and washer.

Quick Reference: When to Use Each Size

• 1/4" bolts: Light duty, brackets, machinery guards
• 3/8" bolts: General structural, most construction work
• 1/2" bolts: Heavy structural, beams, major connections
• 5/8" or larger: Bridge work, industrial applications, specialty structures