How to Calculate the Diameter of Wire

When preparing to choose wire stock for your electrical job you need to know the diameter of the wire to determine what electrical load it can handle and over what distance. If you are purchasing new wire, this is easy as you can just read the label on the package. If you are looking at open wire stock, you need to know how to calculate the diameter of the wire in order to determine its gauge, and from there, its suitability to your project.

Using a caliper will give you an accurate measurement
  1. Zero out your micrometer. A micrometer is a measuring caliper that allows you to measure very small (micro) widths. On both manual and digital versions, you must first close the caliper end of the mircometer completely. Read the measurement gauge attached to your micrometer and make sure it is on "0." If it is not, you must reset it. With the caliper part closed, press "Reset" on the button pad of the digital micrometer. With a manual micrometer, adjust the knob at the base of the handle of your manual micrometer until the cylinder that your turn to operate the calipers is aligned with the "0" hash mark on the handle.

  2. Open the caliper of the micrometer so they are slightly wider then the center width of the wire. Hold the calipers up to the wire and visually inspect the wire to see if enough of the core of the wire is exposed to slightly exceed the width of the caliper end. The core is the wire inside the insulating cover; it is usually copper. If there is not enough exposed core, strip off enough insulation so you are only measuring the bare core when you tighten the caliper on the wire.

  3. Place the bare core end of the wire between the two halves of the caliper end (the halves that move as you open and close the caliper). Slowly close the caliper ends on the wire. Do not place the calipers exactly on the end of the wire if you just stripped the insulation off or the wire has been cut; the diameter of the wire is distorted from the pressure of the cut or strip. Move the caliper back a little on the wire to an undamaged part.

  4. Tighten the caliper until it is holding the bare core of the wire firmly, but is not compressing the shape of the wire. Read the measurement on your micrometer. Micrometers are designed with caliper ends that will measure the widest part of a round object. The measurement you read will be the exact diameter of the wire. Consult a wire gauge chart to find which gauge your diameter wire is and what load it can carry.


  • Make sure you consult a wire gauge selection table to make sure the diameter of the wire you are planning on using can handle the load over the distance you will be running the wire. The wrong diameter wire can overheat quickly and start a fire, or if it is too large, can diminish the load capacity and not give you enough power at your connection.