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. 

US Socket Cap screw types and need Hex Key size

Inch Fasteners - Socket Screws Hex Key Drive Size

Screw

Size

Socket Cap Low Head Socket

Button Head 

Flat Head Socket Set Screw
#0 0.050"   0.035" 0.035" 0.028"
#1 1/16"   0.050" 0.050" 0.035"
#2 5/64"   0.050" 0.050" 0.035"
#3 5/64"   1/16" 1/16" 0.050"
#4 3/32" 0.050" 1/16" 1/16" 0.050"
#5 3/32" 1/16" 5/64" 5/64" 1/16"
#6 7/64" 1/16" 5/64" 5/64" 1/16"
#8 9/64" 5/64" 3/32" 3/32" 5/64"
#10 5/32" 3/32" 1/8" 1/8" 3/32"
1/4" 3/16" 1/8" 5/32" 5/32" 1/8"
5/16" 1/4" 5/32" 3/16" 3/16" 5/32"
3/8" 5/16" 3/16" 7/32" 7/32" 3/16"
1/2" 3/8" 1/4" 5/16" 5/16" 1/4"
5/8" 1/2" 5/16" 3/8" 3/8" 5/16"

 

Inch Fasteners - Wrench size needed for Driving common Nuts

Screw Size

Standard Nuts

Wrench Size *

Heavy Hex

Wrench Size**

Alt Size, ***

#0 5/32"    
#1 5/32"    
#2 3/16"    
#3 3/16"    
#4 1/4"    
#5 5/16"    
#6 5/16"    
#8 11/32"    
#10 3/8"    
1/4" 7/16" 1/2"  
5/16" 1/2" 9/16"  
3/8" 9/16" 11/16"  
1/2" 3/4" 7/8"  
5/8" 15/16" 1-1/16"  

* Standard size Nuts: 

  • Hex Nut
  • Hex Jam Nut
  • Square Nut
  • Flange Nuts
  • Nylon Insert (Nyloc) Lock Nut, Regular
  • Jam Nylon Insert (Nyloc) Lock Nut
  • Acorn Nut
  • Stover Equivalent Nuts

** Alternate Heavy Hex:

  • A 194 - For structural bolts
  • A 563 - For structural bolts
  • Heavy Hex Jam Nut

*** Alternative Size