Bolt grades ISO vs SAE comparison
The ISO (metric) and SAE (imperial) systems are the two dominant standards for classifying the mechanical strength of bolts and screws. This comparison brings together approximate equivalences, mechanical properties, allowable loads, and recommended tightening torques to facilitate selection between both systems in engineering applications.
Main features
Section titled “Main features”The ISO designation is defined by standard ISO 898‑1 and uses a property class (e.g., 8.8, 10.9). The first number multiplied by 100 gives the minimum tensile strength in MPa; the product of both numbers multiplied by 10 gives the approximate minimum yield strength. SAE grades (J429) are identified by a number (Grade 2, Grade 5, Grade 8) and are based on imperial units (psi). The following table shows the generally accepted equivalences in industry: an 8.8 ≈ Grade 5, a 10.9 ≈ Grade 8, and a 12.9 exceeds any common SAE grade.
Equivalences between standards
Section titled “Equivalences between standards”| ISO class | Equivalent SAE grade | Associated DIN standard | Typical material | Common application |
|---|---|---|---|---|
| 4.6 | – | DIN 931 / 933 | Low/medium carbon steel | Light loads, non-critical structures |
| 4.8 | Grade 2 | DIN 931 / 933 | Low/medium carbon steel | Low-demand general fastening |
| 5.8 | Grade 2 | DIN 931 / 933 | Medium carbon steel | General use in machinery |
| 8.8 | Grade 5 | DIN 931 / 933 / 912 | Medium carbon steel, quenched and tempered | Automotive, metal structures |
| 10.9 | Grade 8 | DIN 931 / 933 / 912 | Alloy steel, quenched and tempered | High-strength connections |
| 12.9 | Higher than Grade 8 | DIN 912 / 7991 | Alloy steel, quenched and tempered | Critical applications with high dynamic loads |
Mechanical properties
Section titled “Mechanical properties”| Class/Grade | Proof strength (min) | Tensile strength (min) | Rockwell hardness (HRC) | Applicable diameters |
|---|---|---|---|---|
| 4.6 | 225 MPa / 32 600 psi | 400 MPa / 58 000 psi | B67 max. | M5 – M36 |
| 4.8 | 310 MPa / 45 000 psi | 420 MPa / 60 900 psi | B71 max. | M1.6 – M16 |
| 5.8 | 380 MPa / 55 100 psi | 520 MPa / 75 400 psi | B82 max. | M5 – M24 |
| 8.8 | 600 MPa / 87 000 psi | 830 MPa / 120 350 psi | 22 – 32 | M1.6 – M36 |
| 10.9 | 830 MPa / 120 400 psi | 1 040 MPa / 150 800 psi | 32 – 39 | M5 – M36 |
| 12.9 | 970 MPa / 140 700 psi | 1 220 MPa / 176 900 psi | 39 – 44 | M1.6 – M36 |
| SAE Grade 2 | 230 MPa / 33 000 psi (up to ¾″) | 410 MPa / 60 000 psi (up to ¾″) | B70 max. | ¼″ – ¾″ (up to 1½″ with adjustment) |
| SAE Grade 5 | 586 MPa / 85 000 psi (¼″–1″) | 827 MPa / 120 000 psi (¼″–1″) | 25 – 34 | ¼″ – 1″ |
| SAE Grade 8 | 827 MPa / 120 000 psi (¼″–1½″) | 1 034 MPa / 150 000 psi (¼″–1½″) | 33 – 39 | ¼″ – 1½″ |
Allowable loads
Section titled “Allowable loads”Recommended proof loads (based on tensile stress area and 75% of proof load for commercial applications with basic safety factor).
| Thread / Diameter | Class 8.8 / Grade 5 | Class 10.9 / Grade 8 |
|---|---|---|
| M6 / ¼″‑20 UNC | 6.6 kN / 1 480 lbf | 10.0 kN / 2 250 lbf |
| M8 / 5/16″‑18 UNC | 12.0 kN / 2 700 lbf | 18.2 kN / 4 090 lbf |
| M10 / 3/8″‑16 UNC | 19.0 kN / 4 270 lbf | 29.0 kN / 6 520 lbf |
| M12 / 7/16″‑14 UNC | 28.0 kN / 6 300 lbf | 42.5 kN / 9 550 lbf |
| M14 / ½″‑13 UNC | 38.0 kN / 8 540 lbf | 57.5 kN / 12 920 lbf |
| M16 / 9/16″‑12 UNC | 52.0 kN / 11 690 lbf | 79.0 kN / 17 750 lbf |
| M20 / 5/8″‑11 UNC | 81.0 kN / 18 200 lbf | 123.0 kN / 27 650 lbf |
| M24 / ¾″‑10 UNC | 117.0 kN / 26 300 lbf | 177.0 kN / 39 800 lbf |
Recommended tightening torques
Section titled “Recommended tightening torques”Values for coarse metric thread and imperial UNC, dry assembly (μ ≈ 0.15). Converted to ft·lb from N·m using factor 0.738.
| Thread | Class 8.8 / Grade 5 | Class 10.9 / Grade 8 |
|---|---|---|
| M6 / ¼″‑20 | 9.5 N·m / 7.0 ft·lb | 14.5 N·m / 10.7 ft·lb |
| M8 / 5/16″‑18 | 23 N·m / 17 ft·lb | 35 N·m / 26 ft·lb |
| M10 / 3/8″‑16 | 46 N·m / 34 ft·lb | 68 N·m / 50 ft·lb |
| M12 / 7/16″‑14 | 80 N·m / 59 ft·lb | 115 N·m / 85 ft·lb |
| M14 / ½″‑13 | 126 N·m / 93 ft·lb | 180 N·m / 133 ft·lb |
| M16 / 9/16″‑12 | 190 N·m / 140 ft·lb | 270 N·m / 200 ft·lb |
| M18 / – (– no exact equivalent) | 260 N·m / 192 ft·lb | 380 N·m / 280 ft·lb |
| M20 / 5/8″‑11 | 370 N·m / 273 ft·lb | 520 N·m / 384 ft·lb |
| M22 / – (–) | 500 N·m / 369 ft·lb | 720 N·m / 531 ft·lb |
| M24 / ¾″‑10 | 640 N·m / 472 ft·lb | 910 N·m / 671 ft·lb |
Tap drill sizes
Section titled “Tap drill sizes”Pre‑drill bit for ISO coarse metric thread and imperial UNC thread, based on 75% thread engagement.
| Thread | Pitch | Metric drill (mm / ″) |
|---|---|---|
| M3 | 0.5 mm | 2.5 mm / 0.0984 in |
| M4 | 0.7 mm | 3.3 mm / 0.130 in |
| M5 | 0.8 mm | 4.2 mm / 0.165 in |
| M6 | 1.0 mm | 5.0 mm / 0.197 in |
| M8 | 1.25 mm | 6.8 mm / 0.268 in |
| M10 | 1.5 mm | 8.5 mm / 0.335 in |
| M12 | 1.75 mm | 10.2 mm / 0.402 in |
| M14 | 2.0 mm | 12.0 mm / 0.472 in |
| M16 | 2.0 mm | 14.0 mm / 0.551 in |
| M20 | 2.5 mm | 17.5 mm / 0.689 in |
| M24 | 3.0 mm | 21.0 mm / 0.827 in |
| ¼″‑20 UNC | 20 TPI | 5.1 mm / 0.201 in (No. 7) |
| 5/16″‑18 UNC | 18 TPI | 6.5 mm / 0.257 in (F) |
| 3/8″‑16 UNC | 16 TPI | 7.9 mm / 0.312 in (5/16) |
| 7/16″‑14 UNC | 14 TPI | 9.3 mm / 0.368 in (U) |
| ½″‑13 UNC | 13 TPI | 10.7 mm / 0.421 in (27/64) |
| 5/8″‑11 UNC | 11 TPI | 13.5 mm / 0.531 in (17/32) |
| ¾″‑10 UNC | 10 TPI | 16.5 mm / 0.649 in (21/32) |
Frequently Asked Questions (FAQ)
Section titled “Frequently Asked Questions (FAQ)”What is the difference between an 8.8 bolt and an SAE Grade 5?
Section titled “What is the difference between an 8.8 bolt and an SAE Grade 5?”Although an 8.8 has a minimum tensile strength of 830 MPa / 120 350 psi and a Grade 5 of 827 MPa / 120 000 psi, they are practically interchangeable in most applications; the 8.8 has a slightly higher yield strength (600 MPa / 87 000 psi vs. 586 MPa / 85 000 psi).
Can I replace a 10.9 bolt with a Grade 8 without modifying the torque?
Section titled “Can I replace a 10.9 bolt with a Grade 8 without modifying the torque?”Yes, because the 10.9 has a proof strength of 830 MPa / 120 400 psi and the Grade 8 of 827 MPa / 120 000 psi, very close values. However, it is recommended to apply the torque specified for the lower grade if in doubt.
Does a 12.9 bolt have a direct SAE equivalent?
Section titled “Does a 12.9 bolt have a direct SAE equivalent?”No. With 970 MPa / 140 700 psi proof and 1 220 MPa / 176 900 psi tensile, it exceeds Grade 8. It should only be used when the drawings expressly specify it due to its lower ductility.
What tightening torque is required for a dry M12 8.8?
Section titled “What tightening torque is required for a dry M12 8.8?”Under unlubricated assembly conditions and coefficient of friction 0.15, the recommended torque is 80 N·m / 59 ft·lb.
What is the correct drill bit for tapping M10 x 1.5 with a tap?
Section titled “What is the correct drill bit for tapping M10 x 1.5 with a tap?”The pre‑drill bit for 1.5 mm pitch is 8.5 mm / 0.335 in, guaranteeing approximately 75% thread height.
What is the equivalence between class 4.6 and SAE grades?
Section titled “What is the equivalence between class 4.6 and SAE grades?”Class 4.6 offers a tensile strength of 400 MPa / 58 000 psi, lower than Grade 2 (410 MPa / 60 000 psi for small diameters), so there is no direct equivalent and it is typically used in joints without structural load.