We highly recommend measuring metric screws by the screw thread diameter, but there are situations that come up where you might need to measure a screw size based upon limited information. Here we address two useful ways to find the screw size, or at least get you close enough that you can make an informed decision about what size screw you need.
First option is using a known size hex key to measure the screw diameter based upon the information below. The second way would be to use the size of the wrench used to drive the nut to estimate the screw size based upon the external nut size. A common issue with measuring this way is not accounting for US / Metric screws that take the similar hex key or wrench (but different threads) or nuts with alternative sizes than what is common to name a few.
Metric Fasteners - Socket Screws Hex Key Drive Size
|Socket Cap||Low Head Socket||Flat Head||Socket Set Screw|
Metric Fasteners - Wrench size needed for Driving common Nuts
|Screw Size||Wrench Size *||Alt Size, Flange Nuts **||
Alt Size, Acorn ***
* Standard size Nuts:
** Alternate size Nuts:
*** Alternative Size, Acorn:
DIN 1587 Acorn Nut
DIN 917 Low Cap Hex Cap Acorn