Understanding bolt and nut sizes is essential for engineering, construction, machinery repair, and DIY projects. A wrong fit can lead to loose connections, equipment failure, or safety risks. This guide provides a clear bolt and nut size chart with standard metric and imperial measurements.
How Bolt and Nut Sizing Works
Bolt and nut sizes are determined by:
- Diameter (thread size) – thickness of the bolt shaft
- Pitch (thread spacing) – distance between threads
- Length – total bolt length
- Head size – tool size required (wrench/socket)
👉 The metric system uses millimeters (mm), while the imperial system uses inches (in).
Imperial Bolt Size Chart
| Nominal Size | Bolt Diameter ‘D’ | Bolt Diameter ‘D’ | Across Flats ‘A’ | Across Flats ‘A’ | Across Flats ‘A’ | Across Corners ‘B’ | Across Corners ‘B’ | Head Height ‘C’ | Head Height ‘C’ | Head Height ‘C’ | Thread Length ‘L’ | Thread Length ‘L’ | Transition Length ‘Y’ |
| Max (in) | Min (in) | Nominal (in) | Max (in) | Min (in) | Max (in) | Min (in) | Nominal (in) | Max (in) | Min (in) | ≤6 in | >6 in | Max (in) | |
| 1/4 | 0.25 | 0.245 | 7/16 | 0.438 | 0.428 | 0.505 | 0.488 | 5/32 | 0.163 | 0.15 | 0.75 | 1 | 0.25 |
| 5/16 | 0.3125 | 0.3065 | 1/2 | 0.5 | 0.489 | 0.577 | 0.557 | 13/64 | 0.211 | 0.195 | 0.875 | 1.125 | 0.278 |
| 3/8 | 0.375 | 0.369 | 9/16 | 0.562 | 0.551 | 0.65 | 0.628 | 15/64 | 0.243 | 0.226 | 1 | 1.25 | 0.312 |
| 7/16 | 0.4375 | 0.4305 | 5/8 | 0.625 | 0.612 | 0.722 | 0.698 | 9/32 | 0.291 | 0.272 | 1.125 | 1.375 | 0.357 |
| 1/2 | 0.5 | 0.493 | 3/4 | 0.75 | 0.736 | 0.866 | 0.84 | 5/16 | 0.323 | 0.302 | 1.25 | 1.5 | 0.385 |
| 9/16 | 0.5625 | 0.5545 | 13/16 | 0.812 | 0.798 | 0.938 | 0.91 | 23/64 | 0.371 | 0.348 | 1.375 | 1.625 | 0.417 |
| 5/8 | 0.625 | 0.617 | 15/16 | 0.938 | 0.922 | 1.083 | 1.051 | 25/64 | 0.403 | 0.378 | 1.5 | 1.75 | 0.455 |
| 3/4 | 0.75 | 0.741 | 1-1/8 | 1.125 | 1.1 | 1.299 | 1.254 | 15/32 | 0.483 | 0.455 | 1.75 | 2 | 0.5 |
| 7/8 | 0.875 | 0.866 | 1-5/16 | 1.312 | 1.285 | 1.516 | 1.465 | 35/64 | 0.563 | 0.531 | 2 | 2.25 | 0.556 |
| 1 | 1 | 0.99 | 1-1/2 | 1.5 | 1.469 | 1.732 | 1.675 | 39/64 | 0.627 | 0.591 | 2.25 | 2.5 | 0.625 |
| 1-1/8 | 1.125 | 1.114 | 1-11/16 | 1.688 | 1.631 | 1.949 | 1.859 | 11/16 | 0.718 | 0.658 | 2.5 | 2.75 | 0.714 |
| 1-1/4 | 1.25 | 1.239 | 1-7/8 | 1.875 | 1.812 | 2.165 | 2.066 | 25/32 | 0.813 | 0.749 | 2.75 | 3 | 0.714 |
| 1-3/8 | 1.375 | 1.363 | 2-1/16 | 2.062 | 1.994 | 2.382 | 2.273 | 27/32 | 0.878 | 0.81 | 3 | 3.25 | 0.833 |
| 1-1/2 | 1.5 | 1.488 | 2-1/4 | 2.25 | 2.175 | 2.598 | 2.48 | 1-5/16 | 0.974 | 0.902 | 3.25 | 3.5 | 0.833 |
| 1-3/4 | 1.75 | 1.738 | 2-5/8 | 2.625 | 2.538 | 3.031 | 2.89 | 1-3/32 | 1.134 | 1.054 | 3.75 | 4 | 1 |
| 2 | 2 | 1.988 | 3 | 3 | 2.9 | 3.464 | 3.306 | 1-7/32 | 1.263 | 1.175 | 4.25 | 4.5 | 1.111 |
| 2-1/4 | 2.25 | 2.238 | 3-3/8 | 3.375 | 3.262 | 3.897 | 3.719 | 1-3/8 | 1.423 | 1.327 | 4.75 | 5 | 1.111 |
| 2-1/2 | 2.5 | 2.488 | 3-3/4 | 3.75 | 3.625 | 4.33 | 4.133 | 1-17/32 | 1.583 | 1.479 | 5.25 | 5.5 | 1.25 |
| 2-3/4 | 2.75 | 2.738 | 4-1/8 | 4.125 | 3.988 | 4.763 | 4.546 | 1-11/16 | 1.744 | 1.632 | 5.75 | 6 | 1.25 |
| 3 | 3 | 2.988 | 4-1/2 | 4.5 | 4.35 | 5.196 | 4.959 | 1-7/8 | 1.935 | 1.815 | 6.25 | 6.5 | 1.25 |
Thread Pitch
| Bolt Size | Thread Pitch | Thread Pitch |
| (in) | Coarse | Fine |
| 1/4 | 20 | 28 |
| 5/16 | 18 | 24 |
| 3/8 | 16 | 24 |
| 7/16 | 14 | 20 |
| 1/2 | 13 | 20 |
| 9/16 | 12 | 18 |
| 5/8 | 11 | 18 |
| 3/4 | 10 | 16 |
| 7/8 | 9 | 14 |
| 1 | 8 | 14 |
| 1-1/8 | 7 | 12 |
| 1-1/4 | 7 | 12 |
| 1-3/8 | 6 | 12 |
| 1-1/2 | 6 | 12 |
| 1-3/4 | 5 | – |
| 2 | 4-1/2 | – |
| 2-1/2 | 4 | – |
| 3 | 4 | – |
Metric Bolt Size Chart
| Size | Pitch | Across Flats ‘A’ | Across Flats ‘A’ | Across Corners ‘B’ | Head Height ‘C’ | Bolt Diameter ‘D’ | Bolt Diameter ‘D’ |
| (mm) | Max (mm) | Min (mm) | Min (mm) | Max (mm) | Max (mm) | Min (mm) | |
| M5 | 0.8 | 8 | 7.78 | 9.2 | 3.5 | 5 | 4.82 |
| M6 | 1 | 10 | 9.78 | 11.5 | 4 | 6 | 5.82 |
| M8 | 1.25 | 13 | 12.73 | 15 | 5.3 | 8 | 7.78 |
| M10 | 1.5 | 16 | 15.73 | 18.4 | 6.4 | 10 | 9.78 |
| M12 | 1.75 | 18 | 17.73 | 20.7 | 7.5 | 12 | 11.73 |
| M14 | 2 | 21 | 20.67 | 24.2 | 8.8 | 14 | 13.73 |
| M16 | 2 | 24 | 23.67 | 27.7 | 10 | 16 | 15.73 |
| M18 | 2.5 | 27 | 26.67 | 31.2 | 11.5 | 18 | 17.73 |
| M20 | 2.5 | 30 | 29.67 | 34.6 | 12.5 | 20 | 19.67 |
| M22 | 2.5 | 34 | 33.38 | 39.3 | 14 | 22 | 21.67 |
| M24 | 3 | 36 | 35.38 | 41.6 | 15 | 24 | 23.67 |
| M27 | 3 | 41 | 40.38 | 47.3 | 16.7 | 27 | 26.67 |
| M30 | 3.5 | 46 | 45 | 53.1 | 18.7 | 30 | 29.67 |
| M33 | 3.5 | 50 | 49 | 57.7 | 20.5 | 33 | 32.61 |
| M36 | 4 | 55 | 53.8 | 63.5 | 22.5 | 36 | 35.61 |
Thread Pitch
| Size | Coarse (mm) | Fine (mm) | Super Fine (mm) |
| M2 | 0.4 | – | – |
| M2.5 | 0.45 | – | – |
| M3 | 0.5 | – | – |
| M3.5 | 0.6 | – | – |
| M4 | 0.7 | – | – |
| M5 | 0.8 | – | – |
| M6 | 1 | 0.75 | – |
| M7 | 1 | – | – |
| M8 | 1.25 | 1 | – |
| M10 | 1.5 | 1.25 | 1 |
| M12 | 1.75 | 1.5 | 1.25 |
| M14 | 2 | 1.5 | – |
| M16 | 2 | 1.5 | – |
| M18 | 2.5 | 1.5 | – |
| M20 | 2.5 | 1.5 | – |
| M22 | 2.5 | 1.5 | – |
| M24 | 3 | 2 | – |
| M27 | 3 | 2 | – |
| M30 | 3.5 | 2 | – |
| M33 | 3.5 | 2 | – |
| M36 | 4 | 3 | – |
| M42 | 4.5 | 3 | – |
| M48 | 5 | 3 | – |
| M56 | 5.5 | 3 | – |
What are Bolts and Nuts?
Before diving into size charts, it’s important to understand the basic components and functions of bolts and nuts in various applications. Bolts are threaded fasteners used to join materials together, while nuts are counterparts that secure bolts in place. Choosing the right type and size of both is essential for strength, stability, and durability.
However, the tradeoff between size, material, and application requirements can be challenging, as selecting the wrong combination may lead to weak connections or premature wear. Balancing these factors is key to ensuring your project’s safety and longevity.
Common Bolt and Nut Size Standards
There are several standards for bolt and nut sizes across different industries, including metric and imperial systems, that you need to know. First, metric sizes, commonly used in Europe and other parts of the world, are based on millimeters and use a decimal system.
In contrast, imperial sizes, common in the United States, are measured in inches. Furthermore, the ISO standard thread sizes offer a universal approach, ensuring compatibility across different manufacturers.
Additionally, UNC (Unified National Coarse) and UNF (Unified National Fine) threads represent different thread pitches, impacting the strength and tightness of the connection. When deciding between these options, you must balance factors like location, material compatibility, and strength requirements, as the wrong choice can lead to problems like loosening or structural failure.
Bolt Size Measurements
The main dimensions to consider when choosing a bolt are diameter, length, and thread pitch, each contributing to the bolt’s strength and compatibility. The diameter refers to the outer measurement of the bolt, and selecting the right size ensures the bolt can withstand the required load.
Next, the length is the measurement from the bolt’s head to its tip, determining how deep it will fasten into materials. Finally, the thread pitch is the distance between threads, affecting how tightly the bolt grips and holds materials together.
Balancing these factors is crucial; choosing too small a diameter or incorrect thread pitch can weaken the connection, while excessive length might lead to unnecessary protrusion or instability. Therefore, it’s important to carefully assess your project’s needs to ensure a secure and durable fastening solution.
How to Choose the Right Bolt and Nut Size for Your Project
Choosing the correct bolt and nut size requires considering several factors, including the load, environment, and material compatibility. First, the load and stress requirements are crucial; a bolt that is too small may fail under heavy pressure, while a larger one could be unnecessarily bulky.
Next, environmental considerations, such as exposure to moisture or extreme temperatures, influence the material choice and thread protection. Finally, material selection plays a key role in ensuring the bolt and nut are compatible with the materials being fastened together.
Balancing these factors can be tricky, as using the wrong size or material may compromise the integrity and safety of the connection. It’s important to weigh these tradeoffs carefully to achieve the most reliable and durable result.
Bolt Types You Should Know
- Hex bolts – most common general-purpose bolts
- Carriage bolts – used in wood connections
- Flange bolts – built-in washer for grip
- Socket head bolts – used in machinery and tight spaces
- Anchor bolts – used in concrete structures
How to Choose the Right Bolt Size
Follow this simple rule:
- Measure the hole diameter
- Match it with bolt diameter (M or inch size)
- Check thread compatibility
- Ensure proper nut size match
- Confirm load requirements
Common Mistakes to Avoid
- Mixing metric and imperial threads
- Ignoring thread pitch
- Using the wrong spanner size
- Over-tightening bolts (can damage threads)
- Choosing a bolt length too short for a secure grip
FAQs
1. What is the most common bolt size?
M8 and M10 bolts are the most commonly used in construction and automotive work.
2. How do I identify bolt size?
You can measure the diameter and thread pitch using a caliper or thread gauge.
3. Are all nuts compatible with bolts?
No. Nuts must match both the diameter and thread pitch of the bolt.
4. What is the difference between metric and imperial bolts?
Metric uses millimeters (M6, M8), while imperial uses inches (1/4″, 3/8″).
5. Can I mix metric and imperial bolts?
No, they are not interchangeable due to different thread systems.
6. What happens if I use the wrong nut size?
It can cause loose fitting, thread damage, or complete joint failure.
Final Verdict
Bolt and nut sizing is standardized, but precision matters. Always match diameter, thread pitch, and nut compatibility to ensure safety and durability in any project.