Choosing the correct bolt size is essential for strength, safety, and proper fit. Whether you’re working on automotive repairs, construction projects, machinery, woodworking, or DIY installations, understanding bolt dimensions helps prevent stripped threads, loose connections, and premature failure.
This updated bolt size guide explains standard bolt measurements, metric and imperial sizing systems, and how to choose the right bolt for your application.
Bolt Size Chart
| Size | Nominal (Major) | Coarse Thread (UNC) | Fine Thread (UNF) | ||||
| Diameter [in] | Threads Per Inch | Tensile Stress Area [in2] | Minor Area [in2] | Threads Per Inch | Tensile Stress Area [in2] | Minor Area [in2] | |
| #0 | 0.06 | — | — | — | 80 | 0.0018 | 0.0015 |
| #2 | 0.086 | 56 | 0.0037 | 0.0031 | 64 | 0.0039 | 0.0034 |
| #4 | 0.112 | 40 | 0.006 | 0.005 | 48 | 0.0066 | 0.0057 |
| #5 | 0.125 | 40 | 0.008 | 0.0067 | 44 | 0.0083 | 0.0072 |
| #6 | 0.138 | 32 | 0.0091 | 0.0075 | 40 | 0.0102 | 0.0087 |
| #8 | 0.164 | 32 | 0.014 | 0.012 | 36 | 0.0147 | 0.0129 |
| #10 | 0.19 | 24 | 0.0175 | 0.0145 | 32 | 0.02 | 0.0175 |
| 1/4″ | 0.25 | 20 | 0.0318 | 0.0269 | 28 | 0.0364 | 0.0326 |
| 5/16″ | 0.3125 | 18 | 0.0524 | 0.0454 | 24 | 0.058 | 0.0524 |
| 3/8″ | 0.375 | 16 | 0.0775 | 0.0678 | 24 | 0.0878 | 0.0809 |
| 7/16″ | 0.4375 | 14 | 0.1063 | 0.0933 | 20 | 0.1187 | 0.109 |
| 1/2″ | 0.5 | 13 | 0.1419 | 0.1257 | 20 | 0.1599 | 0.1486 |
| 9/16″ | 0.5625 | 12 | 0.182 | 0.162 | 18 | 0.203 | 0.189 |
| 5/8″ | 0.625 | 11 | 0.226 | 0.202 | 18 | 0.256 | 0.24 |
| 3/4″ | 0.75 | 10 | 0.334 | 0.302 | 16 | 0.373 | 0.351 |
| 7/8″ | 0.875 | 9 | 0.462 | 0.419 | 14 | 0.509 | 0.48 |
| 1″ | 1 | 8 | 0.606 | 0.551 | 12 | 0.663 | 0.625 |
| 1-1/8″ | 1.125 | 7 | 0.763 | 0.693 | 12 | 0.856 | 0.812 |
| 1-1/4″ | 1.25 | 7 | 0.969 | 0.89 | 12 | 1.073 | 1.024 |
| 1-3/8″ | 1.375 | 6 | 1.155 | 1.054 | 12 | 1.315 | 1.26 |
| 1-1/2″ | 1.5 | 6 | 1.405 | 1.294 | 12 | 1.581 | 1.521 |
| 1-3/4″ | 1.75 | 5 | 1.9 | 1.74 | — | — | — |
| 2″ | 2 | 4.5 | 2.5 | 2.3 | — | — | — |
Metric Bolt Thread Size
| Nominal (Major) | Coarse Pitch | Fine Pitch | ||||
| Diameter [mm] | Pitch | Tensile Stress | Minor | Pitch | Tensile Stress | Minor |
| [mm] | Area [mm2] | Area [mm2] | [mm] | Area [mm2] | Area [mm2] | |
| 1.6 | 0.35 | 1.27 | 1.076 | — | — | — |
| 2 | 0.4 | 2.073 | 1.789 | — | — | — |
| 2.5 | 0.45 | 3.391 | 2.98 | — | — | — |
| 3 | 0.5 | 5.031 | 4.473 | — | — | — |
| 3.5 | 0.6 | 6.775 | 6 | — | — | — |
| 4 | 0.7 | 8.779 | 7.75 | — | — | — |
| 5 | 0.8 | 14.18 | 12.68 | — | — | — |
| 6 | 1 | 20.12 | 17.89 | — | — | — |
| 8 | 1.25 | 36.61 | 32.84 | 1 | 39.17 | 36.03 |
| 10 | 1.5 | 57.99 | 52.29 | 1.25 | 61.2 | 56.3 |
| 12 | 1.75 | 84.27 | 76.25 | 1.25 | 92.07 | 86.04 |
| 14 | 2 | 115.4 | 104.7 | 1.5 | 124.5 | 116.1 |
| 16 | 2 | 156.7 | 144.1 | 1.5 | 167.2 | 157.5 |
| 20 | 2.5 | 244.8 | 225.2 | 1.5 | 271.5 | 259 |
| 24 | 3 | 352.5 | 324.3 | 2 | 384.4 | 364.6 |
| 30 | 3.5 | 560.6 | 519 | 2 | 621.2 | 596 |
| 36 | 4 | 816.7 | 759.3 | 2 | 914.5 | 883.8 |
| 42 | 4.5 | 1121 | 1045 | 2 | 1264 | 1228 |
| 48 | 5 | 1473 | 1377 | 2 | 1671 | 1629 |
| 56 | 5.5 | 2030 | 1905 | 2 | 2301 | 2252 |
| 64 | 6 | 2676 | 2520 | 2 | 3031 | 2975 |
| 72 | 6 | 3460 | 3282 | 2 | 3862 | 3799 |
| 80 | 6 | 4344 | 4144 | 1.5 | 4851 | 4798 |
| 90 | 6 | 5591 | 5364 | 2 | 6099 | 6020 |
| 100 | 6 | 6995 | 6740 | 2 | 7562 | 7473 |
| 110 | — | — | — | 2 | 9182 | 9084 |
Why Bolt Sizes Matter
Choosing the correct bolt size goes beyond fit; it directly affects structural integrity, safety, and service life. Moreover, the right diameter and thread pitch distribute loads evenly, thereby preventing fatigue cracks that spread silently until a joint fails.
Conversely, an oversized bolt may seem safer, yet it can concentrate stress in surrounding materials, add unnecessary weight, and inflate costs. Meanwhile, undersized fasteners save money up front but often stretch, loosen, or shear under dynamic loads, leading to costly downtime or catastrophic collapse.
Therefore, you must weigh each trade‑off—strength, weight, cost, and ease of assembly—before settling on a size. However, that decision rarely stands alone: you must also match strength grade, material, and proper torque, since even a perfectly sized bolt will fail if over‑ or under‑tightened.
Ultimately, a reliable bolt size chart brings these variables together in one place, guiding you through competing priorities and revealing how a seemingly small choice can determine the long‑term performance and safety of the entire structure.
Understanding Bolt Size Notation (Diameter × Pitch × Length)
Before you consult any chart, it helps to decode the three key numbers that define every bolt. First, diameter dictates how much load a fastener can shoulder; a larger shank resists shear but requires a wider hole and heavier tooling.
Next, thread pitch controls how force translates into clamping pressure; a coarse pitch tightens quickly and tolerates dirt, whereas a fine pitch delivers precise preload but risks stripping if installers rush.
Finally, length determines whether the threads engage fully without bottoming out, but excessive overhang wastes material and invites corrosion on exposed threads. Therefore, you must juggle these three variables—diameter, pitch, and length—against space constraints, assembly speed, and long‑term service conditions.
Moreover, each adjustment cascades through the design: switching to a finer pitch may allow higher preload, yet it can force a jump to stronger (and pricier) alloy grades.
Consequently, a well‑structured bolt size chart is more than a reference; it clarifies how adjusting one dimension reshapes the entire fastening strategy, ultimately helping you strike the optimal balance among performance, cost, and reliability.
Metric vs. Imperial: Key Differences
Although both metric and imperial systems measure the same physical dimensions, they label and tolerate bolts quite differently. Metric bolts use millimeters for diameter, pitch, and length, offering a straightforward, decimal-based system widely adopted worldwide.
In contrast, imperial bolts list diameter in fractional inches and thread count in threads per inch (TPI), which can complicate conversions and create confusion. This difference forces designers to balance trade-offs: metric bolts simplify global sourcing and reduce errors, but may not fit older equipment designed for imperial fasteners.
Imperial bolts maintain compatibility with legacy tools but limit international options and require careful sizing checks. Additionally, metric and imperial threads follow different tolerance standards, making it crucial to consult a bolt size chart that compares both systems.
Such charts help avoid mismatches, reveal compatible sizes, and guide engineers through challenges when working in mixed-standard environments. Ultimately, understanding these differences ensures the right bolt choice for performance and compatibility.
Standard Metric Bolt Size Chart (M3–M48)
Below is the go‑to reference for common metric bolts, from the tiny M3 used in electronics to the hefty M48 found in heavy machinery. Firstly, smaller sizes like M3 and M4 offer precision and lightweight strength, ideal for delicate assemblies, but they can’t handle heavy loads or harsh environments.
On the other hand, larger bolts such as M24 and above provide exceptional strength and durability, yet they require more space and heavier tools, increasing costs and installation time.
Balancing these factors means selecting a size that meets load requirements without overengineering or overspending. Moreover, pitch variations within each size affect thread engagement and clamping force, so consulting a detailed bolt size chart becomes essential.
This chart not only ensures the right diameter and length but also guides you through pitch choices and strength grades, helping avoid mismatches that compromise safety and performance.
Standard Imperial Bolt Size Chart (¼″–1 ½″)
For projects rooted in U.S. standards, this chart covers the most frequently specified imperial bolt diameters, ranging from ¼″ to 1 ½″. Firstly, smaller sizes like ¼″ and ⅜″ are perfect for light-duty applications, offering ease of installation and cost savings, but they may lack the strength needed for heavy loads.
Conversely, larger diameters such as 1″ and above deliver robust performance in structural and industrial settings, yet they demand more clearance and specialized tools, increasing complexity. Balancing these trade-offs requires careful consideration of load, space, and budget constraints.
Furthermore, thread pitch and grade variations influence the bolt’s holding power and durability. Consulting a detailed imperial bolt size chart helps ensure you select the correct diameter, length, and thread type, ultimately preventing costly mismatches and enhancing overall safety and reliability.
Strength Grades and Head Markings
Those little dashes or digits on a bolt head aren’t decorative; they tell you exactly how much load the fastener can carry. Firstly, understanding these markings helps you choose the right strength grade, balancing durability with cost.
For example, higher-grade bolts can withstand greater stress, but they are often more expensive and may require special tools for installation. On the other hand, lower-grade bolts are cheaper and easier to work with, yet they might fail under heavy loads or harsh conditions.
Therefore, selecting the appropriate grade involves weighing safety requirements against budget and application needs. Moreover, a comprehensive bolt size chart includes strength grades alongside dimensions, making it easier to match the right bolt to your project.
Ultimately, ignoring these markings risks structural failure, while proper attention ensures long-lasting, reliable connections.
Common Mistakes When Reading Size Charts
Misreading pitch or assuming thread compatibility across systems are two of the errors that most frequently lead to stripped threads. Firstly, confusing metric and imperial pitches can cause mismatched threads that damage both bolt and hole, resulting in costly repairs and downtime.
Additionally, overlooking the importance of length can cause bolts to bottom out or fail to engage fully, compromising joint strength. Balancing these factors requires careful attention to detail and a clear understanding of sizing standards.
Furthermore, relying on incomplete or outdated charts increases the risk of errors, especially when dealing with mixed standards or unusual sizes.
Therefore, consulting a comprehensive, up-to-date bolt size chart is crucial; it helps you verify pitch, diameter, and length simultaneously, ultimately preventing common mistakes that jeopardize safety and performance.
Conclusion
Bolt sizing involves more than choosing a diameter. Thread pitch, length, material, and strength grade all influence performance. Whether you use metric or imperial hardware, matching specifications correctly helps create stronger, safer, and longer-lasting connections.
FAQs
What does M8 mean on a bolt?
M8 means the bolt has an 8 mm outside thread diameter.
How do I know if a bolt is metric or imperial?
Metric bolts use millimeters and thread pitch, while imperial bolts use inches and threads per inch.
Is thread pitch important?
Yes. Even if diameters match, incorrect thread pitch prevents proper installation.
What is the most common household bolt size?
M6, M8, and 1/4-inch bolts are commonly used in household projects.
Can I replace a metric bolt with an imperial bolt?
Only if all dimensions and thread specifications match exactly. Mixing systems is generally not recommended.