We highly recommend measuring 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 Socket Screw Types and Hex Key drive size needed

Metric Fasteners - Socket Screws Hex Key Drive Size

Screw Size

Socket Cap Low Head Socket

Button Head 

Flat Head Socket Set Screw
M2 M1.5   M1.3 M1.3 M0.9
M2.5 M2   M1.5 M1.5 M1.3
M3 M2.5 M2.5 M2 M2 M1.5
M4 M3 M2.5 M2.5 M2.5 M2
M5 M4 M3 M3 M3 M2.5
M6 M5 M4 M4 M4 M3
M8  M6 M5 M5 M5 M4
M10 M8 M7 M6 M6  M5
M12 M10 M8 M8 M8 M6
M16 M14 M12 M10  M10   

Metric Fasteners - Wrench size needed for Driving common Nuts

Screw Size Wrench Size * Alt Size, Flange Nuts **

Alt Size, Acorn ***

M2 4mm    
M2.5 5mm    
M3 5.5mm 5.5mm   
M4 7mm 7mm  7mm
M5 8mm 8mm  8mm
M6 10mm 10mm 10mm
M8  13mm 13mm 13mm
M10 17mm 15mm 16mm
M12 19mm 18mm 18mm
M16 24mm 24mm   
M20 30mm 30mm  

* Standard size Nuts: 

DIN 934 Hex Nut
DIN 439B Hex Jam Nut
DIN 557 Square Nut
DIN 985 Nylon Insert (Nyloc) Lock Nut, Regular
DIN 982 Nylon Insert (Nyloc) Lock Nut, High Type
DIN 980V Prevailing Torque metal Lock Nut

** Alternate size Nuts:

DIN 6926 Prevailing Torque Nylon Insert Hex Flange Nut
DIN 6927 Prevailing Torque Hex Flange All Metal Locknut
DIN 6923 Serrated and Non-Serrated flange nuts

*** Alternative Size, Acorn:

DIN 1587 Acorn Nut
DIN 917 Low Cap Hex Cap Acorn