The grade of a bolt can be determined by __________.

The grade of a bolt is indicated by the markings found on the head of the bolt. These markings often consist of numbers, letters, and sometimes symbols, which signify the strength and material specifications of the bolt according to industry standards. For example, a grade 5 bolt typically has three radial lines on its head, while a grade 8 bolt has six lines. This system of markings allows users to quickly assess the appropriate applications of the bolt in terms of strength and load-bearing capacity without needing to perform additional measurements or calculations.

Measuring the diameter, length, or counting threads per inch might provide information about the bolt's dimensions or thread specifications, but these factors do not determine the grade or strength classification of the bolt. The grade is specifically tied to the material properties and is standardized, which is effectively communicated through the markings on its head.