Bolt Grade Chart

Identify bolt grades from head markings and reference tensile strength for metric and SAE fasteners. Use as a workshop guide — always verify before use in structural or safety-critical applications.

Metric Bolt Grades

Grade	Tensile Strength (MPa)	Proof Load (MPa)	Yield Strength (MPa)	Head Marking
4.6	400	225	240	4.6
4.8	420	310	340	4.8
5.8	520	380	415	5.8
8.8	800	600	640	8.8
10.9	1040	830	940	10.9
12.9	1220	970	1100	12.9

SAE / Imperial Bolt Grades

SAE Grade	Material	Tensile Strength (psi)	Proof Load (psi)	Head Marking
Grade 2	Low carbon steel	74,000	55,000	No marks
Grade 5	Medium carbon steel	120,000	85,000	3 radial lines
Grade 8	Alloy steel	150,000	120,000	6 radial lines

Stainless steel bolts (A2-70, A4-80) have 700 MPa and 800 MPa tensile strength respectively. They are not interchangeable with alloy grades in high-temperature or high-vibration applications.

How to Use This Reference

Metric bolt grades are stamped directly on the head — e.g. "8.8" or "10.9". The first number × 100 gives approximate tensile strength in MPa. The two numbers multiplied × 10 gives approximate yield strength. For example, an 8.8 bolt: 8 × 100 = 800 MPa tensile; 8 × 8 × 10 = 640 MPa yield.

SAE grades are identified by radial lines on the bolt head: Grade 2 has no markings, Grade 5 has three lines, Grade 8 has six. Higher grade does not always mean more suitable — match the grade to the design requirement and apply the correct torque values for that grade.

Related References

Tap Drill Chart Find the correct drill size before tapping metric, UNC, and UNF threads. View chart → Nm to lb-ft Converter Convert torque units and reference common fastener torque values. Open converter → Feeds & Speeds Reference HSS cutting speeds by material and RPM formula for machining operations. View reference →

Torque Toolbox for Android includes guided calculators, saved setups, offline references, and workshop utility tools in one app.

Get it on Google Play