Complete Bolt Grade Chart

Complete Bolt Grade Chart

SAE Grades 2, 5, 8, 9 | Metric Classes 4.6–12.9 | Stainless A2/A4 — Tensile strength, head markings, materials & applications

Understanding bolt grades is essential for selecting the right fastener for your application. This comprehensive guide covers every common bolt grade—from basic Grade 2 hardware store bolts to high-strength Grade 9 and metric Class 12.9 fasteners used in critical applications.

Quick Reference: Bolt Grade Comparison

Grade Tensile Strength Yield Strength Material Head Marking
Grade 2 74,000 PSI (510 MPa) 57,000 PSI Low carbon steel No marks
Grade 5 120,000 PSI (827 MPa) 92,000 PSI Medium carbon Q&T 3 radial lines
Grade 8 150,000 PSI (1,034 MPa) 130,000 PSI Medium carbon alloy Q&T 6 radial lines
Grade 9 180,000 PSI (1,241 MPa) 160,000 PSI Alloy steel Q&T 6 lines + "9"
Class 8.8 116,000 PSI (800 MPa) 91,000 PSI Medium carbon Q&T "8.8"
Class 10.9 150,000 PSI (1,040 MPa) 122,000 PSI Alloy steel Q&T "10.9"
Class 12.9 174,000 PSI (1,220 MPa) 158,000 PSI Alloy steel Q&T "12.9"
A2-70 102,000 PSI (700 MPa) 45,000 PSI 304 Stainless "A2-70"
A4-80 116,000 PSI (800 MPa) 64,000 PSI 316 Stainless "A4-80"

US SAE Grades (SAE J429 Standard)

The SAE J429 standard defines bolt grades for inch-series fasteners used throughout North America. These grades are identified by radial lines on the bolt head—more lines indicate higher strength.

Grade 2 Bolts — The Basic Utility Fastener

Grade 2 is the most common and economical bolt grade, typically found at hardware stores for general-purpose applications where high strength isn't required.

Property Value
Tensile Strength 74,000 PSI (510 MPa) — sizes ¼" to ¾"
Yield Strength 57,000 PSI (393 MPa)
Material Low or medium carbon steel (AISI 1006-1022)
Heat Treatment None required
Head Marking No markings (plain head)

Common Applications: General construction, non-structural wood connections, light machinery mounting, furniture assembly, DIY projects

Grade 5 Bolts — The Automotive & Industrial Standard

Grade 5 bolts are the workhorse of the fastener industry—strong enough for most automotive and industrial applications while remaining cost-effective.

Property Value
Tensile Strength 120,000 PSI (827 MPa) — sizes ¼" to 1"
Yield Strength 92,000 PSI (634 MPa)
Material Medium carbon steel (AISI 1038-1055)
Heat Treatment Quenched and tempered
Head Marking 3 radial lines (120° apart)

Common Applications: Automotive suspension, steering, and drivetrain; agricultural equipment; industrial machinery; structural steel connections

Grade 8 Bolts — High-Strength for Critical Applications

Grade 8 bolts provide 25% more tensile strength than Grade 5, making them the standard choice for critical structural connections, heavy equipment, and high-stress applications.

Property Value
Tensile Strength 150,000 PSI (1,034 MPa)
Yield Strength 130,000 PSI (896 MPa)
Material Medium carbon alloy steel (AISI 4037, 4340)
Heat Treatment Quenched and tempered
Head Marking 6 radial lines (60° apart)
⚠️ Note on Brittleness: Grade 8 bolts are harder and therefore more brittle than Grade 5. In applications with shock loading or vibration, Grade 8 may be more prone to sudden failure.

Grade 9 Bolts — Maximum Strength for Extreme Applications

Grade 9 bolts represent the highest strength commonly available in SAE grades—20% stronger than Grade 8. Used in performance automotive and heavy equipment applications.

Property Value
Tensile Strength 180,000 PSI (1,241 MPa)
Yield Strength 160,000 PSI (1,103 MPa)
Material Alloy steel (typically 4140, 4340)
Head Marking 6 radial lines + numeral "9"

Metric Property Classes (ISO 898-1)

Metric bolts use a numbering system (like 8.8 or 10.9) stamped on the head. The first number × 100 = approximate tensile strength in MPa. The second number × first number × 10 = approximate yield strength in MPa.

Example: Class 10.9 → 10 × 100 = 1,000 MPa tensile; 10 × 9 × 10 = 900 MPa yield

Class 8.8 — The Metric Equivalent of Grade 5

Class 8.8 is the most common metric bolt grade for automotive and industrial applications.

Property Value
Tensile Strength 116,000 PSI (800 MPa)
Yield Strength 91,000 PSI (640 MPa)
Head Marking "8.8"

Class 10.9 — High-Strength Metric Standard

Class 10.9 is the metric equivalent of SAE Grade 8 for high-strength applications.

Property Value
Tensile Strength 150,000 PSI (1,040 MPa)
Yield Strength 122,000 PSI (940 MPa)
Head Marking "10.9"

Class 12.9 — Maximum Metric Strength

Class 12.9 is the highest standard metric property class—stronger than SAE Grade 8.

Property Value
Tensile Strength 174,000 PSI (1,220 MPa)
Yield Strength 158,000 PSI (1,100 MPa)
Head Marking "12.9"

Stainless Steel Grades (ISO 3506-1)

Stainless fasteners use a different marking system: A2 = 304 stainless (18-8), A4 = 316 stainless (marine grade). The number indicates tensile strength in MPa ÷ 10.

Grade Tensile Strength Material Best For
A2-70 102,000 PSI (700 MPa) 304 Stainless General outdoor, food equipment
A2-80 116,000 PSI (800 MPa) 304 Stainless (cold worked) Higher strength + corrosion resistance
A4-70 102,000 PSI (700 MPa) 316 Stainless Marine, chemical, chlorine exposure
A4-80 116,000 PSI (800 MPa) 316 Stainless (cold worked) Marine + higher strength

How to Identify Bolt Grade

SAE Grade (Inch Bolts)

Count the radial lines on the bolt head: 0 lines = Grade 2, 3 lines = Grade 5, 6 lines = Grade 8

Metric Class

The property class is stamped numerically: 8.8, 10.9, 12.9

Stainless

Look for alphanumeric markings: A2-70, A4-80, or just A2/A4

Nut-Bolt Grade Matching

Critical Rule: The nut must be equal or higher strength than the bolt.

Bolt Grade Minimum Nut Grade
Grade 2 Grade 2
Grade 5 Grade 5 (3 marks)
Grade 8 Grade 8 (6 marks)
Class 8.8 Class 8
Class 10.9 Class 10
Class 12.9 Class 12

Frequently Asked Questions

What's the difference between tensile strength and proof load?
Tensile strength is the maximum stress before breaking. Proof load is the maximum stress without permanent deformation—your "safe working limit." Never exceed proof load in normal use.
Are Grade 8 bolts always better than Grade 5?
No. Grade 8 bolts are stronger but more brittle. For applications with vibration or shock, Grade 5's extra ductility may prevent sudden failure. Use what's specified.
Can I substitute metric for SAE grades?
Yes, with appropriate matching. Class 8.8 ≈ Grade 5, Class 10.9 ≈ Grade 8. But thread pitches differ—you need the correct thread standard regardless of grade.

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