gauge inches   millimeters

20    0.032    0.812
18    0.040    1.024
16    0.051    1.291
14    0.064    1.628
12    0.081    2.053
10    0.102    2.588
8     0.128    3.264
6     0.162    4.115
4     0.204    5.189
2     0.258    6.544
0     0.325    8.251
00    0.365    9.266

Example: If a shotgun barrel was found to hold 12 lead spheres that weighed one pound, it would be a 12-gauge shotgun.

Gauge is a measure of the diameter of wire or jewelry. For example, gauge is used to describe the thickness of piercings. Measures in gauge can be confusing because, counterintuitively, a higher unit of gauge indicates a smaller diameter. So, 14 gauge piercing is thicker than 20 gauge.

An extremely important point that is often forgotten about gauge is that there are at least two different measures of gauge: the British (Standard Wire Gauge, SWG) and American (American Wire Gauge, AWG) measures. Although these measures are both referred to as "gauge", they indicate different actual widths: for example, 14 gauge SWG is .080 inches, while 14 gauge AWG is .0640 inches.

(I've been informed that there is a third measure of gauge, abreviated BWG, but I don't know what it is used for.)

One might think that it is appropriate to use SWG when measuring wire in Britain, and to use AWG when in America. However, this is wrong. At least in the United States, the appropriate gauge system to use depends on the type of metal you are measuring. Convention dictates to use AWG measurements for non-ferrous wire (copper, aluminum), and to use SWG for ferrous wire (steel).

So, getting 14 gauge steel wire and 14 gauge copper wire from the same company will mean you will have two different thicknesses of wire.

Also note that under AWG, there is a mathematical relationship between the gauge number and the thickness; that is, given one, you can derive the other. As far as I know, there is no system dictating SWG, and the correlation between gauge and thickness seems derived from historical practice.

Shotgun gauges in common use

The bore sizes given are their nominal imperial equivalent.

• 10 gauge = .775 inch - Fairly rare now, was used for larger birds, e.g. geese.
• 12 gauge = .729 inch - The 'standard' shotgun gauge. If you were to be handed a shotgun, chances are it'll be a 12 gauge. For all-round usage.
• 16 gauge = .662 inch - Less common lighter variant of the 12 gauge. Would be used by someone wanting a lighter gun, therefore easier to maneuver.
• 20 gauge = .615 inch - Lighter still than a 16 gauge, preferred as an alternative because of the maneuvering advantage. Used for smaller game: woodcock, widgeon etc.
• 28 gauge = .550 inch - Noticeably smaller, generally used by children, or for the smallest game.
• .410 is the smallest gauge in common usage, and doesn't have a gauge name, it is referred to by its bore size. Children's gun, or for pest control, jokingly called 'bumble-bee' guns.

One of the most heartwrenching experiences in knitting is finishing a challenging project—say an intricate Fair Isle pullover—only to discover that it's way bigger than it should have been. You used the yarn and needles that the pattern called for, and used the numbers for your size, but it's ten inches bigger around than the pattern said it would be. You try it on, but no matter how much you try to rationalise about the need for extra ease or the convenience of being able to layer six sweaters underneath it because it gets cold in the winter, you know, and layering is good in the cold, it just doesn't look good. At all. Layers or no.

Shamefaced, you're left with the task of either unravelling the entire thing and starting again, or finding someone who will fit into a pullover sized to fit the Incredible Hulk if he had strangely foreshortened arms and torso.

What went wrong?

Swatch and swatch

Gauge, also called tension, is the number of knitted stitches and rows over a given width or length of knitted fabric, worked in a given pattern. For instance, a pattern might tell you that the gauge at which it is to be knitted is twenty stitches and twenty-five rows over four inches in stockinette stitch. (This would probably be written in abbreviated form, thus: 20 sts and 25 rows = 4" in st st; gauge also might be measured over centimetres instead of inches, as 20 sts and 25 rows= 10cm in st st.) "Stitch gauge" refers specifically to stitches per inch, and "row gauge" refers to rows per inch.

Gauge is just as critical in crochet as it is in knitting, and it is measured the same way.

Gauge depends on a number of factors: the type of yarn you're using, the size of the needles you're working with, the stitch pattern you're following, and your own tension as a knitter, to name a few. As a general rule, bulky yarns that don't have a lot of loft to them tend to produce bigger stitches; this makes the gauge come out to fewer stitches and rows per inch. Thinner yarns, or loftier yarns, will work up with more stitches and rows to the inch. Switching to bigger needles will produce fewer stitches and rows to the inch; smaller needles will yield more stitches and rows to the inch. A naturally tight knitter might have to use larger needles than a loose knitter, to achieve the same gauge with the same yarn. If you had a stressful day you might have to go up a needle size—a knitter's tension will fluctuate based on her mood or tiredness more drastically than you might expect. Ribbing and seed stitch will draw in a bit and therefore will have more stitches to the inch than stockinette stitch. The yarnovers in a lace pattern might cause the knitting to stretch out a bit, and there will be fewer stitches to the inch than a swatch of stockinette. Sometimes, a knitter's tension differs between knitting and purling, so a swatch of stockinette might have more stitches to the inch if she worked it in the round than if she worked it back and forth. Et cetera.

The upshot is that you can't really predict with any accuracy what your gauge is going to be unless you've made a sample piece and measured with a ruler how many stitches and rows there are to the inch, or the centimetre, or any other appropriate unit of length.

The sample piece is called a gauge swatch or tension swatch, and "swatching" is the process of making it. (Or them. Sometimes you need to make more than one swatch until you end up with the appropriate gauge.) Because your tension might be a bit different toward the edges of a knitted piece, you want a swatch to be amply-sized; loosely cast on at least four inches' worth of stitches. The sample gauge above says that four inches would be twenty stitches; when I'm swatching, I cast on six inches' worth for good measure. Where four inches equals twenty stitches, there are five stitches to the inch, so six inches is thirty stitches. (This is about as complicated as knitting math gets.) Work in whatever stitch pattern you want to measure your gauge over—usually stockinette stitch, sometimes a lace or textured pattern, sometimes garter stitch or ribbing, or whatever the pattern specifies—until the swatch is roughly square, then cast off as loosely as you can.

Knitted items usually end up being washed at one point or another, so wash your swatch as you would wash a finished item before you measure gauge. Doing this will help you be prepared for surprises, like cotton that shrinks or wool that blooms. At this point you may as well block it, too, again as you'd block a finished item. When the swatch is completely dry, measure gauge by laying a tape measure across a row of stitches, sticking pins in the knitting exactly four inches apart; do the same for the rows. Then count the stitches and rows between the pins, rounding to the nearest half or quarter of a stitch if you can. Write down those numbers, and the fact that you measured over four inches; this is your gauge.

Measuring over four inches is wiser than measuring over one inch because it's more likely to be accurate: if you round to the nearest half-stitch over one inch, that errant third of a stitch or whatever was rounded out of existence will come back to haunt you. Measuring over four inches minimises its impact.

When you're following a pattern, usually gauge will be provided for you. This means that you need to swatch until you are able to match that gauge: if you are getting too few stitches to the inch, go down a needle size; if you're getting too many, go up a needle size. Lather, rinse, and repeat until your gauge matches what the pattern says; bad things happen if you don't.

When gauge goes wrong

Suppose that my hypothetical Fair Isle pullover is designed to measure 40" around the bust, with a gauge of 5 stitches to the inch. Multiplying those two numbers means that I need to cast on 200 stitches, which is what the pattern calls for. But if I'm getting 4 stitches to the inch—unwittingly, since I didn't bother to swatch and see—those 200 stitches will make a sweater body fifty inches around. Disastrous!

It could just as easily go the other way: if I got 6 stitches to the inch, my 200 stitches would make a sweater about 33" around. Another disaster!

Row gauge is just as important as stitch gauge, though it often fades into the background. Knitting patterns usually specify the length of a piece to be knitted in inches or centimetres, not in rows—but when it comes to things like sleeve caps or v-necks or waist shaping, everything would be thrown off-kilter if your row gauge was wrong. Measure row gauge with the same attention to detail as you measure stitch gauge.

Often, knitted sweaters have bands of ribbing or something similar along their cuffs and hems and collars. Ribbing tends to look sloppy if it's worked on the same size needles that you'd normally use with a given yarn for a flat stitch like stockinette, but it looks tidy and "finished" if it's worked on needles that are a size or two smaller. Patterns will sometimes suggest that you do this, without giving a suggested gauge for the sections worked on smaller needles; this is usually pretty safe because it's worked over a small area and the designer has taken the gauge difference into account. But if you're curious about how it will turn out, it is safer still to swatch and see for yourself.

This is a pretty tedious process to endure if all you want to do is start knitting the goddamned project already. There are a few different ways to circumvent swatching; all of them are to be undertaken at your peril.

Shortcuts

I don't like making gauge swatches, and I'll go to great lengths to avoid them. My two favourite tactics are making things where exact size isn't terribly important—shawls or scarves or curtains or tablecloths—with texture and drape in mind instead of stitches per inch, or making garments without swatching and finding people who fit into them after they're finished. Sometimes neither of these solve anything: perhaps you want to knit something for yourself or another specific person, and perhaps you (or they) are tired of shawls and scarves.

Knitting maven Elizabeth Zimmerman suggested another tactic for dodging gauge swatches, this time when making a sweater: start in on a sleeve with needles you think are the right size, and measure after a few inches to see if you're getting gauge. If you are, congratulations; you're a few inches into a sleeve. If you aren't, rip back and try again with different needles; you aren't out too much time and effort.

This is a fine strategy if you're pressed for time and think you'll be close enough to gauge for the gamble to pay off. Unfortunately, it's still safer—and probably faster in the long run—to bite the bullet and make a proper gauge swatch before you start in on the project; it might be irritating, but you'll know for sure that the end result will turn out exactly the size that you want it.

for hapax, with thanks to DejaMorgana for input.

There are three main differences between both rifles and pistols, on the one hand, and shotguns on the other. The first major difference would be that of the construction of the barrels. In shotguns, much like in ancient cannon, the bore in which the projectile travels and eventually leaves the gun is smooth and lacks any sort of rifling. Rifling, or the grooves cut in the inside of a rifle or pistol barrel, is used to give the bullet spin. And by giving a bullet spin, one gives the bullet accuracy. Except for cases in which shotgun barrels are specifically manufactured to shoot slugs and therefore rifled (to be used for deer hunting in certain areas), all shotguns are smoothbores.

Another main difference between said firearms is that of the projectile which they shoot. Rifles and pistols shoot bullets, or solid metal projectiles usually made of lead and more often than not jacketed in some fashion by copper. Bullets come in different calibers and are made larger or smaller, and by proxy heavier or lighter in weight, for their intended target. Except in the case of shotgun slugs as mentioned above, shotguns shoot many small round metal pellets, collectively and singularly called "shot," instead of a single projectile. A musket could be used in the same way: fill the barrel with the required powder charge and proceed to fill the barrel with whatever one could find to stuff in it and off you go. A shotgun is meant to produce a great deal of damage one little bit at a time and to cover a wider area than a single bullet in order to deal this damage.

The third and final difference between the aforementioned firearms is that of how one measures the bore. The bore is the internal diameter of the barrel in which the bullet or shotshell's contents travel from the cartridge to the target. For those in the UK, a shotgun "bore" is the same thing as we in America call the "gauge"; and by gauge, I mean the measurement of the diameter of the internal part of the barrel(s) themselves, excluding the choke. Except in the case of the .410 bore, all shotguns are measured not by their diameter in inches, as are rifles and pistols, but by another measure entirely.

Shotgun gauge is measured by the amount of spherical balls that fit the diameter of the bore which equal one pound in weight. Once done, the resulting number of balls is the resulting gauge. Please recall that I mentioned ancient cannon. The reason for measuring gauge in this way is because in ancient times, cannon, too, was also a smoothbore. In the case of cannon, which also fired spherical shot in a smooth barrel, one measured the size of the cannon by the amount of weight that comprised the cannonball. Thus, a 10 pound cannon shot a 10 pound cannon ball. This 10 pound cannonball was of such a diameter that a relationship could be established: all 10 pound cannonballs could be made and be loaded into a 10 pound cannon, itself which had a certain diameter. So, too, it was with muskets. Obviously not as large as cannon, muskets were also referred to by the weight of the projectiles they shot. Because of their diminutive size compared to cannon, one had to fit a smaller projectile into a smaller tube. Therefore, the old way of measuring what kind of bore was being used was turned into fractions of a pound in weight. Thus, if a musket shot a spherical projectile that was 1/16th of a pound, it was called a 16 bore because it would take 16 such projectiles to equal one pound in weight. And in America, the 16 bore was translated into the 16 gauge.

Since modern day shotguns are the direct descendants of ancient cannon and muskets, it is no wonder that shotguns are measured in this way.

Gauge (?), v. t. [imp. & p. p. Gauged (?); p. pr. & vb. n. Gauging (?)] [OF. gaugier, F. jauger, cf. OF. gauge gauge, measuring rod, F. jauge; of uncertain origin; perh. fr. an assumed L. qualificare to determine the qualities of a thing (see Qualify); but cf. also F. jalon a measuring stake in surveying, and E. gallon.] >[Written also gage.]

1.

To measure or determine with a gauge.

2.

To measure or to ascertain the contents or the capacity of, as of a pipe, barrel, or keg.

3. Mech.

To measure the dimensions of, or to test the accuracy of the form of, as of a part of a gunlock.

The vanes nicely gauged on each side.
Derham.

4.

To draw into equidistant gathers by running a thread through it, as cloth or a garment.

5.

To measure the capacity, character, or ability of; to estimate; to judge of.

You shall not gauge me
By what we do to-night.
Shak.

© Webster 1913.

Gauge, n. [Written also gage.]

1.

A measure; a standard of measure; an instrument to determine dimensions, distance, or capacity; a standard.

This plate must be a gauge to file your worm and groove to equal breadth by.
Moxon.

There is not in our hands any fixed gauge of minds.
I. Taylor.

2.

Measure; dimensions; estimate.

The gauge and dimensions of misery, depression, and contempt.
Burke.

3. Mach. & Manuf.

Any instrument for ascertaining or regulating the dimensions or forms of things; a templet or template; as, a button maker's gauge.

4. Physics

Any instrument or apparatus for measuring the state of a phenomenon, or for ascertaining its numerical elements at any moment; -- usually applied to some particular instrument; as, a rain gauge; a steam gauge.

5. Naut.

1. Relative positions of two or more vessels with reference to the wind; as, a vessel has the weather gauge of another when on the windward side of it, and the lee gauge when on the lee side of it.
2. The depth to which a vessel sinks in the water.

Totten.

6.

The distance between the rails of a railway.

⇒ The standard gauge of railroads in most countries is four feet, eight and one half inches. Wide, or broad, gauge, in the United States, is six feet; in England, seven feet, and generally any gauge exceeding standard gauge. Any gauge less than standard gauge is now called narrow gauge. It varies from two feet to three feet six inches.

7. Plastering

The quantity of plaster of Paris used with common plaster to accelerate its setting.

8. Building

That part of a shingle, slate, or tile, which is exposed to the weather, when laid; also, one course of such shingles, slates, or tiles.

Gauge of a carriage, car, etc., the distance between the wheels; -- ordinarily called the track. -- Gauge cock, a stop cock used as a try cock for ascertaining the height of the water level in a steam boiler. -- Gauge concussion Railroads, the jar caused by a car-wheel flange striking the edge of the rail. -- Gauge glass, a glass tube for a water gauge. -- Gauge lathe, an automatic lathe for turning a round object having an irregular profile, as a baluster or chair round, to a templet or gauge. -- Gauge point, the diameter of a cylinder whose altitude is one inch, and contents equal to that of a unit of a given measure; -- a term used in gauging casks, etc. -- Gauge rod, a graduated rod, for measuring the capacity of barrels, casks, etc. -- Gauge saw, a handsaw, with a gauge to regulate the depth of cut. Knight. -- Gauge stuff, a stiff and compact plaster, used in making cornices, moldings, etc., by means of a templet. -- Gauge wheel, a wheel at the forward end of a plow beam, to determine the depth of the furrow. -- Joiner's gauge, an instrument used to strike a line parallel to the straight side of a board, etc. -- Printer's gauge, an instrument to regulate the length of the page. -- Rain gauge, an instrument for measuring the quantity of rain at any given place. -- Salt gauge, or Brine gauge, an instrument or contrivance for indicating the degree of saltness of water from its specific gravity, as in the boilers of ocean steamers. -- Sea gauge, an instrument for finding the depth of the sea. -- Siphon gauge, a glass siphon tube, partly filled with mercury, -- used to indicate pressure, as of steam, or the degree of rarefaction produced in the receiver of an air pump or other vacuum; a manometer. -- Sliding gauge. Mach. (a) A templet or pattern for gauging the commonly accepted dimensions or shape of certain parts in general use, as screws, railway-car axles, etc. (b) A gauge used only for testing other similar gauges, and preserved as a reference, to detect wear of the working gauges. (c) Railroads See Note under Gauge, n., 5. -- Star gauge Ordnance, an instrument for measuring the diameter of the bore of a cannon at any point of its length. -- Steam gauge, an instrument for measuring the pressure of steam, as in a boiler. -- Tide gauge, an instrument for determining the height of the tides. -- Vacuum gauge, a species of barometer for determining the relative elasticities of the vapor in the condenser of a steam engine and the air. -- Water gauge. (a) A contrivance for indicating the height of a water surface, as in a steam boiler; as by a gauge cock or glass. (b) The height of the water in the boiler. -- Wind gauge, an instrument for measuring the force of the wind on any given surface; an anemometer. -- Wire gauge, a gauge for determining the diameter of wire or the thickness of sheet metal; also, a standard of size. See under Wire.

© Webster 1913.