How to Find a Screw's Thread Size

Written by susan revermann Google
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How to Find a Screw's Thread Size
Take a few measurements to determine you thread size. (Hemera Technologies/AbleStock.com/Getty Images)

Screws are one of the most popular devices used to attach and secure items. They can be used in almost any material, such as wood or metal. If you need to purchase more screws but do not know the exact screw or thread size, there are two main methods to find out. One method is to measure the screws that are in millimetres; the other is for screws in inches.

Skill level:
Easy

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Things you need

  • Ruler
  • Caliper
  • Permanent marker

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Instructions

  1. 1

    Measure the diameter of the screw body with a caliper or ruler in millimetres. Do not include the screw head for this measurement, just the width of the screw body. This will let you know the first number of a screw thread designation. It will be the "M3" in the screw size M3-.50 x 10. The "M" stands for major diameter or outside diameter and is measured in millimetres.

  2. 2

    Hold a millimetre ruler flush to the side of the screw, along the length of the screw. Measure the distance in millimetres from the top of one thread to the top of the next thread. If the threads are do not have a point but rather have a flatter thread, measure from the top edge of thread (if you are holding it vertically) to the same position on the next thread. The distance between threads is also called thread pitch. The pitch may or may not be included on the thread designation. If it is, it will be the middle number before the "x".

  3. 3

    Measure the length of the screw from the tip to the bottom of the screw head. If you have a flathead screw, however, measure from the tip of the screw to the top of the screw head. This is the last number in your thread designation and will come after the "x" in the designation. For example, it would be the "10" in M3-.50 x 10.

  1. 1

    Measure the diameter of the screw body with a caliper or ruler. Do not include the screw head for this measurement, just the screw body. This will let you know the first number of a screw thread designation. It will be the "1/4" of ¼-20 x 5/8 in the screw size. The diameter may also be designated my a number between 0 and 10 instead of a fraction. There will be a "#" before the number designation. The fraction designations are usually used for diameters above the "#10" diameter.

  2. 2

    Use a ruler to measure an inch on your screw. Mark it with a permanent marker. Unified threads is the term used for threads per inch.

    Count the number of individual threads within that one-inch length. Start with the number zero, not one, to get the correct thread count. This will be the "20" in a ¼-20 x 5/8 screw size.

  3. 3

    Measure the length of the screw from the tip to the bottom of the screw head. If you have a flathead screw, however, measure from the tip of the screw to the top of the screw head. This is the last number in your thread designation and will come after the "x" in the designation.

Tips and warnings

  • If you don't know if your screw is in millimetres or inches, do the measurements for both. You could also take your screw to the store to use it as a reference when you are trying to figure out which measurement is the correct one.

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