If you want to understand how to determine a bolt or nut thread size then you need to read this article by RCF Bolt & Nut.
1/4-20 x 1 Flat Head Socket Cap Screws, Allen Socket Drive, Stainless Steel 18-8 (304), Full Thread, Bright Finish, 25 PCS : Industrial & Scientific
Bolt Size Chart Bolt Dimensions, Thread, How to measure Bolt Size?
Determining Bolt & Nut thread sizes by RCF Bolt & Nut
How to Measure the Size of a Bolt
Removing a bolt that snapped inside carbon fiber wing - ClubLexus - Lexus Forum Discussion
Socket CSK Screws Quality Fasteners: RCF Bolt & Nut
How to Measure Size
Copper Bolts and Cu-ETP Stud/ Nuts/ Hex Bolt/ Fasteners manufacturer
How to Measure the Size of a Bolt
Determining Bolt & Nut thread sizes by RCF Bolt & Nut
How to Measure Bolts, Nuts and Screws Using a Thread Gauge Size Tool