Converting units

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Converting a quantity means writing the same amount in a different unit: five kilometres as miles, a hundred centimetres as metres, a temperature in Celsius as Fahrenheit. The amount does not change, only the unit it is expressed in.

to, in and into all convert.

5 km to miles // 3.11 miles
100 cm into m // 1.00 m
1 hour to minutes // 60 minutes
72F to C // 22.22 C
20C in F // 68.00 F

A conversion between two different dimensions has no answer, so it is refused rather than guessed. The message names the dimensions rather than the units, so 1 hour in metres reports a duration cannot be converted to a length and 5 kg in m reports a mass cannot be converted to a length. Combining two different dimensions is refused the same way: 5 kg + 3 m reports mass and length cannot be added.

Only a number or a quantity has an amount to convert. A piece of text or a colour does not, so converting one is refused rather than answered as zero of the unit. A list converts every cell, since each cell is an amount (see lists and units):

"two" in miles // ERROR: Text has no single amount to convert to miles: only a number or a quantity can be converted.
(1, 2) in miles // [1.00 miles, 2.00 miles]

A time written as a word agrees with its count, the way it is said: one hour, two hours. A count of exactly one takes the singular and every other count the plural, whichever spelling was typed, so converting 3,600 seconds into hours gives one hour rather than one hours. A symbol such as h or min has no plural and is written as it is.

3600 seconds in hours // 1 hour
2 hour // 2 hours
90 minutes in hours // 1.50 hours
0 hours // 0 hours

The boundary: only the time words (second to year, the words a duration is already written in) change their spelling. Other unit words are written as the value carries them, so 1609.344 m in miles is 1.00 miles, and a count shown with places is plural even when it rounds to one, because the places say it is a measurement rather than a count.

Asked the other way round

“How many metres are in a kilometre?” is a conversion asked the other way round: the unit you want comes first, and the amount comes after in. Write it that way and it is the same question as 1 km in m, with the same answer.

km in 1 mile // 1.61 km
m in 1 furlong // 201.17 m
km in a furlong // 0.20 km
g in 1 carat // 0.20 g
kJ in 1 kcal // 4.18 kJ
mW in 1 W // 1,000.00 mW
seconds in a day // 86,400 seconds
km in -1 mile // -1.61 km

Every unit a conversion reads can be asked for this way, the less common units such as the furlong and the carat included, and a unit is read in its own case, as everywhere else, so mW is the milliwatt and MW the megawatt. A currency works too, once its rate is known (USD in 1 EUR).

The form is kept narrow on purpose: the line must start with the unit, and what follows in must be a plain amount and a unit, a signed amount (-1 mile), or a or an standing in for one. So days in February 2020, which asks how long a named month is, is left as its own question.

Units written in more than one word

Some units are named in two or three words, or with a hyphen: a nautical mile, a square foot, a cubic metre, an imperial gallon, a light-year. Each reads as the one unit it names, after an amount or after the conversion word.

5 km in nautical miles // 2.70 nautical miles
1 nautical mile in km // 1.85 km
5 cubic metres in litres // 5,000.00 litres
3 imperial gallons in litres // 13.64 litres
10 US fluid ounces in ml // 295.74 ml
2 troy ounces in g // 62.21 g

The spellings are the unit table’s own, so nothing is guessed at: the words are separated the way the table writes them, one space, or a hyphen in light-years, and a spelling the table does not carry stays unread. The one allowance is a plural. A few table entries have only the singular (troy ounce, watt-hour), and the plural a reader writes reads as that unit. A symbol takes no plural, so kW h is the kilowatt-hour and kW hs is not a unit.

A few two-word spellings are the engine’s own rather than the table’s, each naming a size the plain word leaves ambiguous: the metric and imperial cups (see cooking), miles per imperial gallon (see fuel economy), and the typographic point below.

The typographic point

Type is sized in points: a 12-point font is 12 of them tall, and a point is a 72nd of an inch (the desktop publishing point, which is the one CSS and every word processor use). The short spellings are taken, pt by the pint and point by ordinary English (“scored 12 points”), so after a number the point is written typographic point:

12 typographic points in mm // 4.23 mm
72 typographic points in inches // 1.00 inches
1 inch in typographic points // 72.00 typographic points
1 pica in typographic points // 12.00 typographic points

pt stays the US pint, so 12 pt in mm is refused as a volume that cannot become a length rather than read as type. points still works as a conversion target, where there is no prose for it to collide with:

12 pt in mm // a volume cannot be converted to a length
1 pica in points // 12.00 points

Litres with a capital L

The litre is written l or L, and the capital is the symbol most bottles and cartons print, since a lower-case l is easily read as the digit one. Both are the same unit, as are ml and mL.

2 L in ml // 2,000.00 ml
1.5L + 500 ml // 2.00 L
500 ml in L // 0.50 L

The boundary: the capital is a unit only straight after a number, so a variable named L keeps working on its own line and after an operator (see variables). No other single capital letter is added.

Micro, with either µ or μ

The micro prefix means a millionth: a microsecond is a millionth of a second, a microgram a millionth of a gram. Its symbol is the Greek letter mu, and it reaches a line as one of two characters that look the same: the micro sign µ (U+00B5, what Option-M types on a Mac) and the Greek small letter μ (U+03BC, what a Greek keyboard types). Both are read as the prefix.

5 µs in ns // 5,000.00 ns
5 μs in ns // 5,000.00 ns
1 mL in µL // 1,000.00 µL
3 µm in nm // 3,000.00 nm

The ASCII stand-in us is not read as microseconds, because it is also an ordinary word. Write µs, or microseconds in full.

Temperatures, with or without the degree sign

°C and °F read as the units they obviously are, which is what a phone keyboard, a weather app and a recipe all write. The precomposed ℃ and ℉ that some keyboards emit read the same way, and °K is kelvin, which has no degree sign of its own.

20°C in F // 68.00 F
100°F in C // 37.78 C
180°C in gas mark // gas 4
37°C // 37.00 °C

The scale letter is what makes it a temperature, so the bare symbol is still an angle:

90° // 90.00 degrees

Every spelling of the same question agrees, so use whichever you have to hand:

20 C in F // 68.00 F
20 degrees C in F // 68.00 F
20° C in F // 68.00 F

The boundary is the symbol forms only. C is Celsius, and no ordinary word is claimed as a unit: the cooking cup is spelled cup, not c, which is not a unit at all. The case sensitivity of the unit table is unchanged.

1 cup in ml // 236.59 ml

Where two scales meet, and adding across them

Celsius and Fahrenheit put their zero in different places, so a conversion between them adds or takes away an offset as well as scaling. Where the two scales meet a single point, the answer is that point exactly: 32 °F is the freezing point, 0 °C, not a tiny number a hair away from it.

32 °F in °C // 0.00 °C
273.15 K in °C // 0.00 °C
32.0018 °F in °C // 0.001 °C

Adding to a temperature adds a difference of temperature, a number of degrees warmer, rather than a second reading. Ten Fahrenheit degrees are 5.56 Celsius degrees, so 20 °C + 10 °F is 25.56 °C, and a kelvin is a Celsius degree. The answer is in the scale of the temperature on the left, as every sum of two quantities is.

20 °C + 10 °F // 25.56 °C
20 °C + 10 K // 30.00 °C
68 °F + 10 °C // 86.00 °F
20 °C + 10 °C // 30.00 °C

Subtraction is the boundary. It still reads both sides as temperatures and converts the right one as a reading, and a difference converted afterwards is converted as a reading too. Changing either changes answers people already rely on, so it waits for the next major version, 3.0:

20 °C - 10 °F // 32.22 °C
(30 °C - 20 °C) in F // 50.00 F

Inches, where the abbreviation is also the word for converting

in is how the engine spells the conversion itself, so it cannot simply be a unit as well: 12 in ft has to keep meaning “twelve, in feet”. The word is read as inches where there is plainly nothing to convert into, which is at the end of a line, before an operator, or before a second in or a to.

12 in in cm // 30.48 cm
2 in + 3 in // 5.00 in
12 in to cm // 30.48 cm

Everywhere else it is still the preposition, including when the thing being converted into is itself inches:

3 ft in in // 36.00 in

The full spellings never have to be reasoned about at all, so they are the safer thing to write in a document somebody else will read:

12 inches in cm // 30.48 cm
1 inch in mm // 25.40 mm