One agreement behind every factor
Length is the easy dimension, and it is worth saying why, because the reason is historical rather than mathematical. In 1959 the national standards bodies of the United States, the United Kingdom, Canada, Australia, New Zealand and South Africa signed an agreement fixing the inch at exactly 25.4 millimetres and the pound at exactly 0.453 592 37 kilograms. Before that, an American inch and a British inch were defined independently and differed in the seventh decimal place.
Everything imperial on this page descends from that single constant. A foot is twelve inches, a yard is three feet, a mile is 5 280 feet — so once the inch is exact, every one of them is exact too, and the same on both sides of the Atlantic. That is the opposite of the situation in volume, where the US and imperial gallons were never reconciled and remain about 20% apart to this day.
This converter therefore offers one set of imperial lengths rather than two, and says so rather than leaving you to wonder whether a US inch was meant. The one historical exception is the US survey foot, which persisted in American surveying until the end of 2022 and was larger than the international foot by two parts per million — negligible over a room, several metres across a state.
The mil that is not a millimetre
The genuine trap in length is a naming collision rather than a system disagreement. A mil — called a thou in British engineering — is one thousandth of an inch, which is 0.0254 mm. It is not a millimetre, and it is not a millimetre’s thousandth either. Reading “5 mil” as five millimetres overstates the dimension roughly twenty-five-fold.
This matters because mils are the working unit for exactly the things where being out by 25× is expensive: plastic sheeting and film thickness, printed circuit board copper weight and trace clearance, paint and powder-coating dry film thickness, plating, and machining tolerances on American drawings. A 6-mil polythene sheet is 0.15 mm thick; a 6-millimetre sheet is a structural panel.
The converter lists it as thou with the symbol mil and a note on the option itself, so it cannot be picked by accident from a dropdown while scanning for millimetres. If you are reading a specification and cannot tell which was meant, the context usually settles it: film and coating thicknesses quoted in whole numbers between about 1 and 100 are almost always mils, because the millimetre equivalents would be implausible.
From nanometres to nautical miles
The units here span fifteen orders of magnitude, and the ones at each end belong to different worlds. At the small end, the nanometre is the unit of semiconductor process nodes and optical wavelengths, and the micrometre — still widely called the micron — is the unit of filtration ratings, machining surface finish and particle size. A human hair is roughly 70 µm; visible light runs about 380 to 700 nm.
At the large end sits the nautical mile, which is not a rounded version of anything. It is exactly 1 852 metres by international agreement, chosen because it approximates one minute of latitude — so a degree of latitude is about sixty nautical miles anywhere on Earth, which is why marine and air navigation use it and why a knot is one nautical mile per hour. A statute mile is 1 609 m, about 13% shorter, and the two are not interchangeable in any navigational context.
Because the tool shows every unit at once rather than one pairing, the orders of magnitude are visible together — which is the quickest way to catch an input that landed in the wrong box.
Feet and inches as one measurement
Human heights and room dimensions are usually written as a compound: 5 ft 11 in, not 5.917 ft. That is a display convention rather than a different unit, but it is where decimal conversions go wrong, because the fractional part of a decimal foot is not the inches. 5.9 ft is 5 ft 10.8 in, not 5 ft 9 in — the digit after the point is tenths of a foot, and a foot has twelve inches rather than ten.
To convert a compound height by hand, turn it into inches first: multiply the feet by twelve, add the inches, then convert once. 5 ft 11 in is 71 in, which is 180.34 cm. Going the other way, divide by 2.54 to get inches, take the whole number of feet, and the remainder is the inches.
Carpentry adds fractions on top: American plans mark inches in sixteenths rather than decimals, so 3.4375 in is 3 7⁄16 in. A tape measure has no decimal markings, so a decimal answer has to be turned back into the nearest sixteenth before anyone can cut to it.
Doing it in a spreadsheet
Excel and Google Sheets both have CONVERT, and for length the unit codes are unambiguous: =CONVERT(A1,"in","cm"), "ft", "yd", "mi", "Nmi" for the nautical mile, and the metric ones by SI prefix. Unlike volume there is no US-versus-imperial pair to pick wrongly here.
For a compound height, the formula worth keeping is the one that splits back out: with total inches in A1, =INT(A1/12) gives the feet and =MOD(A1,12) gives the inches. Rounding to the nearest sixteenth is =MROUND(A1,1/16), which is what a tape measure can actually be read to.
Sources and methodology
Nothing here is fetched and there is no data feed: every figure is computed from what you type. The arithmetic needs no authority; the definitions do, and that is what the sources settle — the 1959 agreement fixing the inch, the retirement of the US survey foot, the metre’s own definition, and the nautical mile as a separate exact integer.