Bolts are measured using two standards; metric and SAE (Society of Automotive Engineers). SAE bolts are often referred to as standard bolts and are ranked by bolt size, coarse threads per inch, and SAE Grade. The torque a standard bolt is capable of withstanding depends on several factors, including base material, thread pattern, and bolt size.
Most SAE grades stamped onto the head of the bolt range from 0 to 8. Grades 0 to 8 are typically available through hardware stores. The grade is determined based upon the type of material used. Grade 0 through 2 uses low carbon steel and is the weakest bolt available. Grade 5 is the average using iron or carbon heat-treated steel. Grade 8 is the most industrial bolt made out of a carbon alloy steel with a tensile strength up to 150,000 PSI (pounds per square inch). Grades 0 through 2 have a tensile strength of up to 74,000 PSI.
The measurements of a standard bolt follow an established layout. For example, 1/4-20 is a bolt that is one-quarter inch long with 20 threads per inch. The sizes range from as small as one-quarter inch up to several inches. The threads range between 10 and 20 threads per inch on most bolts however some industrial and high-pressure bolts use fewer threads per inch. One of the larger and more durable bolts is a 2-4.5 bolt, with 4.5 threads per inch. The higher number of threads leads to a weaker bolt.
The torque for a specific bolt is directly dependent upon the grade, or materials used, and the measurement of a bolt. A 1/4-20 grade 2 bolt can only handle 6 foot-pounds of torque. A 1-8 grade 5 bolt can withstand up to 530 foot-pounds of torque. One of the most industrial bolts is the 2-4.5 grade 8, which is capable of withstanding up to 8,200 foot-pounds of torque. The torque applicable to the majority of bolts ranges between 12 and 400 foot-pounds of torque. Most applications that use bolts require between 10 and 150 foot-pounds of torque. Use at least a 5/8-11 grade 8 bolt for applications that require higher levels of torque. The torque for that bolt is 230 pounds per square inch.