Manual: Units of Measurement

How do I correctly show the temperature in a weather app: +30 C? +30° C? +30 °С? What about displaying the flight time in a travel search engine: 2h 30min? 2 h. 30 min.? 2 h 30 min? The apartment size in a housing rental service: 50 sq m? 50 m² ? 50 sq.m? We are to address these and many other questions in our new manual.

Some general rules

Any abbreviation of a measurement unit should start with a lowercase letter — besides those formed from last names. For example, the unit of force, the newton, is to be abbreviated as N, even though the full word begins with a lowercase n.

You should insert a non-breaking space between the numerical value and its name, as there mustn’t be a line break between them.

Abbreviated names of measurement units are written without a period unless they sit at the end of the sentence.

One should opt for upright styles to write down units of measurement, whereas italics are typically used to write down the properties of an object. For example, mass, which is considered a property in physics, is denoted by an italic letter m, while meters are abbreviated as an upright letter m.

In texts set in Belarusian, Tadjik, Kazakh, Kyrgyz, Ukrainian, and Russian (even those that must meet the requirements of the GOST standard), one may use the international designations in Latin.

If calculations are made in thousands, say, meters, you can lose the last three zeros and add the letter k, which stands for the prefix kilo- (which is Greek for ‘thousand’). The same can be done if you calculate in thousandths of meters — by adding the letter m, which stands for the prefix milli-. In this case, the letter k or the letter m are to be lowercase. If you operate with millions or billions, the letters that stand for the unit prefixes mega- and giga- should be capitalized.

If the unit of measurement features the prefix micro-, the international abbreviation starts with the letter μ. The letter m is used to abbreviate words with the prefix milli-.

Abbreviations mln (mn, M) and bn (B) are written without a period, but if you deal with regular text rather than technical documentation, it makes sense to spell out the entire numeral. In a long text, ‘20 million’ is easier to perceive than ‘20,000,000’, but if you’re making a pitch deck for startup investors, a number with a large amount of zeros will look more impressive. You should write out numbers in nontechnical writing — twenty billion, one hundred million.

The book cover for Twenty Thousand Leagues Under the Sea, 1871

Time

There must be no period after min, sec (or s), or h in English. At the same time, the century (c.), which is not regarded as a physical quantity, should be followed by a period. Whenever you use the word in the plural, meaning centuries, place a period after the letter s (cs.) The same rule applies to French and German.

It isn’t right to use the prime symbol or double prime symbol instead of ‘minutes’ or ‘seconds’ when we talk about time. Minutes and seconds of arc are a different thing, those are units of angular measurement, and that’s where we’re supposed to use the prime symbol.

When describing antiquities, it is common practice to indicate not only the century but also the era to which a work of art or an artifact belongs. German and Russian use shortened phrases for this end. In the church context, it’s abbreviated phrases v. Chr. “before Christ (was born)”, and n. Chr. “after Christ (was born)”. In the secular context, it’s “before our era” — v. u. Z., and “of our era”, u. Z. A period is needed after each abbreviated word and the words should be spaced.v. Chr. and n. Chr. used to be the preferred abbreviations before 1992, but nowadays the indication of the era is not regulated by any standards.

English uses acronyms to indicate the era. AD (anno Domini, Latin for “after Christ”) and BC (“before Christ”) are the most commonly used. However, in sciences, such as astronomy, it is common practice to use the acronyms CE (of the Common Era) and BCE (before the Common Era).

Oxford and Merriam-Webster recommend writing these abbreviations without periods, but using periods is also common. AD, BC, CE, and BCE are often set in small caps.

Era designations, at least in the Western world, are usually expressed in one of two ways: either CE (“of the Common Era”) and BCE (“before the Common Era”), or AD (anno Domini, “in the year of the Lord”) and BC (“before Christ”). Other forms include AH (anno Hegirae, “in the year of [Muhammad’s] Hegira,” or anno Hebraico, “in the Hebrew year”); AUC (ab urbe condita, “from the founding of the city [Rome]”); — for archaeological purposes — BP (“before the present”). Note that the Latin abbreviations AD and AH precede the year number, whereas the others follow it. (All abbreviations follow a spelled-out century, as in “the third century AD.”) The University of Chicago Chicago Manual of Style

Page from the San Francisco Oracle, 1968. Image: Letterform Archive

The identity of the Museum of Archaeology, 2020. Image: Looch

Although the Latin abbreviation AD should precede the year, it is common to see it following the year in today’s journalistic writing. Length and distance

Length and distance

The international units of length and distance are meters (m), millimeters (mm), centimeters (cm), and kilometers (km). In the British system of measurement (also known as the Imperial System), used in the UK, USA, Myanmar, and to some extent in Canada, the units for length or distance measurement are the inch (2.54 cm), the foot (30.48 cm), the yard (91.44 cm), and the mile (1609.344 m). There’s also the nautical mile used in air and marine navigation, equal to 1,852 metres.

The inch is shortened as in or denoted by a double prime (″), the feet are ft or a prime (′), yards are abbreviated as yd, miles and nautical miles as mi and nmi respectively. The prime symbols used to denote arcseconds and arcminutes are the same symbols used to represent inches and feet.

Single and double primes should not be confused with quotation marks, which in some faces (frakturs especially) also take the shape of sloped primes. Robert Bringhurst The Elements of Typographic Style

Typography purists would point out that proper foot and inch marks have a slight northeast-to-southwest slope to them. HTML purists would further point out that these characters have their own escape codes (&primе; and &Primе;). True, and you are welcome to seek them out. Matthew Butterick Butterick’s Practical Typography

Speed

Since speed is a distance divided by the time during which the distance is travelled, the units for measuring speed are the units of distance divided by the units of time: for example, kilometers per hour (km/h), and meters per second (m/s). Those are written using the slash and without a space. According to the current typographic convention, the speed in miles per hour is abbreviated as mph, however, you can also see mi/h in technical writing.

Area

The area is measured in square millimeters, centimeters, meters, kilometers, or hectares, while in the UK, USA, and (to some extent) Canada, they also use acres (ac), square miles (sq mi or, less commonly, mi² ), square feet (ft² ) and square inches (in² ). The square meter is abbreviated as m to the second power, which is m² (and it is km² when we talk square kilometers). It is not advisable to use the form ‘sq m’ in English. All the abbreviations mentioned above are written without a period at the end.

If you’re supposed to deal with text containing superscripts, make sure the font you choose features the appropriate set of figures. If there is no such set, the graphic editor software automatically reduces the size of lining figures. A reduced figure will look considerably lighter than the preceding m and the superscript — due to automatically reduced letter spacing — may collide with the following symbol (such as a bracket).

There are also properly designed numerators and denominators to set fractions (as opposed to figures automatically reduced to fraction size or just the figures for quarter, half and three quarters), specially designed figures for sub- and superscript, even figures to go with small caps, and there again lining or oldstyle. Erik Spiekermann Stop Stealing Sheep & Find Out How Type Works

Newsreader — with an index (on the left) and an automatically reduced numeral two (on the right).

Weight

When we say ‘weight’ in our everyday life, we most often mean mass. In physics, you multiply mass by the gravitational acceleration to get weight. However, this substitution is not considered a mistake in colloquial usage.

In the SI, mass is measured in kilograms (kg), grams (g), and tonnes (t). The carat, which most people associate with diamonds, is also a unit of mass. One carat is equal to 0.2 g.

In the British imperial measurement system, the mass is also measured in ounces (oz) and pounds (lb). One ounce equals 28.35 grams, and one pound is 453.59 grams or 16 ounces.

One may write abbreviations for ounces and pounds either without or with a period.

Volume

The internationally accepted measure of volume is a cubic meter. Its symbol is the letter m raised to the third power (m³ ). Volume is also measured in cubic millimeters, centimeters, and kilometers.

The volume of liquid is measured in liters (L, l). The USA and the UK also use fluid ounces — one liquid ounce equals 30 ml. It is abbreviated as oz or fl oz. It is common to see it written with a period in cookbooks.

Teen Cuisine. French cuisine beginners guide. Image: Elizabeth Goodspeed

In the US, Canada, and Australia, the uppercase L for liters is preferred since it is easily distinguished from the digit 1. United States government Writing with Metric System Units

l, L and 1 in Rooftop Mono

Density

Density is a substance’s mass per volume it occupies, measured in kilograms or grams per cubic meter (kg/m3, g/m³ ).

However, paper density is the ratio of its mass to its area and not to its volume. It is measured in grams per square meter (g/m² ). Indicating the density of a sheet of paper, we basically say how many grams a sheet of such paper with an area of one square meter would weigh.

Munken papers catalogue

Electric energy consumption

A kilowatt-hour is a non-SI unit of energy. It is used in many countries as a billing unit for electrical energy supplied by utilities. One kilowatt-hour equals the amount of energy used by a one-kilowatt electrical device during one hour of operation.

Since we address power and not the distance travelled in a certain period, writing kW/h is incorrect. In Cyrillic, the amount of energy is spaced from the time by an interpunct (U+00B7) — кВт⋅ч, while the Latin uses a dot operator (U+22C5), kW⋅h, or a space, kW h.

Electric energy consumption meter. Image: Amazon

The symbol for a compound unit that is a product of two units is indicated by a multiplication dot (dot operator) or by a space. United States government Physical Measurement Laboratory. Writing with Metric System Units

Despite the fact that this violates the guidelines, kilowatt-hours are most often written without a space — kWh, with the proper form of this abbreviation found only in documents and on electricity meters.

Technical Specifications of the Bosch Refrigerator. Image: Bosch

Image resolution

Resolution indicates the number of dots per unit area or length. The more dots per unit, the more accurate the image will be. The resolution of print layouts and printers is measured in dots per inch (dpi). Display resolution is measured in pixels per inch (ppi).

Image size

The size of a digital or printed image is described in the same way — the product of length and width, but you use pixels (px) instead of centimeters or millimeters if it’s digital. In both cases, be it a photograph 10×15 cm or 1000×1500 px, using the symbol × rather than the letter x is crucial. Numbers should be separated from the × sign by thin spaces.

When indicating the size of a digital image, it’s common practice to omit the unit of measurement.

Type size

Type is measured in (typographic) points. The abbreviation for points is pt. At various times and in different countries, the point was equal to 0.3473 mm, 0.3759 mm, 0,3514 mm, and 0.375 mm. In today’s Adobe software, one point equals 0.3527777… mm That said, not every 144-point font will have the highest character of 50.8 mm — the character height depends on the typeface’s design and the number of languages it supports.

Unlike Adobe software, Figma by default uses a unit called a device-independent pixel (dip, dp) to define the font’s size. For instance, in Android OS, 1 dip equals one pixel on a screen with a resolution of 160 ppi. At the same time, if you use Figma to handle printed formats (A4, A5, A6), the software will render the font in points, and if you’re working on a layout for digital purposes, it will use pixels.

Litogaphia by Julius Klinghart founry, 1906. Cast in two sizes: 16 pt and 28 pt. Image: Letter Library

Blood alcohol level, grade of a railway, water salinity

Blood alcohol concentration, water salinity, and railway incline (in Europe) are expressed in per mille. It is not a unit of measurement but one thousandth (‰) or 0.1 percent of a number. The sign was formed from the percent sign (%) by adding another zero to the denominator.

According to the ISO standard and the style guide of the European Parliament, the per mille symbol (as well as the percent symbol) should be spaced with an en space. However, the style guide for authors and translators working for the European Commission and Chicago Manual of Style state that there’s no need for any space. In practice, the per mille symbol (as well as the percent sign) is most often not spaced from the preceding number.

Temperature and angle

The degree symbol is used to express temperature and is also an angular measure. In music, it is used to designate the note on which a diminished triad is built.

Diminished triad beginnig with F♯ and G♯ in The Well-tempered Klavier

If the temperature scale is not specified, there should be no space between the number and the degree symbol (30°). If the scale is indicated (for example, Celsius, 30 °C, or Fahrenheit, 30 °F), there should be an en space between the degree symbol and the number. The number, the scale and the degree symbol should not be placed on differernt lines.

°С and °F have their own Unicodes (U+2103 and U+2109), but since 2017, the Unicode Consortium advises against introducing these symbols into typefaces and suggests using the degree symbol and the Latin letter instead. Since the unit of temperature in the Kelvin scale is not the degree Kelvin but simply ‘kelvin’, no degree symbol is needed.

The degree sign with an abbreviated temperature scale designation is separated from the preceding digit by an en space. Arkady Milchin, Lyudmila Cheltzova Publisher and Author’s Handbook

Whenever one needs to specify whether the temperature is positive or negative, the minus and the plus signs are not spaced from the numerical value (+30 °C).

The degree of arc is also not spaced from the number. Minutes and seconds of arc (units equal to 1/60 and 1/3600 of one degree respectively) are represented by single and double primes respectively and are written with a numerical value unspaced (30° 17′ 19″). You can type the degree symbol by pressing Alt and typing 0176 on a numerical pad on Windows. Use Option+Shift+8 for that end if you’re working on a MacOS device.

From the «Geometry for Enjoyment and Challenge» book

Not to be confused with the superior o used in abbreviations such as the number sign, nor with the ring, which is a diacritic. Robert Bringhurst The Elements of Typographic Style

Margin of error

If the numerical value is inaccurate or can vary, the value and its margin of error should be bracketed, with the unit of measurement indicated after the bracket. For example, if the length of the chain links varies from 9.8 mm to 10.2 mm, one link is (10 ± 0,2) mm long.

When a numerical value has a tolerance or limit deviation in combination with a physical quantity unit designation, it is required to either enclose the value in parentheses or place the unit designation both after the numerical value and after the tolerance or limit deviation. Arkady Milchin, Lyudmila Cheltzova Publisher and Author’s Handbook

How to type the mentioned symbols

Prime	Mac: Character Viewer (Control + Command + Space) → ′ (U+2032) Windows: Alt + 8242 (on numpad) HTML: &primе;
Double Prime	Mac: Character Viewer (Control + Command + Space) → ″ (U+2033) Windows: Alt + 8243 (on numpad) HTML: &Primе;
Interpunct	Mac: Alt + Shift + 9 Windows: Alt + 0183 (on numpad) HTML: &#183 or &middot, · · Dot operator: ⋅ or ⋅
Superscript 2	Mac: Option + 2 Windows: Alt + 253 (superscript will appear when you release Alt) HTML: &suр2;
Superscript 3	Mac: Option + 3 Windows: Alt + 0179 (superscript will appear when you release Alt) HTML: &suр3;
Multiplication Sign	Mac: Character Viewer (Control + Command + Space) → × (U+00D7) Windows: Alt and = HTML: &timеs;
Per Mille	Mac: Shift+Option+R Windows: Alt+0137 (on numpad) HTML: &pеrmil;
Degree symbol	Mac: Option+Shift+8 Windows: Alt + 0176 (on numpad) HTML: °