How to Read and Understand Bolt Grades
If you've ever looked closely at the head of a bolt, you've probably noticed some markings or numbers on the top side—and you may have even come across the term "bolt grades" in your work. Whether you're running a small business or working out of your truck, it can be useful to understand the meanings of these markings and grades—especially when it comes to choosing the right fasteners and gear for your project.
If you're new to reading bolt heads or shopping for bolts by grade, this guide and the included chart will demystify the process. We'll go over head markings and bolt grades, as well as why each one matters—and how metric vs. SAE sizing will also impact your purchasing decisions. You can always refer back to the chart in the future, too, if you need to check the specifications of a fastener for your current project.
Understanding Head Markings and Bolt Grades
Bolt head markings allow you to visually identify bolt grades quickly and easily. But first, you'll need to know the difference between standard (SAE) and metric fasteners.
Remember: the ratings for standard bolts (often identified as SAE) and metric hardware are not interchangeable. Metric and SAE bolts are rated and read in different ways, so you'll need to know which one you're dealing with.
Whether metric or standard, all bolts are sized by diameter, thread pitch, and length—in that order. So, for example, an SAE 1/2-16 x 2 bolt is a standard bolt that is ½" in diameter, has 16 threads per inch, and is 2" long. An M12-1.5 x 50 metric bolt is 12mm in diameter, has a thread pitch of 1.5mm, and is 50mm long.
Using the Bolt Grade Chart
As mentioned above, bolts are available in both SAE and metric sizes, and each is graded differently.
Consult the chart below for the most common bolts and their grades, including SAE and metric sizes, along with their possible markings and what each one means.
| Head | Grade/ Specification |
Material | Nominal Size | Proof Load (psi) | Yield Min (psi) | Tensile Min (psi) | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
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Grade 2 | Low/Medium Carbon Steel |
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Grade 5 | Medium Carbon Steel, Quenched & Tempered |
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Grade 8 | Medium Carbon Alloy Steel, Quenched & Tempered | 1/4" - 1 1/2" | 120,000 | 130,000 | 150,000 | ||||||||
| Markings Vary | 18-8 & 316 Stainless | Steel Alloy with Chromium & Nickel | Up to 1" | N/A | 20,000 Min., 65,000 Typical |
65,000 Min., 100,000 – 150,000 Typical |
| Head | Grade/ Specification |
Material | Nominal Size | Proof Load (MPa) | Yield Strength Minimum (MPa) | Tensile Strength Minimum (MPa) | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
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Class 8.8 | Medium carbon steel, quenched and tempered |
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Class 10.9 | Alloy steel, quenched and tempered | 5mm - 100mm | 830 | 940 | 1,040 | ||||||||
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Class 12.9 | Alloy steel, quenched and tempered | 1.6mm - 100mm | 970 | 1,100 | 1,220 | ||||||||
| Usually Stamped A-2 or A-4 | A-2 & A-4 Stainless | Steel alloy with chromium and nicke | All sizes thru 20mm | N/A | 210 Min., 450 Typical |
500 Min., 700 Typical |
Selecting the Correct Fastener
In addition to bolt head markings, there are other critical mechanical properties of bolts that are helpful to understand in order to select the correct fastener.
For example, proof load measures the amount of weight the bolt itself can support. Minimum yield strength is the load at which a bolt shows a specific permanent deformation, such as stretching, cracking, or bending. Tensile strength refers to the maximum load in tension that a bolt can withstand before actually breaking or fracturing. Depending on the application, these can all play an important role when it comes to choosing the right fastener for the job.
Finding the Right Tools for Every Job
For small businesses working with bolts and fasteners, the task becomes a lot simpler when you can fully understand the markings and grades that apply to each and every bolt. If you ever find yourself confused about bolt grades, consult the handy chart provided in this guide to steer your project in the right direction.
Shopping for bolts, fasteners, and related tools, such as ratcheting wrenches? Find everything you need for every job at Zoro.com!
Product Compliance and Suitability
The statements contained in this guide are intended for general informational purposes only. Such statements do not constitute a product recommendation or representation as to the appropriateness, accuracy, completeness, correctness, or currentness of the information provided. Information provided in this guide does not replace the use by you of any manufacturer instructions, technical product manual, or other professional resource or adviser available to you. Always read, understand, and follow all manufacturer instructions.
