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Identify a Bolt Grade by Its Head Marking
Match the marking you can actually see, then confirm the diameter and governing specification before selecting a replacement.
Important: head markings identify a specification system, not a safe working load. An unmarked head should not be assumed to be Grade 2 without traceable identification, and SAE, ISO metric and ASTM grades are not automatically interchangeable.
SAE inch-series
SAE Bolt Grade Strength Chart
Use the grade and diameter range together. The same grade can have different minimum strength requirements at larger diameters.
Values are minimum reference strengths in ksi (1 ksi = 1,000 psi). They are not allowable assembly loads or tightening torques. SAE J429 covers inch-series steel externally threaded fasteners through 1-1/2 in; the published mechanical-property summary below preserves the size bands rather than flattening each grade to one number. Source [1] Source [4]
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Scroll within this table to view additional columns. The first column identifies each grade and size band.
| SAE Grade 2 | No radial grade lines* | 1/4–3/4 in | 55 | 57 | 74 |
|---|---|---|---|---|---|
| SAE Grade 2 | No radial grade lines* | Over 3/4–1-1/2 in | 33 | 36 | 60 |
| SAE Grade 5 | 3 radial lines | 1/4–1 in | 85 | 92 | 120 |
| SAE Grade 5 | 3 radial lines | Over 1–1-1/2 in | 74 | 81 | 105 |
| SAE Grade 8 | 6 radial lines | 1/4–1-1/2 in | 120 | 130 | 150 |
*A blank or unmarked head by itself is not proof that an unknown fastener is SAE Grade 2. Manufacturer identification, procurement documentation and the governing application specification still matter.
Bolt Grade Chart | Turn2Engineering | Strength values are reference minima; verify the governing specification and fastener documentation before use.
Visual identification
SAE Bolt Head Marking Chart
ISO metric property classes
Metric Bolt Grade Chart: 8.8, 10.9 and 12.9
Metric property classes use a different marking and specification system than SAE grades. Similar strength does not make two fasteners interchangeable.
| Property class | Common head marking | Reference size band | Proof min. (MPa) | Yield min. (MPa) | Tensile min. (MPa) |
|---|---|---|---|---|---|
| 8.8 | 8.8 | Below 16 mm | 580 | 640 | 800 |
| 8.8 | 8.8 | 16 mm and above* | 600 | 660 | 830 |
| 10.9 | 10.9 | M5–M39 | 830 | 940 | 1,040 |
| 12.9 | 12.9 | M1.6–M39 | 970 | 1,100 | 1,220 |
The class designation encodes nominal mechanical-property relationships: the first number relates to nominal tensile strength and the second to the yield-to-tensile ratio. The actual acceptance values are the tabulated ISO requirements, which can vary by size band.
No. Their strength levels are often compared, but the thread system, dimensions, marking rules and governing standards differ. Treat any SAE-to-metric comparison as a strength reference, not a substitution rule.
*Within the applicable ISO 898-1 size and thread-pitch scope. ISO 898-1:2013 remains the current published ISO standard and was reviewed and confirmed in 2025. Source [2] The tabulated common-size values above are a practical published summary of ISO 898-1 mechanical properties. Source [5]
Strength comparison only
SAE vs Metric Bolt Grades
| US reference | Metric class often compared | Why the comparison is limited |
|---|---|---|
| SAE Grade 2 | Lower metric property classes | No single metric class is a universal Grade 2 substitute. |
| SAE Grade 5 | 8.8 | Strength levels can be similar, but thread geometry, dimensions and standards differ. |
| SAE Grade 8 | 10.9 | Both are high-strength fasteners, but they are not dimensionally or specification-equivalent. |
| — | 12.9 | 12.9 is a higher metric property class and should not be relabeled as an SAE grade. |
Do not use this comparison to authorize a replacement. Match the complete fastener specification, including thread, dimensions, coating, material, mating hardware, service conditions and any OEM or project requirements.
Beyond SAE J429
Other Common US Bolt Specifications
| Specification | Common marking | Nominal size range | Tensile reference | Important distinction |
|---|---|---|---|---|
| ASTM A307 Grade A | 307A | 1/4–4 in | 60 ksi minimum | General-purpose carbon-steel fastener specification; not SAE Grade 2. |
ASTM A307 Grade A is useful to include because users often encounter it alongside SAE hardware, but it belongs to a different specification system. Do not convert an A307 fastener into an SAE grade label simply because the strength appears similar. Source [4]
Structural fasteners
ASTM Structural Bolt Grades
A325 and A490 are now grades within ASTM F3125/F3125M rather than standalone current specifications.
| F3125 designation | Style / family | Minimum tensile strength | Key note |
|---|---|---|---|
| A325 | Heavy hex | 120 ksi | Current grade under F3125; common structural bolting. |
| F1852 | Twist-off / TC | 120 ksi | Structural tension-control assembly family. |
| Grade 144 | Heavy hex or twist-off | 144 ksi | Inch-series grade added to the consolidated specification. |
| A490 | Heavy hex | 150 ksi minimum | Higher-strength structural grade under F3125. |
| F2280 | Twist-off / TC | 150 ksi minimum | Structural tension-control assembly family. |
ASTM F3125/F3125M-26 states that the specification consolidated and replaced A325, A325M, A490, A490M, F1852 and F2280, and it includes the 144 ksi inch-series grade. Source [3]
Anchor rods and anchor bolts
ASTM F1554 Anchor Bolt Grades
| Grade | Nominal size range | Yield min. (ksi) | Tensile range (ksi) | Typical use context |
|---|---|---|---|---|
| F1554 Grade 36 | 1/2–4 in | 36 | 58–80 | Lower-strength structural anchorage. |
| F1554 Grade 55 | 1/2–4 in | 55 | 75–95 | Intermediate-strength anchorage; weldable supplementary requirement may apply when specified. |
| F1554 Grade 105 | 1/2–3 in | 105 | 125–150 | High-strength structural anchorage. |
ASTM F1554 covers steel anchor bolts/rods in Grades 36, 55 and 105 for anchoring structural supports to concrete foundations. Source [6] The mechanical-property ranges shown above are from a published fastener-industry specification summary. Source [7]
Different material system
Stainless Bolt Markings Are Not SAE Grades
These identify stainless material families used in inch fasteners. They do not by themselves mean “Grade 5” or “Grade 8,” and one universal strength value should not be assigned without the applicable product specification.
These are common metric stainless material designations. The full fastener marking and property class are needed to determine mechanical requirements; appearance alone cannot identify strength.
Do not identify stainless strength from silver color, magnetism or material family alone. Match the complete marking and supplier documentation to the governing standard or OEM requirement.
Read the numbers correctly
Proof, Yield and Tensile Strength Are Different
The specified stress a fastener must withstand in the proof-load test without evidence of permanent set.
The stress associated with the onset of a specified permanent deformation. It is not the same as a recommended working stress.
The maximum tensile stress reached before fracture in the specified test. It is not a safe continuous assembly load.
For example, the common Grade 5 small-diameter row has 120 ksi minimum tensile strength, while Grade 8 has 150 ksi. That is a 25% higher tensile minimum, but it does not mean every Grade 8 joint is 25% stronger. Threads, nut strength, tapped material, preload, fatigue, shear planes, bearing area and geometry can control the joint.
Identification workflow
How to Identify an Unknown Bolt
- Read the head. Clean only enough to see the grade marks and manufacturer identification without removing material.
- Identify the system. Radial lines usually point to SAE J429 grades; numbers such as 8.8, 10.9 and 12.9 are metric property classes; structural and anchor fasteners use their own ASTM designations.
- Measure the nominal thread diameter. Do not use wrench size as the bolt diameter.
- Confirm pitch or thread series. SAE and metric fasteners that look similar can have incompatible threads.
- Match the governing specification. Preserve material, finish/coating, head style, length, thread engagement and mating hardware.
Common comparison
Grade 5 vs Grade 8: Stronger Does Not Always Mean Correct
| Property | SAE Grade 5 | SAE Grade 8 |
|---|---|---|
| Head marking | 3 radial lines | 6 radial lines |
| Proof | 85 ksi | 120 ksi |
| Yield minimum | 92 ksi | 130 ksi |
| Tensile minimum | 120 ksi | 150 ksi |
| Replacement rule | Do not substitute on grade number alone; follow the engineered/OEM fastener specification. | |
Torque is not determined by grade alone. Thread diameter and pitch, finish, lubrication, nut factor/friction and target preload all affect tightening. Use the specified tightening procedure for critical joints.
Data basis
Standards and Sources
Reference basis checked September 30, 2026. Standards are identified by edition/status where the publisher provides it publicly. This page summarizes selected reference values and does not reproduce a standard in full; the contract, drawing, OEM requirement and applicable purchased standard control.
- SAE J429_201405 — Mechanical and Material Requirements for Externally Threaded Fasteners. Scope covers inch-series steel bolts, screws, studs and related externally threaded fasteners through 1-1/2 in.
- ISO 898-1:2013 — Mechanical properties of carbon and alloy steel fasteners. ISO lists Edition 5 as current after review and confirmation in 2025.
- ASTM F3125/F3125M-26 — High Strength Structural Bolts and Assemblies. Consolidates the A325/A490 families and includes the 144 ksi inch-series grade.
- Bolt Depot — Bolt Grade Markings and Strength Chart. Practical published summary for SAE size-dependent Grade 2/5/8 values and common markings.
- Fastenal Engineering — Mechanical Properties of Metric Fasteners. Published ISO 898-1 property-class summary used for the common 8.8, 10.9 and 12.9 size bands.
- ASTM F1554 — Anchor Bolts, Steel, 36, 55 and 105 ksi Yield Strength. Governing anchor-bolt family reference.
- Portland Bolt — ASTM F1554 specification summary. Published mechanical-property table for Grades 36, 55 and 105.
Bolt Grade Chart FAQ
What do the lines on a bolt head mean?
On common SAE J429 fasteners, three radial grade lines identify Grade 5 and six radial grade lines identify Grade 8. Grade 2 commonly has no radial grade lines, but an unidentified unmarked bolt should not automatically be assumed to meet Grade 2 requirements.
Is Grade 8 stronger than Grade 5?
For the common size ranges shown here, yes: Grade 8 has higher minimum proof, yield and tensile strengths. That does not automatically authorize replacing a specified Grade 5 fastener with Grade 8 in an engineered joint.
What does 8.8 mean on a bolt?
8.8 is an ISO metric property class, not a bolt diameter. The designation relates to nominal tensile strength and yield-to-tensile ratio; the actual specified proof, yield and tensile values depend on the applicable ISO requirements and size band.
Is metric 10.9 the same as SAE Grade 8?
No. They are often compared because their strength levels are similar, but they use different thread systems, dimensions, markings and governing standards. They are not automatic substitutes.
What is the difference between A325 and A490 bolts?
Both are structural-bolt grades within ASTM F3125/F3125M. A325 is the 120 ksi minimum tensile family, while A490 is the higher-strength 150 ksi minimum tensile family. Installation and joint requirements still come from the applicable structural specification.
Can bolt grade determine tightening torque?
No. Torque also depends on thread size and pitch, friction/lubrication, coating or finish, target preload and the joint specification. Use the governing equipment, OEM or engineered tightening procedure.
Does bolt color tell me the grade?
No. Finish or plating color is not a reliable universal grade identifier. Use the specified head marking, dimensions and traceable documentation.
What if my bolt has no head marking?
An unmarked bolt cannot be reliably assigned a grade solely from the absence of radial marks. Verify the part specification, supplier documentation or application requirements before selecting a replacement.