Bolt Grade Markings & Strength Chart

Bolt Grade Markings & Strength Chart

Mutual Screw & Supply · mutualscrew.com · (800) 222-0324

Head markings identify a bolt's grade. US bolts use radial lines on the hex head — count the lines and add 2 to get the grade number. Grade 2 has no markings. Structural grades (A325, A490) use their own letter/number stamps.
no marks
Grade 2
SAE J429 Grade 2
Tensile: 60,000–74,000 psi Yield: 36,000–57,000 psi Material: Low carbon steel
General purpose, low-stress applications. Not for structural use.
3 lines
Grade 5
SAE J429 Grade 5
Tensile: 120,000 psi Yield: 92,000 psi (proof: 85,000) Material: Medium carbon steel, Q&T
Most common in automotive and machinery. Good balance of strength and cost.
6 lines
Grade 8
SAE J429 Grade 8
Tensile: 150,000 psi Yield: 130,000 psi (proof: 120,000) Material: Medium carbon alloy, Q&T
High-stress automotive, suspension, drivetrain, heavy equipment.
A307
A307
ASTM A307 Grade A/B
Tensile: 60,000 psi min Yield: not specified Material: Low carbon steel
Low-carbon anchor bolts and general structural bolts where high strength isn't required.
A325 or F1852
A325
ASTM A325 / F3125 Gr. A325
Tensile: 120,000–105,000 psi Yield: 92,000–81,000 psi Material: Medium carbon steel, Q&T
Standard structural steel connection bolt for building and bridge construction.
A490 or F2280
A490
ASTM A490 / F3125 Gr. A490
Tensile: 150,000–170,000 psi Yield: 130,000–150,000 psi Material: Alloy steel, Q&T
High-strength structural connections. Not to be galvanized (hydrogen embrittlement risk).
Grade Standard Tensile (psi) Yield (psi) Proof Load (psi) Head Marking Common Finish
SAE Grades
Grade 2 SAE J429 60,000–74,000 36,000–57,000 33,000–55,000 None Plain, zinc
Grade 5 SAE J429 120,000 92,000 85,000 3 radial lines Plain, zinc, yellow zinc
Grade 8 SAE J429 150,000 130,000 120,000 6 radial lines Plain, yellow zinc
ASTM Structural Grades
A307 ASTM A307 60,000 min — — "A307" Plain, HDG
A325 ASTM F3125 120,000–105,000 92,000–81,000 85,000–74,000 "A325" or "F1852" Plain, HDG, mechanically galv.
A490 ASTM F3125 150,000–170,000 130,000–150,000 120,000–130,000 "A490" or "F2280" Plain only (no galvanizing)
Metric bolt class is stamped directly on the head as two numbers separated by a dot (e.g. 8.8). The first number × 100 = tensile strength in MPa. The two numbers multiplied × 10 = yield strength in MPa. Example: 8.8 → tensile 800 MPa, yield 640 MPa.
4.6
Class 4.6
ISO 898-1
Tensile: 400 MPa (58,000 psi) Yield: 240 MPa (34,800 psi) Material: Low / med carbon steel
Comparable to Grade 2. General, non-critical applications.
8.8
Class 8.8
ISO 898-1
Tensile: 800 MPa (116,000 psi) Yield: 640 MPa (92,800 psi) Material: Medium carbon steel, Q&T
Comparable to Grade 5. The most common metric bolt class worldwide.
10.9
Class 10.9
ISO 898-1
Tensile: 1,040 MPa (150,800 psi) Yield: 940 MPa (136,300 psi) Material: Alloy steel, Q&T
Comparable to Grade 8. Automotive, heavy machinery, high-stress joints.
12.9
Class 12.9
ISO 898-1
Tensile: 1,220 MPa (176,900 psi) Yield: 1,100 MPa (159,500 psi) Material: Alloy steel, Q&T
Highest common metric class. Aerospace, motorsport, critical assemblies. Often socket cap screws.
A2-70 18-8 / 304 SS
A2-70 Stainless
ISO 3506 (304 / 18-8 SS)
Tensile: 700 MPa (101,500 psi) Yield: 450 MPa (65,300 psi) Material: Austenitic 304 stainless
General corrosion resistance. Food service, marine (mild), outdoor hardware.
A4-80 316 SS
A4-80 Stainless
ISO 3506 (316 SS)
Tensile: 800 MPa (116,000 psi) Yield: 640 MPa (92,800 psi) Material: Austenitic 316 stainless
Superior corrosion resistance. Marine, chemical processing, coastal environments.
Class Standard Tensile (MPa) Tensile (psi) Yield (MPa) Proof Load (MPa) US Equiv.
Carbon & Alloy Steel — ISO 898-1
4.6 ISO 898-1 400 58,000 240 225 ≈ Grade 2
4.8 ISO 898-1 420 60,900 340 310 ≈ Grade 2
5.8 ISO 898-1 520 75,400 420 380 Between 2 & 5
8.8 ISO 898-1 800 116,000 640 600 ≈ Grade 5
9.8 ISO 898-1 900 130,500 720 650 Between 5 & 8
10.9 ISO 898-1 1,040 150,800 940 830 ≈ Grade 8
12.9 ISO 898-1 1,220 176,900 1,100 970 Above Grade 8
Stainless Steel — ISO 3506
A2-50 ISO 3506 500 72,500 210 — 304 SS, annealed
A2-70 ISO 3506 700 101,500 450 — 304 SS, cold worked
A4-70 ISO 3506 700 101,500 450 — 316 SS, cold worked
A4-80 ISO 3506 800 116,000 640 — 316 SS, high strength
Tensile strength comparison — bars represent minimum tensile strength. US grades shown in psi; metric classes shown in MPa alongside approximate psi equivalent. Stainless grades shown separately.
US / Imperial Grades — Tensile Strength (psi)
Grade 2
60,000 psi
A307
60,000 psi
Grade 5
120,000 psi
A325
120,000 psi
Grade 8
150,000 psi
A490
150,000–170,000 psi
Metric Classes — Tensile Strength (MPa / psi equiv.)
Class 4.6
400 MPa / 58,000 psi
Class 5.8
520 MPa / 75,400 psi
Class 8.8
800 MPa / 116,000 psi
Class 10.9
1,040 MPa / 150,800 psi
Class 12.9
1,220 MPa / 176,900 psi
Stainless Steel — Tensile Strength
A2-50
500 MPa / 72,500 psi
A2-70
700 MPa / 101,500 psi
A4-70
700 MPa / 101,500 psi
A4-80
800 MPa / 116,000 psi
Grade / Class System Tensile Yield Comparable To Typical Use
Grade 2 / A307 US 60,000 psi 36,000 psi Class 4.6 Light general purpose
Grade 5 / A325 US 120,000 psi 92,000 psi Class 8.8 Automotive, structural
Grade 8 / A490 US 150,000 psi 130,000 psi Class 10.9 High-stress, heavy equipment
Class 4.6 Metric 400 MPa 240 MPa Grade 2 General, non-critical
Class 8.8 Metric 800 MPa 640 MPa Grade 5 Standard metric bolt
Class 10.9 Metric 1,040 MPa 940 MPa Grade 8 High-strength metric
Class 12.9 Metric 1,220 MPa 1,100 MPa Above Grade 8 Aerospace, motorsport
A2-70 / A4-80 Metric SS 700–800 MPa 450–640 MPa Grade 5 range Corrosion-resistant
Nut grade must match or exceed bolt grade. Using an understrength nut is the most common assembly error — it can strip the nut threads before the bolt reaches its rated clamping load. Always confirm nut grade when ordering matched sets.
Bolt Grade / Class Required Nut Grade Acceptable Substitutes Nut Markings Notes
US / Imperial
Grade 2 Grade 2 Hex Nut (no mark) Grade 5, Grade 8 No marks SAE J995 Grade 2. Unmarked nuts are typically Grade 2.
Grade 5 Grade 5 Hex Nut (3 marks) Grade 8 3 circumferential marks SAE J995 Grade 5. Do not use Grade 2 nuts on Grade 5 bolts.
Grade 8 Grade 8 Hex Nut (6 marks) No substitutes recommended 6 circumferential marks SAE J995 Grade 8. Must use Grade 8 nut to achieve full bolt proof load.
A307 Grade 2 Hex Nut Grade 5, Grade 8 No marks Low-strength bolt; standard hex nut acceptable.
A325 A563 Grade C, DH, or DH3 A194 Grade 2H "C", "DH", or "2H" RCSC requires heavy hex nuts for structural connections per AISC.
A490 A563 Grade DH or DH3 Do not use Grade C "DH" or "DH3" A194 Grade 2H also acceptable. Never use standard hex nuts on A490 bolts.
Metric
Class 4.6 ISO 4032 Class 4 Nut Class 5, 6, 8, 10 "4" on face Standard hex nut (ISO 4032). Unmarked nuts are typically Class 4–5.
Class 8.8 ISO 4032 Class 8 Nut Class 10, 12 "8" on face Most common pairing worldwide. Verify nut marking before assembly.
Class 10.9 ISO 4032 Class 10 Nut Class 12 "10" on face Do not use Class 8 nuts on Class 10.9 bolts under full preload.
Class 12.9 ISO 4032 Class 12 Nut No substitutes recommended "12" on face Class 12 nuts are specialty items — verify availability when specifying.
A2-70 / A4-80 SS ISO 3506 A2 or A4 Nut Do not mix SS grades with carbon steel nuts "A2" or "A4" Mixing stainless bolts with carbon steel nuts causes galvanic corrosion risk and grade mismatch.
Heavy Hex vs. Standard Hex: Structural applications (A325, A490) require heavy hex nuts, which are wider and taller than standard hex nuts for the same nominal diameter. Never substitute a standard hex nut in a structural joint.

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Strength values per SAE J429, ASTM F3125, ISO 898-1, and ISO 3506. Structural nut requirements per AISC and RCSC specifications. Always verify with product certifications for critical applications.