Understanding the right bolt and nut sizes is crucial for mechanical and construction projects. By following this guide, you’ll learn how to read a bolt and nut size chart and make better choices. Keep in mind that balancing size, strength, and material is key to ensuring a secure and lasting connection.
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 |
| 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.
FAQs
What happens if I choose the wrong bolt and nut size?
Choosing the wrong bolt and nut size can lead to weak or unstable connections. If the bolt is too small, it may not be able to handle the required load, leading to failure. On the other hand, if the bolt is too large, it may cause unnecessary stress on the materials or create an awkward fit. This can result in damage to the materials or even potential safety hazards. Always ensure the correct size is selected to avoid these issues.
FAQ 2: How do I know if I need a coarse or fine thread for my bolts?
The choice between coarse (UNC) or fine (UNF) threads depends on the specific application. Coarse threads are better for applications that require quick assembly and are ideal for materials like wood or softer metals. Fine threads, however, are preferred for precision applications and provide better grip, making them ideal for harder materials or when a tighter, stronger connection is needed. Consider the material and the forces acting on the bolt to make the right choice.
FAQ 3: Can I use metric bolts with imperial nuts or vice versa?
It’s not recommended to mix metric bolts with imperial nuts or vice versa, as they may not fit properly. Metric bolts have different thread pitches and diameters compared to imperial bolts, so pairing the wrong combination can lead to loose connections or failure. Always ensure that both the bolt and nut are compatible, either by selecting both in metric or both in imperial sizes for a secure fit.