How to Determine Machine Screw Size

Updated February 21, 2017

Machine screws have many different measurements that must be correct to ensure the proper screw is being used. They have different lengths and heads with different sizes. Determining machine screw size is critical in picking out the screw for your application. A screw can accurately be measured and the size can be determined with the use of only a tape measure with millimetre measurements. Once the proper size screw is determined, you can get on with your project.

Measure the length of the screw. Attach a tape measure to the head at the top of the screw and run the tape measure down to the bottom of the screw. Write down the measurement and convert it to a decimal. fraction. If the measurement is 2-1/2-millimeters, then it will be 2.5 millimetres.

Wrap a tape measure around the threads of the screw at any point and measure the diameter of the screw. Record the measurement and convert it to a decimal once again.

Measure the width of the head of the screw. Run a tape measure across the head of the screw and record the measurement. Convert this measurement to decimals.

Measure the length of just the threaded part of the machine screw and write down the measurement. Count the number of threads on the entire screw and divide that number by how many millimetres the thread is. This will give you the number of threads per millimetre.

Combine all of the measurements and take it to a hardware store to match the measurements with the proper size machine screw.


A tape measure that has millimetre measurements must be used. Some tape measures are not as accurate and should not be used for measuring machine screws.

Things You'll Need

  • Tape measure
  • Pencil
  • Paper
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About the Author

Alexander Callos began writing in 2005 for "The Lantern" at The Ohio State University and has written for various websites, including Bleacher Report, Top Ten Real Estate Deals and Columbus Sports. He has published articles for CBS Sports, and other websites. He graduated in 2007 from The Ohio State University with a bachelor's degree in public affairs journalism.