Measure the diameter of one of the individual wires to be placed in the wire harness. This can be done by closing the caliper onto the wire and reading the measurement. If there are multiple sizes of wires, you will need to measure the diameter of each size of wires.
Calculate the radius of the wire(s) by dividing the diameter by two. As an example, if you measured a diameter of 2 mm, the radius would be 1 mm.
Calculate the cross-section area of the wire(s) by squaring the radius and multiplying by pi' which is 3.14. In the example, the area of the wire would be 1 mm times 1 mm times 3.14, which equals 3.14 square millimeters.
Multiply the area of the wire(s) by the number of wires of that size. If there are different sizes of wires, add all the totals together. In the example, if there were 19 same-size wires, you would have a total area of 19 times 3.14 square millimeters, which equals 59.7 square millimeters.
Calculate the radius of the entire bundle of wires by dividing the total area by 3.14 and then taking the square root of the quotient. In the example, 59.7 divided by 3.14, results in just 19. Taking the square root of this number gives you a radius of 4.36 mm.
Measure the width of the wrapping or cover, which will be added to contain the bundle. You can do this using the caliper on the width of the cover material.
Add the width of the cover material to the radius of the bundled wires. In the example, if the width of the cover material was 1 mm, the total radius of the harness would be 4.36 plus 1 mm, which equals 5.36 mm.
Multiply the total harness radius by two to calculate the diameter of the harness. In the example, 5.36 mm times two results in a harness diameter of 10.72 mm.