The letters in stand on the ruler exactly at the first inch, and a caliper comes in from the right and clamps them, and 25.40 lights up on the jaw's little screen. On the millimetre scale below, a red marker moves to the same mark, along the second inch three barleycorns are laid one after another, a glint runs along the steel beam, and in the corner the lathe gears with 127 and 50 teeth turn, linking an inch lead screw to a metric thread.
Quantity data sheet
length · imperial measure sheet 1/1 · rev. 2026-09Inch in · ″
Non-SI unit · quantity: length; exactly 25.4 mm since 1959
The unit the whole world measures in millimetres
A plumber in Madrid and a plumber in Toronto ask their supplier for the same “one-inch” pipe, and neither of them measures it in inches, because it is 33.7 millimetres across the outside and about twenty-seven inside. The pipe inherited its name from the cast-iron water mains of the nineteenth century, whose inch was the inner bore; the walls have since grown thinner but the name has stayed, so the inch lives on in a world that switched to the metre long ago.
The real inch, meanwhile, is simpler than any metric unit: since 1959 it has been exactly 25.4 millimetres, with no tail after the decimal point, and this roundness is not natural but agreed. Before the agreement the British and American inches differed by almost four parts per million, and gauge blocks made on opposite sides of the Atlantic did not match at the factory check.
The sheet shows which inches are honest and which survive only in a name, why the inch has twelve parts, and why every lathe with an inch lead screw keeps a gear with one hundred and twenty-seven teeth in its drawer.
The symbol in is written in lower case and without a full stop, like any unit symbol: 12 in, not 12 In. or 12 ins. On drawings and packaging the double prime ″ lives alongside it, placed right after the number with no space, 12″, and it is not replaced by two apostrophes or a keyboard quotation mark.
The inch of length versus the inch in a product's name: a one-inch water pipe is more than 33 millimetres across the outside, a one-inch G 1 thread is 33.25 mm, and a “two-by-four” board is really 38 by 89. Nor should it be confused with the inches of other countries, from the Paris pouce to the Swedish tum: before metric reform every capital had its own, from 24 to 27 millimetres long.
01 · Definition
yard · metre · nominalThe inch is a unit of length equal to one thirty-sixth of the international yard, and since the yard has been defined since 1959 as 0.9144 metre, the inch is exactly 25.4 millimetres. The inch has no standard of its own: it rests entirely on the metre, and the metre on the speed of light, so all its accuracy is borrowed.
Two traditions of division live inside the inch. In the workshop it is halved again and again, into halves, quarters, eighths and sixty-fourths, because that makes it easy to mark a ruler or size a drill; in engineering it is divided into a thousand, and the thousandth of an inch, the “mil” or thou, equals 25.4 micrometres.
Apart from the unit there is the nominal inch, the inch in a product's name: for a pipe it points to an old bore, for a board to its size before planing and drying, for a floppy disk to a rounded width. Such an inch cannot be converted to millimetres by multiplication; it is looked up in the standard's table.
How many inches in a one-inch pipe
At the top the object itself is drawn to scale, pressed against the zero of a tape measure, and beneath it a steel tape with two scales: inches with divisions down to sixteenths on top, millimetres below. The tape runs out to the edge of the object; the blue line marks what was measured, the brass dotted line what the name promises, and the red fill between them is the difference between the nominal inch and the real one.
Inch and ounce are the same word
The Romans divided almost everything into twelve parts, from the foot to the pound to an inheritance, and one twelfth was called uncia; from it English got both inch and ounce. Twelve suits a workshop better than ten because it divides without remainder by two, three, four and six, which is why a carpenter dividing a board into three equal parts still prefers feet, while a designer adding up dimensions in a column prefers millimetres.
02 · Conversion
units · old inches · nominalsFrom sixteenths to millimetres
Enter a length in any of the units, and the sheet will show it in metric and imperial measures with the nearest sixteenth, convert it into the old inches of various countries and compare it with the inches in product names. The length is the same in every tab; only what it is measured with changes.
A rule to remember: four inches is a little over ten centimetres, and five inches is exactly 127 millimetres.
The old inches of various countries are rounded to a hundredth of a millimetre: they changed from century to century and from standard to standard, and one country often had two or three inches at once, for cloth, for land and for timber. Nominal product sizes are taken from common standards and may differ by fractions of a millimetre between manufacturers.
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04 · Measuring instruments
tape measure · vernier · micrometer · gauge blocksFrom a sixteenth to a millionth
The hook that wobbles
The tape measure is the world's main inch instrument, and the hook at its end deliberately sits on rivets with play exactly equal to the hook's own thickness: measuring outside, it slides out; pushed against an inside surface, it slides in, and zero is in the right place either way. A carpenter who “fixes” the loose hook will be off by a millimetre at the next measurement.
Twenty-five divisions over twenty-four
On an inch caliper the main scale divides the inch into forty parts of 0.025, and the vernier fits twenty-five of its divisions over twenty-four main ones, so the difference between them is one thousandth of an inch. The number of the vernier line that lines up gives the third decimal place, with no electronics and no battery.
Forty threads per inch
The micrometer spindle is a precision screw with forty threads per inch, so one turn of the thimble advances it exactly 0.025 inch, and twenty-five divisions on the thimble cut that step into thousandths. The inch micrometer proved convenient precisely because of this arithmetic, and its metric sibling repeated it with a half-millimetre pitch and fifty divisions.
Johansson blocks
The Swedish machinist Carl Edvard Johansson invented a set of steel blocks that wring together so tightly they hold without glue, and from a couple of dozen pieces any size can be assembled to a ten-thousandth. For his inch sets he used a ratio of exactly 25.4 millimetres, and industry adopted that figure long before governments ratified it.
05 · Writing rules
symbol · double prime · fractionsTwo primes, not two quotation marks
In text the inch is written with the symbol in after a space, on drawings and products with a double prime and no space, and fractions are kept as common fractions if the size is marked in sixteenths and as decimals if it comes from a machine.
The first line on the right adds a plural and a full stop to the symbol, the second types the double prime as two apostrophes and a straight quote, and the third rounds an exact value, although 25.4 is not an approximation but a definition. The fourth turns the symbol into an abbreviation of the word, the fifth converts a pipe's nominal size as if it were a real inch and is off by eight millimetres, and the sixth writes six feet two inches as a decimal, although 6.2 feet is six feet and 2.4 inches.
06 · Neighbouring units
yard · mile · pointThe inch is the youngest member of the imperial ladder, in which twelve inches make a foot, three feet a yard and 1760 yards a mile; below it lie the typographic point, one seventy-second of an inch, and the web page pixel, one ninety-sixth.
The inch is defined through the yard: thirty-six inches are exactly 0.9144 metre.
The top of the ladder, where the two-parts-per-million difference between inches already shows in millimetres.
The typographic small change of the inch, in which almost every font on screen is set.
07 · Historical section
archive · from barley to the metreHow the inch became metric
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A difference only a gauge block can see
The bar shows how much each inch differed from today's 25.4 millimetres, in nanometres: up means longer, down means shorter. The British imperial inch was 44 nanometres shorter, the American 51 longer; the frame visits them in turn.
For a joiner this is nothing, but two parts per million over a thousand kilometres of survey add up to two metres, which is why American surveyors kept the old “survey” foot until the end of 2022.
One hundred and twenty-seven teeth between two worlds
The industrial inch was set at 25.4 millimetres not only for its roundness: 254 factors as 2 × 127, so 25.4 is exactly 127 fifths. On a lathe with an inch lead screw, a 127-tooth gear paired with a 50-tooth gear converts an inch thread pitch into a metric one without any error, and such a pair lay in every turner's drawer for decades.
Had the inch stayed at the old 25.399956 millimetres, an exact conversion would have needed a gear with more than two million teeth, and threads would have had to be cut with accumulated error. Thus the metrology of the lathe got ahead of diplomacy: factories adopted 25.4 in the thirties, and governments signed it only in 1959.
Since then the inch has had no length of its own: all its standards are metre standards multiplied by 0.0254, and the accuracy of any inch gauge block in the world traces back to the same speed of light as the accuracy of a millimetre.