inch
Lengthexact by definition
The inch is exactly 0.0254 metres, fixed by the international yard and pound agreement of 1959 and adopted by the United States, the United Kingdom, Canada, Australia, New Zealand and South Africa. Twelve inches make a foot and thirty-six make a yard. The factor is exact by definition, not a measured approximation.
Watch out: Before 1959 the American inch was 1/39.37 of a metre, about two parts per million longer than today's. That difference is invisible on a workshop drawing but not on a survey baseline, which is why the survey foot and survey inch persisted separately in US geodesy until they were retired.
| 1 in | 0.0254 m |
The name descends from the Latin uncia, a twelfth part, the same root as "ounce". Older English statutes defined it against grain: a length of "three barleycorns, round and dry", laid end to end, is the definition commonly attributed to a medieval statute, and the barleycorn survives today as the one-third-inch step between British shoe sizes.
About the inch
The inch is the last unit anyone should expect to be exactly defined, and yet it is: exactly 25.4 mm, no rounding. That number was agreed in 1959 by the six main English-speaking standards bodies, who noticed that their national inches had drifted apart by a few parts per million and settled on a value close to the average. Machine shops had in fact been working to 25.4 mm for decades before the treaty made it official, because the industrial standard published in the 1930s already used it.
What makes the inch awkward is not its size but its subdivisions. Traditional practice halves it repeatedly, giving 1/2, 1/4, 1/8, 1/16, 1/32 and 1/64, so a tape measure is a binary instrument while a caliper reading 0.4375 in is a decimal one, and both mean 7/16. Machining and aerospace drawings use decimal inches with "thou" (thousandths, 0.001 in) as the working precision, so a tolerance of "plus or minus two thou" is 50.8 μm. Reading a fractional dimension into a decimal-inch machine is a classic and expensive mistake.