Released On 26th May 2026

Complete Bolt Size Guide

Selecting the correct bolt size is essential for creating strong, safe and reliable fixings. Whether you are working on construction projects, machinery, automotive repairs or general maintenance, understanding bolt sizing helps ensure the correct fit and performance.

This guide explains:

• Metric bolt sizes
• Common bolt dimensions
• Bolt head types
• Metric vs imperial bolts
• How to measure bolts
• Choosing the right bolt size

How bolt sizing works

Metric bolts are generally shown using:

M8 × 50

Meaning:

• M = Metric thread
• 8 = Diameter in millimetres
• 50 = Bolt length in millimetres

Example:

M10 × 100

• Thread diameter: 10mm
• Length: 100mm

Common Metric Bolt Sizes

Factors when selecting bolt size

Load requirements
Higher loads usually require larger diameter bolts.

Material thickness
Bolt length should provide adequate thread engagement.

Environmental condition
Outdoor use may require stainless steel.

Bolt grade
Strength ratings affect performance.

FAQ

How do I measure bolt size?
Measure thread diameter, thread pitch and overall length.

What is the most common bolt size?
M8 and M10 bolts are commonly used across many industries.

Should I use a larger bolt for extra strength?
Not always. Strength depends on material, grade and application requirements.