Bolts are made of different grades of steel. The grade of a bolt determines the maximum amount of stress (tensile strength) that the bolt can handle. Tensile strength is the amount of pull the bolt can withstand before breaking. The method used for tightening the bolt is also dependent upon the grade of the bolt. The grades are indicated on top of the bolts as raised numbers or dashes. There are three main systems of bolt classification: SAE, Metric and ASTM.

• SAE Bolt Grades—The standards for bolt strength grades in the U.S. are set according to a system developed by the SAE (Society of Automotive Engineers.) The SAE marking system uses raised dashes on the bolt head to indicate strength.
• Metric Bolt Grades—Metric grades are set according to rules set by the ISO (International Standards Organization.) The grades consist of two numbers separated by a dot. The property class is expressed in raised or depressed numerals on top or on the side of the bolt head.
• ASTM Bolt Grades—The ASTM grade standard is set by the American Society for Testing and Materials. Its strength grades are indicated by the letter A plus three numerals stamped on the bolt head.

The most common grading system used for bolts in the U.S. is the SAE system, but you can find metric grading systems as well. There are several grades of bolts but the three common grades for SAE are 2, 5 and 8. The common class (grades) for metric are 5.8, 8.8, 10.9 and 12.9. Each grade has a specific bolt strength. No matter the system used to grade the bolts, the higher the number means the stronger the bolt. For most home and automotive applications, you won’t need anything more than a grade 8 bolt strength or metric class 10.9.

SAE Grade 2 Bolts

• Head Marking: No marking
• Material: Low or medium carbon steel
• Size Range: 1/4-inch to 3/4-inch

        Proof Load (psi):55,000

        Min. Yield Strength (psi):57,000

        Min. Tensile Strength (psi):74,000

• Size Range: 7/8-inch to 1-1/2-inch

        Proof Load (psi): 33,000

        Min. Yield Strength (psi): 36,000

        Min. Tensile Strength (psi):60,000

SAE Grade 5 Bolts

• Head Marking: Three radial lines
• Material: Medium carbon steel, quenched and tempered
• Size Range: 1/4-inch to 1-inch

       Proof Load (psi): 85,000

       Min. Yield Strength (psi): 92,000

       Min. Tensile Strength (psi): 120,000

• Size Range: Over 1-inch to 1-1/2-inch

       Proof Load (psi): 74,000

       Min. Yield Strength (psi): 81,000

       Min. Tensile Strength (psi): 105,000

SAE Grade 8 Bolts

• Head Marking: 6 radial lines
• Material: Medium carbon alloy steel, quenched and tempered
• Size Range: 1/4-inch to 1-1/2-inch
• Proof Load (psi): 120,000
• Min. Yield Strength (psi): 130,000
• Min. Tensile Strength (psi): 150,000

Grade A325 

This bolt is an ASTM specification bolt that’s readily available in many home improvement and automotive stores. It’s virtually identical in terms of chemical and physical strengths and properties as an SAE Grade 5 bolt.

• Head Marking: A325
• Material: Carbon or Alloy Steel with or without Boron
• Size Range: 1/2-inch to 1-1/2-inch
• Proof Load (psi): 85,000
• Min. Yield Strength (psi): 92,000
• Min. Tensile Strength (psi): 120,000

Class 8.8

• Head Marking: 8.8
• Material: Medium carbon steel, quenched and tempered
• Size Range: All Sizes below 16-mm

       Proof Load (MPa): 580

       Min. Yield Strength (MPa): 640

       Min. Tensile Strength (MPa): 800

• Size Range: 16-mm to 72-mm

       Proof Load (MPa): 600

       Min. Yield Strength (MPa): 660

       Min. Tensile Strength (MPa): 830

Class 10.9

• Head Marking: 10.9
• Material: Alloy steel, quenched and tempered
• Size Range: 5-mm to 100-mm
• Proof Load (MPa): 830
• Min. Yield Strength (MPa): 940
• Min. Tensile Strength (MPa): 1040

Class 12.9

• Head Marking: 12.9
• Size Range: 1.6-mm to 100-mm
• Proof Load (MPa): 970
• Min. Yield Strength (MPa): 1,100
• Min. Tensile Strength (MPa): 1,220