Sevres.science / The SI

Writing units right

A space between number and unit

Write 25 kg, 3 m, 230 V and 20 °C, always with a space. The exceptions are the degree, minute and second of plane angle, which follow the number directly: 30°, 22′.

Symbols are fixed

Unit symbols are printed upright, whatever the surrounding text. They never take a plural ending (5 kg, not 5 kgs) and have no full stop after them, except at the end of a sentence. Case matters: kg, not Kg; s, not sec.

Symbols for quantities, by contrast, are written in italics: m for mass, t for time. That is how a reader tells the mass m from the metre m.

Do not mix names and symbols in one expression. Write km/h or kilometres per hour, not km per hour.

Multiplying and dividing units

A product of units is written with a space or a half-high dot: N m or N·m. A quotient uses a solidus or a negative power: m/s or m s⁻¹. Use only one solidus unless brackets make the meaning clear: m/s², not m/s/s.

Prefixes

A prefix symbol is joined to the unit symbol with no space: km, mg, ns. Case changes the meaning: mW is a milliwatt, MW a megawatt, a billion times more.

Never stack prefixes: write nm, not mμm. The kilogram is the one base unit whose name already contains a prefix, so prefixes go on the gram instead: a millionth of a kilogram is a milligram (mg), not a microkilogram.

Writing numbers

Group long numbers in threes with a thin space, on both sides of the decimal marker: 299 792 458 and 6.626 070 15. Never use points or commas to group digits, because in half the world they mark decimals. Four-digit numbers are usually left whole: 1875.

The decimal marker may be a point or a comma, following the custom of the language. English uses a point; Polish uses a comma.

One unit per value

Give a value in a single unit: 10.234 m, not 10 m 23 cm 4 mm. Time and plane angle are the usual exceptions, so 1 h 30 min and 30°22′8″ are fine.

Ranges and products

Repeat the unit for each number, or put the numbers in brackets: 0 V to 5 V, or (0 to 5) V, but not 0 – 5 V. For dimensions write 51 mm × 51 mm × 25 mm, not 51 × 51 × 25 mm. A word such as “to” is clearer than a dash, which can be mistaken for a minus sign.

Numerals and symbols, not words

In technical text, write values with numerals and unit symbols: 5 m, not five metres. Spelled-out names are fine in descriptive or literary text, but never mix the two: “five m” is wrong in any style.

A unit symbol never stands alone without a number or a quantity symbol. Write “the length in metres”, not “the length in m”.

Choosing a prefix

Where practical, choose the prefix so that the number lies between 0.1 and 1000, and use the prefixes that go in steps of a thousand: 12 km rather than 12 000 m, 3.5 mg rather than 0.0035 g. In a table, keep one prefix for the whole column.

Per cent, ppm and other pure numbers

The NIST guide to the SI writes the per cent sign with a space, 0.25 %, and treats it simply as the number 0.01. The guide does not accept abbreviations such as ppm, ppb and ppt with the SI; it recommends writing 3 × 10⁻⁶ or using a ratio of units instead. Legal texts that require ppm are the exception.

More on two questions that come up again and again in the articles: how to write degrees Celsius and whether a kilobyte has 1000 or 1024 bytes.