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General pitfalls added 21 December 2001
Minor revisions 7 July 1999
Revised 20 February 1998
Released 2 February 1998 

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Tip of the month is edited by Peter Ring, PRC (Peter Ring Consultants, Denmark)
- consultants on how to write userfriendly manuals

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The handling of units and their abbreviations causes many problems in technical documentation. There are three main areas of problems:

• Handling of units in systems different from what the technical writer is used to, for example American writers' use of "metric units". Here there are two problems:
• wrong abbreviations , especially incorrect use of uppercase and lowercase letters.
• wrong unit conversion or problems with checking the conversion.
• Handling of bits and bytes .
• General pitfalls.
  
• Inter-language pitfalls .

The "metric" prefixes and abbreviations

The SI system is based on powers of 10 and other simple relations between the units. An abbreviation for a unit typically consists of two parts: an abbreviation for the prefix indicating the power of 10, and the abbreviation for the unit. Example: "km": k= kilo = 1000; m = meter (the basic unitfor length). 1 km = 1000 meter.

The official power of 10 prefixes are:
 
 

Deci, centi, hecto and especially deca (da) are - with a few exceptions like hPa in meteology - not very much used. However, deci and centi is commonly used for a few daily measures, e.g. cm (centi-meter), dl (deci-litre) and cl (centi-litre). Check them before using them.

Only the terms pico to giga (in some professions also tera and/or femto) are generally known, even among technitians.

Note, that the prefixes above 10³ are always written with UPPERCASE letters, the rest are always written with lowercase letters.

All units are based on the seven basic units (length, mass, time, electric current, thermodynamic temperature, matter, light). For practical reasons a large number of non-basic units has been defined, too, based on the seven basic units, e.g. Henry for electrical inductance. As a general rule, the abbreviations for these units are in UPPERCASE if the name of the unitis derived from a name (Henry was a person), and else they are in lowercase. This means, that if you don't know the origin of the unit, you must lookthe abbreviation up in a suitable reference.

Wrong conversion factors

Handling of bits and bytes

The "de facto" standard (in accordance with "The Microsoft Manual of Style"), which has gained more and more general acceptance over the lastyears is as follow:

1 byte = 8 bits.

Bit is abbreviated "b".

Byte is abbreviated "B".

The prefixes for bits and bytes are the same as in the metric system, but here each power of 3 refers to 2¹⁰ = 1024 instead of 1000. This means that 1 kB = 1024 bytes, 1 MB = 1024 kB = 1024² bytes= 1,048,576 bytes, 1 GB = 1024 ³ = 1024 MB = 1,073,741,824 bytes, etc. 

K is often used for kB, see General pitfalls below.

General pitfalls

Uppercase K is a generally accepted abbreviation for kB (example: 360K = 360 kB). But don't use M and G the same way! And please note, that 360 K in other texts means 360 kelvin (360 K = 86.85 °C or 188.33 °F)!

Units may be used in singular or plural form. That means, that 100 metre or 100 metres are equally correct!!! Advice: be consistent. If you have a style-guide, make a rule here.

Inter-language pitfalls

• In most non-English languages, the comma (",") indicates the decimal point, and the point (".") is used as the thousand group separator. In these languages, 1,234 means one-point -two-three-four, and 1.234 means one-thousand-two-hundred-and-thirty-four. Solution: If the number can be misunderstood in e.g. continental Europe, add an extra digit and write 1,234.0 or 1.234,0 or use space " " as thedecimal group separator (1 234.567 89).
• In many languages (incl. formal/older UK English), the names for the high powers of ten are not thousand, million, billion, trillion, ..., but thousand, million, milliard, billion, ... If the number can be misunderstood, write it with numerals, too, explain (e.g.: 1 billion = 10⁹⁾, or use prefixes like giga.
• In the Far East incl. Japan, their languages work with four decimals per group (ten-thousand groups) and not three (thousand-groups) like the Western languages do. This means that you must be extra careful checking large figures in information coming from these countries.
• If date formats are important, please note that to most Europeans, the date & month format is "dd mm" in various forms (examples: 25.12., 25/12, 25-12). December, 25th is of course not a problem, but is 02/05 May 2nd or February 5th? To most Europeans it is May 2nd, or they will be in doubt. Solution: state your format in the introduction chapter, e.g."In this manual, the date format is mm/dd", or (better!) use letters for the month, e.g. Feb/07.

If you disagree with these ideas - or have other relevant points, experiences, or ideas +/-, please e-mail me !

Ideas for new "Tip of the month" subjects are very welcome, too!

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