What’s Up With Measuring So Many Things in “Gauge”?

From the dawn of history we humans have been obsessed with measurement, striving to gauge, rank, and quantify every part of our world from the tallest mountain to the tiniest grain of sand. It is an obsession which led to the development of the precise mathematics and science which underpin our modern world. But one unfortunate result of this millennia-long quest has been to saddle us with a bewildering array of disparate weights and measures, many of which to modern eyes appear completely arbitrary and illogical. Much of this apparent arbitrariness stems from these systems of measurement having been created in an era before precise measuring instruments, when people had to rely on everyday objects and observations to make sense of their world. For example, the mile, now defined as 5,280 feet or 1,609.344 metres, derives its name from the Latin word for “one thousand,” and once corresponded to a thousand standard paces of a marching Roman soldier. Similarly, the acre, now defined as 43,560 square feet or 4,047 square metres, was traditionally the average area of land which could be ploughed by a team of oxen in one day. Slightly more modern but equally based in everyday agrarian life is horsepower, developed by engineer James Watt in the late 18th Century to measure the output of steam engines. Defined today as 745.7 watts, one horsepower originally referred to the mechanical power needed by a draft horse to lift a 550 pound weight a distance of one foot over one second. Essentially Watt wanted a way to show people how economical it would be to replace their draft horses with his engines.

It isn’t known exactly how he came up with the numbers he did, as there are conflicting accounts of the experiments he ran. And, in truth, that’s a very generous estimate as very few horses could maintain that kind of power for a full workday, but getting a perfect figure wasn’t that important to what Watt was trying to do. Further, by overestimating what a horse could do, whether intentionally or not, he made sure that his product would always over deliver what he said when trying to get people to buy it, which is a great word-of-mouth marketing trick.

Moving on from there, other common systems of measurement are baffling on an entirely different level, and one of the most widespread and confusing is the concept of “gauge.” While the term originates from the use of a standardized measuring tool or gauge for inspection, the various measurement systems known as “gauge” are as different as the objects they measure.

The most common use of “gauge” is to measure the thickness of sheet metal and the diameter of metal wire. The gauge scale works opposite to what one might expect, with 1-gauge sheet measuring 0.28 inches thick but 10 gauge sheet only 0.14 inches. Similarly 1-gauge wire measures 0.3 inches in diameter and 10-gauge wire 0.128. This system, however, starts to make a lot more sense once you consider how these materials are actually manufactured. Sheet metal is produced by passing thick sheet steel through heavy rollers, while wire is manufactured by pulling metal rods through a draw plate – a hardened metal die with a series of precise holes drilled in it. The gauge number refers to the number of times a sheet is passed through the rollers or a wire through the draw plate, with the metal getting progressively thinner with each pass. Thus, the higher the gauge, the thinner the metal. Originally, the exact thickness of each gauge number was based on the standard thickness of the original – or 0-gauge – starting material: 0.2813 inches for sheet metal and 0.324 inches for wire. Later, smaller gauge numbers, written as 2/0, 3/0, 4/0 and so on were added to accommodate thicker materials. Nowadays, however, sheet metal and wire are typically measured by simple thickness or diameter in millimetres or fractions of an inch, while the gauge systems still in use have been rationalized to make them more consistent. For example, in the British Standard Wire Gauge or SWG system, introduced in 1883, wire sizes are fitted to a pre-calculated exponential curve wherein the weight per unit length changes by 20% per step. This, in turn means that wire diameter decreases by approximately 10.6% between gauge numbers.

Gauge is also used to measure hollow tubes, though in this case gauge refers not to the diameter of the tube but rather its wall thickness. As in other gauge systems, the higher the gauge number, the thinner the tube walls. This again derives from the manufacturing process, with the walls getting thinner each time a tube is drawn through a tool called a swage. In industry, tube dimensions are typically given as a combination of outer diameter and gauge. However, this applies only to larger structural tube sizes; in the Birmingham Gauge System, commonly used for hypodermic needles, gauge numbers refer not to wall thickness alone but rather to a standardized combination of outer diameter and wall thickness. Wall thickness also does not consistently decrease with diameter, with some higher gauges actually having thicker walls than lower gauges. The Birmingham scale ranges from 00000 gauge, with a diameter of half an inch, down to 36-gauge, with a diameter of 0.003 inches, with 19-26 gauge being the most common diameters for hypodermic needles. Further adding to the confusion, intravenous catheters, though inserted using hypodermic needles, are measured using an entirely different system called French Gauge. Unlike in Birmingham Gauge, a higher French Gauge number indicates a thicker catheter, with the numbers corresponding to three times the diameter of the catheter tube in millimetres. Thus a 3 French Gauge tube has an outer diameter of 1mm.

And if all this wasn’t enough, Gauge only applies to seamless drawn tubing; for welded pipes like those used to carry water and sewage, yet another system known as Pipe Schedule is used. Each standard pipe size is available in up to 11 standard pipe schedules, ranging from 5 to 160, each defined by a particular ratio between the internal pressure of the pipe and maximum allowable stress in the pipe wall. Thus, as schedule increases, so does the thickness of the pipe wall compared to its outer diameter, meaning that the higher the schedule, the more internal pressure a pipe can carry. But as pipe schedule is based on calculations of mechanical stress, the ratio between pipe thickness and pipe diameter is not constant across each schedule. For example, for a 1-inch schedule 30 pipe the ratio is 13.5% while for a 3-inch schedule 30 pipe it is only 5%. Thus, in most practical applications it is easier look up the dimensions of a particular pipe size on a standard chart rather than calculate it.

Yet another way in which gauge is commonly used is to measure the barrel diameter or “bore” of firearms, particularly shotguns. As in other gauge systems, the higher the gauge, the smaller the diameter, with a 16-gauge barrel, for instance, being smaller than a 12-gauge barrel. However, unlike with sheet metal, wire, or tubing, this system is based not on the manufacturing process of the barrel, but a totally different and archaic system involving lead, the metal traditionally used to make shotgun projectiles. If you were to take a pound of lead, divide it into 12 equal pieces, and form those pieces into perfect spheres, each of those spheres will have a diameter of 0.729 inches or 18.53mm – the exact inner diameter of a 12-gauge shotgun barrel. Thus, shotgun gauge refers to the number of pieces into which that pound of lead is divided; the higher the number, the smaller the resulting spheres, and the smaller the barrel diameter. The only common exception to this is .410 gauge, commonly used for hunting small game like rabbits or grouse. In this case .410 is not actually a gauge but simply the calibre or internal diameter of the barrel in inches.

But while gauge is used to measure the diameter of the barrel, the actual pellets or “shot” fired by a shotgun are measured using an entirely different system. As with gauge, a higher number corresponds to a smaller pellet diameter, but as there are multiple different classification systems still in use, things get very confusing very quickly. In the British system shot is classified according to the number of pellets per ounce, with, for example, AAA shot corresponding to 35 pellets per ounce, A shot 50 pellets per ounce, BB shot 72 pellets per ounce, and No.2 shot 87 pellets per ounce. The American and European systems are similar but differ in the actual diameter of each shot type, the American system being based on hundredths of an inch and the European system on millimetres, with each number corresponding to a 0.25mm decrease in diameter. This system, incidentally, is the origin of the “BBs” traditionally used in children’s BB guns. “BB” in this case does not stand for “Ball Bearing” as is sometimes claimed but rather BB-size shot, once defined as .180 inches but now standardized at .177 inches or 4.5mm. Buckshot, used against larger game, is measured using a similar – but once again slightly different – system. As with birdshot, the British system is based on shot weight, with SG or “small game” shot corresponding to 8 pellets per ounce. Each subsequent size, designated as SSG, SSSG and so on, is half the weight of the last. The American system is centred around #0 buck, with larger pellets designated by additional zeroes – as in the ubiquitous “double-aught-buck” – and smaller pellets by larger numbers like #1 buck, #2 buck, and so on. These sizes in turn correspond to an increase of approximately 2-3 pellets per ounce. However, given the long and complex history of these measures and the inexact methods used to create them, most of these sizes are more a matter of tradition than any rational system.

And finally, we come to the last common use of gauge: as a measure of the width of railroad tracks. Unlike other gauge-based systems, railroad gauge is not based on any overarching system of measurement but rather evolved over time to meet the ever-changing needs of railway rolling stock. The earliest common track gauge of around 4 feet 4 inches was based on the width of the wagons and horse teams used to haul coal and other minerals out of mines. But as steam locomotives continued to grow, so too did track gauge. What is now known as a standard gauge of 4 feet, 8-1/2 inches was first used on the Liverpool and Manchester Railway in 1830, while the broad gauge of 7 feet, 1/4 inch was first used on the Great Western Railway in 1833 to give trains greater lateral stability. However, the use of different gauges on competing rail lines lead massive compatibility problems within the British rail system, and in 1846 the Regulating Gauge of Railways Act mandated that all lines be converted to standard gauge. Today, Standard Gauge is the most commonly-used railway gauge in the world, though Broad Gauge – defined as any gauge significantly wider than 4 feet, 8-1/2 inches – is widely used in India and Russia, while Narrow Gauge is used in parts of Africa, Southeast Asia, Central and South America, and in industrial settings like mines and factory complexes.

And so we reach the end of our journey through the arcane and confusing world of gauge. After all that, all I can say is: I, for one, welcome our Metric overlords.

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