The Ten “Commandments” of Writing Metric Symbols in English Grammar

Caveat: The purpose of this series is neither to proclaim myself as a grammarian nor to despise people who are still unfamiliar with common grammar mistakes. It is to serve as a reminder that there are rules in English that refute those mistakes. I am well aware that I still have a lot to learn in English and I will not stop digging those rules to share them here. If you happen to be a guilty party, please remember that I shared these blog posts in the spirit of constructive criticism.

It has been literally ages since I published a blog post for the grammar pet peeves series. Had it not been for a friend who called my attention regarding the metric symbols I used in one of my email messages to him, the grammar ranter in me would still be in hibernation. So he wrote:

“Hi, Sonyboy. Review that email you sent me regarding product sizes yesterday. You did not pluralize ‘kg’ and ‘lb’. Please add ‘s’ to each of them.”

Talking about a grammar ranter becomes the hunted.

Well, almost but not quite. It should be clear by now that because of the request for pluralization, I have turned the table on him—instant fuel to my muse so to speak. I then politely obliged to the calling. This was my “supposed” reply to him:

“Thanks a lot for digging into the email. However, I regret that I will not give in to your request. Let me put it this way, your initiative is certainly appreciated but inappropriate in the context of Standard English not to mention if you have not done your homework first.

For the record, I also did not know about the real deal on the metric units of measurement until 2012 or when I started this series. Thank God for doing this kind of work because it entails being one step or staying ahead of the game. Having said that, I fully acknowledge that my knowledge of the English language as well as this endeavor will always be a work in progress.

Anyway, the most important thing that you need to remember when it comes to these units of measurement (e.g., kg for kilogram, g for gram, and lb or L for pound-mass) is they have unit symbols not abbreviations.  These symbols do not follow the grammatical rules for abbreviations. They follow the mathematical rules for symbols, that we should make a note of, instead. Those rules include the following:

  1. A symbol is never followed by a period. The exception is, if it happens to fall at the end of a sentence.
  2. The letter “s” is never added to a symbol to indicate a plural. In other words, “2 minutes” is written “2 min” not “2 min.” or “2 mins”.
  3. Symbols are case-sensitive and must be written as they are defined.
  4. There is a tradition in the metric system that the first (or only) letter of an unprefixed unit symbol is capitalized if the unit’s name comes from a proper name. Thus “W” is the symbol for the watt and “A” is the symbol for the ampere, because these units are named after scientists.
  5. It makes a big difference whether a symbol is capitalized or not. The same letter represents different units often (e.g., “t” stands for the tonne and “T” for the tesla). There is one loophole in the rule on capitalization: it is acceptable to use the symbol L instead of l for the liter, since the letter l is so easily confused with the number 1.
  6. The case of symbol prefixes is specified, upper and lower, and must not be changed (e.g., the symbol for kilo- is “k-“, so “kW” and not “KW” is the symbol for the kilowatt).
  7. The superscripts 2 and 3 are always used for “square” and “cubic”, respectively. Hence, the square kilometer, for example, is written “km2”, not “sq km”.
  8. A raised dot (also called a middle dot or half-high dot) is recommended when symbols are multiplied. It is permissible to use a space instead, but symbols should not be placed next to one another with nothing between them. (e.g., “A·h” is the recommended symbol for the ampere hour. “A h” is also permitted, but not “Ah” or “amp hr”).
  9. The slash (solidus) / is used for “per”. Furthermore, only one slash is allowed per symbol. This means the SI unit of acceleration is written “m/s2” rather than “m/s/s”, even though it is often spoken “meters per second per second”. (Negative exponents can also be used: “m/s2” can be written “m·s-2”.)
  10. Symbols are separated from the numerical quantity they follow by a space.”
Summary (Image via Google)

I would like to reiterate that this grammar pet peeves series is not a way of proclaiming myself as an authority. If you happen to be a guilty party, always remember that I am sharing the blog posts in the spirit of constructive criticism.

Reference: Using Abbreviations or Symbols (

Related Articles

Units Of Measure (

10 Rules for Writing Numbers and Numerals (

SI Unit rules and style conventions checklist (

One Comment Add yours

  1. Mabel Kwong says:

    Another informative grammatical post from you, Sony. Writing metric symbols can be confusing, and it did seem that the person who wrote you that email was confused about how to write them correctly. They are definitely different from abbreviations – I like to think of abbreviations as short form (e.g. of a word, number) and symbols as a unique market to represent a kind of measurement. Honestly, sometimes I am guilty of using 10 sec 😀 I know it is wrong but sometimes it is a habit of mine to comtinue to use it. Now whenever I use this, I will think of you and your post and correct myself 😀


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