How to Measure Washers, Bolts & Nuts

For a novice working on a project involving bolts, washers or nuts, determining the correct measurements can be challenging. After all, terms like thread pitch, shank diameter and tapered socket can sometimes confuse someone new to the fastener vocabulary. That's why we're here to provide you with a measuring guide to help you get what you need.

how to pair the correct washers with bolts - directions for sizing the washer properly to your bolt

How to Measure a Bolt

When you purchase a bolt, the sizing on the package or container typically reflects three measurements — the diameter, the thread count and the length, in that order. For example, our 1/4-20 x 3 A307A Hex Bolt would entail a 1/4-inch diameter, 20 threads per inch and a 3-inch length. The A307A represents the grade of steel, according to standards from the American Society for Testing and Materials (ASTM).

For those who do need to measure a bolt manually, it's best to use a micrometer or bolt gauge and follow these guidelines:

  • To determine the diameter of a bolt, measure the distance from the outer thread on one side to the other. This calculation is called the major diameter. Do not include the head in your measurement.
  • The inclined plane degree of the bolt's threading indicates the pitch of a bolt. The thread pitch represents the number of threads per inch — there are several ways to obtain this number, such as a thread pitch gauge or the three wire method.
  • The distance from the end to the flat part of the head determines the length. Always measure from underneath the head to the end to calculate this number.

At FMW Fasteners, we offer a large selection of bolts for almost any application or purpose. We list all sizing data in the bolt specifications to ensure quick and easy ordering.

How to Measure Flat Washer Size

Washer size refers to the diameter of the hardware's hole and correlates with the bolt diameter. The outside diameter (OD) of a washer is always wider than the fastener. Most manufacturers usually specify size and OD in fractional inches, although some occasionally use decimal measurements. 

Example: If you're determining the diameter of a washer (without knowing the package specs) and measure 5/16 of an inch, it most likely fits a bolt with a diameter of one-quarter of an inch. 

At FMW Fasteners, our hardware products help you pick a washer size by first specifying the diameter of the bolt a piece will fit, followed by the hardness value and whether it is beveled. For example, our 1/2 F436 Beveled Washer Plain consists of carbon steel and has a hardness that conforms to ASTM F436. It fits a bolt with a 1/2-inch diameter.

How Do I Pair the Correct Washer With the Bolt I've Selected?

Most flat washers are shown with a single sizing option: Diameter.  

This indicates the nominal inner diameter of the washer AND the bolt diameter this washer is designed to work with. The size of a washer indicates the size bolt that it fits, so you will choose your washer hardware to match the diameter of the fastener you selected.

An exception would be Fender Washers. By design, these allow you to select from two size options (i.e. 1/4 x 3/4). The first indicates the nominal inner diameter. This is the bolt diameter it works with. The second indicates the outer diameter of the washer itself.

How to Measure a Nut

When determining nut size, you can calculate the diameter the same way as a washer. However, you must consider the threads of the nut if measuring manually. 

At FMW Fasteners, our nuts first indicate the diameter of the bolt they fit, followed by the thread count and the material. For example, our 3/4-10 Cap Nut 18-8 SS fits a 3/4-inch diameter bolt. It has 10 threads per inch and consists of 18-8 stainless steel — 18% chromium and 8% nickel.

We feature a comprehensive selection of nuts for any application, from cap nuts to flange nuts to jam nuts and more.

Choosing FMW Fasteners as Your Trusted Supplier

At FMW Fasteners, we offer a large selection of bolts, washers, nuts and other fasteners without imposing minimum order quantities. Since 1986, we have been providing high-performance hardware to our long list of dedicated customers. To learn more about our products, browse our extensive online catalog today. 

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