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Bolt Grade Marking Decoder

Read the lines and numbers on a bolt head. SAE, metric, ASTM structural, and stainless, decoded to proof, yield, and tensile.

Field guide: What the Markings on a Bolt Head Mean

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A bolt head carries its grade in code, and four different systems share the hardware store shelf. SAE inch bolts use radial lines, and the grade is the line count plus two, so three lines is a Grade 5 and six lines is a Grade 8. Metric bolts stamp a property class like 10.9, where the first number times 100 is the tensile strength in MPa and the second number sets the yield. Stainless stamps a corrosion group (A2 for 304, A4 for the molybdenum-bearing 316 family) and a strength class. Structural bolts stamp the ASTM grade (A325, A490, now published under F3125). This decoder reads all four, returns the proof load, yield, and tensile, gives the cross-system equivalents, and carries the cautions that matter, like never galvanizing an A490 and never downgrading a structural bolt.

Pro Tip: For SAE bolts, count the radial lines and add two. No lines is a Grade 2, three lines is a Grade 5, six lines is a Grade 8. A plain head with only a maker mark is a Grade 2, not an unmarked mystery. If a structural drawing calls for A325 or A490, do not substitute a hardware-store Grade 5 or 8 by eye: the structural bolt has controlled pretension requirements and the connection was designed around it.

Once you know the grade, get the tightening torque with the

Bolt Torque Calculator →

Convert torque to clamp force and check it against the proof load with the

Bolt Torque-Tension Calculator →

Check the clamping-force safety margin with the

Bolt Clamping Force Calculator →

For stainless fasteners, look up the base alloy (A2 = 304, A4 = 316) in the

Steel Grade Decoder →

How It Works

  1. Enter what you see

    Type the grade (Grade 8), the marking (6 radial lines), the metric class (10.9), the ASTM grade (A325), or the stainless class (A4-80).

  2. Read the marking breakdown

    The decoder shows how the marking resolves: the radial-line rule for SAE, the number rule for metric, and the group and class for stainless.

  3. Check the strength values

    You get proof load, yield, and tensile, plus the size range the values apply to.

  4. Use the cross-system equivalent

    The tool tells you the rough equivalent across SAE, metric, and ASTM so you can substitute knowingly when a spec allows it.

  5. Verify before a structural decision

    The marking decode is exact. Confirm the property values against the governing standard before a structural or safety calculation.

Built For

  • A mechanic staring at a bolt with three lines who needs to know if it is strong enough to reuse.
  • A millwright matching a metric 10.9 to the nearest SAE grade for a mixed-hardware repair.
  • A fabricator confirming an A325 versus A490 callout before torquing a steel connection.
  • Someone picking stainless hardware for a dock and learning why A4 beats A2 in salt water.
  • An estimator sizing fasteners and pulling proof loads for the torque calc.

Features & Capabilities

Four systems, one decoder

SAE inch, metric, ASTM structural, and stainless, all read from the same box.

Decodes the rule, not just a table

The radial-line rule, the metric number rule, and the stainless class rule are explained so you learn the system.

Cross-system equivalents

Grade 5 to 8.8 to A325, Grade 8 to 10.9 to A490, so you can substitute with eyes open.

Field cautions

No-galvanize on A490, no-downgrade on structural bolts, and stainless corrosion class versus strength.

Comparison

Marking System Tensile Rough equivalent Note
No lines SAE Grade 2 60-74 ksi A307, metric 4.6 Low strength general bolt
3 lines SAE Grade 5 120 ksi Metric 8.8, A325 Common automotive/machinery
6 lines SAE Grade 8 150 ksi Metric 10.9, A490 High strength
8.8 Metric class 800 MPa SAE Grade 5 First number x 100 = tensile
10.9 Metric class 1040 MPa SAE Grade 8 Yield = first x second x 10
A2-70 / A4-80 Stainless 700 / 800 MPa n/a Corrosion class, not SAE strength

References

  1. SAE J429, Mechanical and Material Requirements for Externally Threaded Fasteners
  2. ISO 898-1, Mechanical Properties of Fasteners (carbon and alloy steel, metric property classes)
  3. ISO 3506-1, Mechanical Properties of Corrosion-Resistant Stainless Steel Fasteners
  4. ASTM F3125/F3125M, High-Strength Structural Bolts (Grades A325 and A490); ASTM A307
  5. Portland Bolt fastener identification markings reference (corroborating)

Frequently Asked Questions

On an SAE inch bolt the radial lines indicate the grade, and the grade number is the line count plus two. No lines is a Grade 2 (low strength), three lines is a Grade 5 (medium carbon, 120 ksi tensile), and six lines is a Grade 8 (alloy steel, 150 ksi tensile). A plain head with only a manufacturer mark is a Grade 2.
It is not a decimal. The first number times 100 is the nominal tensile strength in MPa, so 10.9 is about 1000 MPa tensile. The yield is the first number times the second times 10, so 10 x 9 x 10 = 900 MPa. The second number is the yield-to-tensile ratio in tenths. Class 8.8 works out to 800 MPa tensile and 640 MPa yield.
The A-number is the corrosion class, not the strength. A2 is the 304 family. A4 adds molybdenum and is the 316 family, which resists chlorides and is the right choice for marine and chemical service. The number after the dash is the strength class: 70 is 700 MPa tensile, 80 is 800 MPa. A stainless fastener is generally not as strong as an SAE Grade 8.
Roughly in strength. A325 (now ASTM F3125 Grade A325) is about the strength of a Grade 5, and A490 (F3125 Grade A490) is about the strength of a Grade 8. But structural bolts are installed to controlled pretension and the connection is designed around the specific grade, so you cannot freely swap a hardware-store Grade 5 or 8 into a structural joint.
No. ASTM A490 bolts are not to be hot-dip galvanized because of the hydrogen embrittlement risk at that strength level. Use a weathering-steel or coated A490, or step down to a galvanized A325 if the connection design allows it.
Disclaimer: Head markings, strength classes, and property values returned by this decoder are reference values from SAE J429, ISO 898-1, ISO 3506-1, and ASTM F3125 and vary by diameter, material, and standard edition. Markings can be worn or counterfeit, and structural bolts are installed to controlled pretension within a designed connection. Use this tool to identify or understand a marking, not to qualify a fastener for a structural or safety-critical joint; always verify against the governing standard and the connection design. This tool is for educational and reference purposes only and is not a substitute for the engineer of record.

Guides for this job

Shops & Outbuildings

What the Markings on a Bolt Head Mean

How to read bolt head markings: SAE radial lines and the count-plus-two rule, the metric property class numbering, stainless A2/A4 corrosion classes, and ASTM structural grades.

Read Guide →

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