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SI units: How to write reports effectively using scientific terminology

The reporting of numbers and quantities requires attention to detail and adherence to conventions, all of which have evolved over time to communicate such data effectively and unambiguously.

Take botanical names, for example, which serve the same purpose, namely unambiguous and universal communication: ‘corn’ can mean wheat in the United Kingdom and maize in the United States, but Triticum aestivum can only refer to wheat of a given type, namely common wheat or bread wheat. 

In the same way, a gallon is 3.785 mL in the United States but 4.546 mL not only in the UK but also in Canada. (The officially recommended symbol for a litre is L, not l, so that it is distinct from the numeral 1 in many typefaces.) Similarly, a tonne is 1000 kg but a ton can be roughly 1016 kg or 907 kg. 

Enabling standardisation

Because of these possible discrepancies, researchers and most journals insist on the international system of units, or SI, short for Le Système International D’unités, the worldwide measurement system coordinated by the International Bureau of Weights and Measures (or BIPM, the Bureau International des Poids et Mesures).

SI units and prefixes

The system is built around seven base units (for length, mass, time, etc.) coupled with appropriate prefixes that can multiply or divide the values of the base units as required to express larger or smaller quantities.

For example, g is the unit for mass. When used with the prefix k (for kilo), as in kg, it means thousand grams or one kilogram; when used with the prefix m (for ‘milli’), as in mg, it means one-thousandth of a gram, or a milligram.

Always remember that all the prefixes that multiply the value beyond 1000 (beyond k) take capital letters, as in M, G (for ‘giga’, or 1 billion, or a thousand million, or 109), or T (for ‘tera’, or a thousand billion, or 1012). Prefixes k and smaller than k, as in µ (Greek mu, or one-millionth, or 10−6) or n (for ‘nano’, or one-billionth, or 10−9) and so on, take lowercase letters. You should never write Kg or KW (which are the most common violations in scientific writing) but instead, write kg or kW.

SI units and symbols

The symbols have no separate plural forms and therefore never take an ‘s’ to form plurals: never write 5 kgs or 10 kms and so on (again, this is among the most common mistakes made by scientists). Instead, you would simply write 5 kg or 10 km.

These symbols are not abbreviations and therefore never take a dot, which indicates abbreviations, such as in lb. (short for ‘libra’, or the pound) or ft. (short for feet): for example, you would not write 10 cm. or 5 kg. because ‘cm’ and ‘kg’ are symbols, not abbreviations.


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