Explaining Diameter Measurements Used in Wire, Nail, and Bullet Sizes

Bullets come in a variety of different calibers. While it might sound like cryptic bullet terminology, the caliber indicates the bullet diameter. Thus the diameter is the bullet size.

Some common bullet calibers include:

The gauge of shotgun shells is also important. The gauge numbers for a shotgun (12 gauge, 14 gauge, etc.) are the numbers of lead balls — of diameters equal to the inside diameter of the gun — required to total up to one pound of lead.

­The measurement system for wire is also interesting. AWG stands for "American Wire Gauge" and is a standard in the United States for wire diameters. In a house, you typically find 10, 12 and 14 gauge wire, and electronics typically use 20 gauge wire. Wire gauges run from 000000 gauge, which is ­about half an inch in diameter, down to 40 gauge, which is 0.001 inches. The higher the AWG number, the thinner the wire.

Where did this odd system come from? The modern method for wire drawing is thought to have emerged in Europe in the latter part of the 13th century. This method involves pulling the metal through a series of progressively smaller holes until it reaches the required diameter. The gauge number stems from how many holes the wire is drawn through. And the more holes it's drawn through, the smaller the diameter.

Then there's the common penny designation used with nails. You can buy nails ranging from twopenny to fiftypenny in the United States (represented as 2d or 50d, respectively).

Why is "d" used instead of "p"? This stems from Roman times. The "d" stood for "denarius," a small coin common throughout the Roman empire. The penny designation can be translated into length. It's not clear that anyone knows where the penny designation comes from — originally, it might have indicated the price of 100 nails.

This article was updated in conjunction with AI technology, then fact-checked and edited by a HowStuffWorks editor.