The Most Common SAE and Metric Bolt Sizes

A bolt is a type of hardware that you need to join or assemble two or more things together. Bolts are similar to screws – the difference is that the former are typically fastened and secured by a nut. 

Manufacturers identify bolts and screws based on type, length, pitch (threads per inch – more on pitch later in the article), the diameter of the head, class or fit, tensile strength, material, and the wrench size required to tighten them. All these elements and features match the fastener to the specific application for which they were designed. 

For example, a bolt that is too short might not be able to sufficiently hold the two parts together, or a bolt that is too long might intrude or obstruct the other parts and won’t be able to utilize its full design. A cap screw which lacks in hardness might snap off before the proper torque is applied. The wrong thread size will cause the nut to strip out and lead to several new problems.

Not all bolts are made the same, because there are different types and sizes of bolts designed for specific applications. Many bolts and cap screws are made of varying quality, materials, as well as tempering. Markings on the face indicate the bolt type, and more specifically, the tensile strength of the fastener. Tensile strength refers to the amount of pull the bolt is able to withstand before breaking. Generally speaking, a bolt or screw with more tensile strength means it has more torque to accept before breaking. Many people look up to torque charts, which are helpful in preventing the bolts from breaking.

SAE and metric bolts

Manufacturers, fortunately, stamp metric bolts differently compared to SAE bolts. They emboss International Standards Organization (ISO) metric bolts much bigger than 6mm-diameter bolts with either “ISO M” or “M” on the head. Also, manufacturers mark most metric bolts with a number on the head (such as “4.6”, “5.8”, “9.8”, “10.9”, “12.9”, and so on). While the number doesn’t have to do with the size of the head, it indicates the bolt’s relative strength – the higher the ISO numbers are, the stronger the bolt. Manufacturers emboss some of the metric nuts with a single-digit number to indicate the strength, while some manufacturers emboss “M” together with strength grade on hex flats.

SAE-grade bolts

SAE bolts are sometimes referred to as “machine bolts.” This section presents the most commonly used small- to medium-sized variants of SAE fastening hardware. There are two main types of SAE bolts:

  • UNF – also known as “fine thread pitch,” this type of SAE bolt has more threads per inch. Fine-threaded hardware tends to take a lot more torque, thus having a somewhat better holding capability.
  • UNC – also known as “coarse thread pitch,” this type of SAE bolt has fewer threads per inch. It is ideal for applications with cast iron and aluminum, as it won’t strip the mating hole as easily as the threads.

Keep in mind that the terms “coarse” and “fine” apply ONLY to SAE hardware – they do not apply to metric hardware.

The “pitch” refers to the distance between the crest of the thread to the same spot on the crest of the proceeding thread. The bigger the pitch, the fewer the number of threads per inch; conversely, the smaller the pitch, the greater the number of threads per inch. Use a thread-pitch gauge to help you determine the exact number of threads per inch.

The SAE marking standard usually starts with grade 1 or 2, indicated by a bolt head with no markings whatsoever. A grade 2 bolt has a tensile strength of 74,000 pounds per square inch (psi) or less.

1) Grade 1 bolt
Size range (inches): ¼ – 1 ½
Minimum proof strength (103 psi): none
Minimum tensile strength: (103 psi): none
Material: Low or medium carbon steel

2) Grade 2 bolt

Size range (inches): ¼ – ¾, and 7/8 – 1 ½
Minimum proof strength (103 psi): 55 (for ¼ – ¾ in.) and 33 (for 7/8 – 1 ½ in.)
Minimum tensile strength: (103 psi): 74 (for ¼ – ¾ in.) and 60 (for 7/8 – 1 ½ in.)
Material: Low or medium carbon steel

3) Grade 5 bolt
Size range (inches): ¼ – 1, and 1 1/8 – 1 ½
Minimum proof strength (103 psi): 85 (for ¼ – 1 in.) and 74 (for 1 1/8 – 1 ½ in.)
Minimum tensile strength: (103 psi): 120 (for ¼ – 1 in.) and 105 (for 1 1/8 – 1 ½ in.)
Material: Medium carbon steel, and Q&T

4) Grade 5.2 bolt
Size range (inches): ¼ – 1
Minimum proof strength (103 psi): 85
Minimum tensile strength: (103 psi): 120
Material: Low carbon martensite steel, and Q&T

5) Grade 7 bolt

Size range (inches): ¼ – 1 ½
Minimum proof strength (103 psi): 105
Minimum tensile strength: (103 psi): 133
Material: Medium carbon alloy steel, and Q&T

6) Grade 8 bolt
Size range (inches): ¼ – 1 ½
Minimum proof strength (103 psi): 120
Minimum tensile strength: (103 psi): 150
Material: Medium carbon alloy steel, and Q&T

7) Grade 8.2 bolt

Size range (inches): ¼ – 1
Minimum proof strength (103 psi): 120
Minimum tensile strength: (103 psi): 150
Material: Low carbon martensite steel, and Q&T

Metric-grade bolts

There is a feature in metric-grade bolts known as “property class”, which is indicated by an embossed or debossed numerals on top or the side of the bolt head, following the ISO standards. The property class comprises of two numbers separated by a dot. The first number indicates megapascals – a pascal unit for internal pressure, which is required to break the bolt. The second number, on the other hand, indicates a ratio between the bending load and breaking load. Again, the higher the ISO numbers are, the stronger the bolt.

1) Property class: 4.6

Size range (mm): M5 – M36
Minimum proof strength (106 Pa): 225
Minimum tensile strength (106 Pa): 400
Material: Low or medium carbon steel

2) Property class: 4.8
Size range (mm): M1.6 – M16
Minimum proof strength (106 Pa): 310
Minimum tensile strength (106 Pa): 420
Material: Low or medium carbon steel

3) Property class: 5.8

Size range (mm): M5 – M24
Minimum proof strength (106 Pa): 380
Minimum tensile strength (106 Pa): 520
Material: Low or medium carbon steel

4) Property class: 8.8

Size range (mm): M1.6 – M36
Minimum proof strength (106 Pa): 600
Minimum tensile strength (106 Pa): 830
Material: Medium carbon steel, Q&T

5) Property class: 9.8

Size range (mm): M1.6 – M16
Minimum proof strength (106 Pa): 650
Minimum tensile strength (106 Pa): 900
Material: Medium carbon steel, Q&T

6) Property class: 10.9

Size range (mm): M5 – M36
Minimum proof strength (106 Pa): 830
Minimum tensile strength (106 Pa): 1040
Material: Low carbon martensite steel, Q&T

7) Property class: 12.9
Size range (mm): M1.6 – M36
Minimum proof strength (106 Pa): 970
Minimum tensile strength (106 Pa): 1220
Material: Alloy steel, Q&T