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Machine Screw Sizes & Head Types

Use this guide to help you find the screw size number or fractional size. Then select the head type that meets your needs.

Background


What is a machine screw?

A machine screw is like a small diameter bolt, like a bolt or hex head cap screw, but is usually driven with a screwdriver instead of a wrench. It features a straight, untampered shank and blunt end. External machine thread runs the entire length of the shank.


How Are Machine Screws Used?

Most commonly used to fasten metal to metal, a machine screw is designed to be inserted into a pre-threaded hole or mated with a nut (in which case it would be called a stove bolt).

Because application thread sizes vary, it is important to select the right machine screw to meet your specific requirements.


EZ-TIP 1: Find Machine Screw Size Number or Fractional Size

Use the conversion charts below to find screw size number or fractional size.


Screw Size Conversion Chart

Screw Size Fractions Decimals Millimeter
#1 1/16" 0.0625" 1.58750 Shop #1 Screws
#2 5/64" 0.078125" 1.98437 Shop #2 Screws
#3 3/32" 0.09375" 2.38125 Shop #3 Screws
#4 7/64" 0.109375" 2.77812 Shop #4 Screws
#5 1/8" 0.125" 3.17500 Shop #5 Screws
#6 9/64" 0.140625" 3.57187 Shop #6 Screws
#8 5/32" 0.15625" 3.96875 Shop #8 Screws
#9 11/64" 0.171875" 4.36562
#10 3/16" 0.1875" 4.76250 Shop #10 Screws
#11 13/64" 0.203125" 5.15937
#12 7/32" 0.21875" 5.55625 Shop #12 Screws
#13 15/64" 0.234375" 5.95312
#14 1/4" 0.250" 6.35000
#16 17/64" 0.265625" 6.74687
#18 19/64" 0.296875" 7.54062
#20 5/16" 0.3125" 7.93750
#24 3/8" 0.375" 9.52500

Standard Threads

UNC UNF
1/4 x 20" 1/4 x 28 Shop 1/4 x 20" Screws
5/16 x 18" 5/16 x 24 Shop 5/16 x 18" Screws
3/8 x 16" 3/8 x 24 Shop 3/8 x 16" Screws
7/16 x 14" 7/16 x 20
1/2 x 13" 1/2 x 20 Shop 1/2 x 13" Screws
5/8 x 11" 5/8 x 18
3/4 x 10" 3/4 x 16
7/8 x 9" 7/8 x 14
1 x 8" 1 x 14


EZ-TIP 3: Tamper-Resistant Machine Screws

Security fastners, as the name indicates, provide an added level of security by their drive design — preventing removal with ordinary screwdrivers.

Screws are available in sheet metal and machine threads.


Still Need Help?

If you are still having trouble choosing a machine screw, contact our Customer Service team at askzoro@zoro.com or call 855-289-9676.


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