Expressing SI Units on a Keyboard: Rules and Keyboard Shortcuts
Anyone who has ever written an article, manuscript, website copy, copy for their calibration certificates, or any writing involving proper SI usage has likely had some confusion on how to make sure what they are writing is technically correct.
We see it all of the time, and it makes sense as there are rules that may not make sense to most of the people who write or edit content for a living.
Luckily, NIST has a special publication 811 titled Guide for the Use of the International System of Units (SI).
Guide for the Use of the International System of Units (SI) is a comprehensive reference for using the International System of Units (SI) correctly in scientific and technical writing. It is intended to help everyone use SI units properly in their work and publications.
In this publication is a Checklist for Reviewing Manuscripts which is essentially a two-page guide for anyone reviewing any publication for proper SI usage.
I know some people might question why does this matter.
Going from a recent podcast I had the pleasure of being part of the reason it matters is if the company performing your calibrations does not know about the proper way for expressing SI units, what else are they not following?
Below is a summary of some of the rules listed in NIST SP 811:
A. Use of SI units:
- Make sure you are expressing SI units for all NIST publications, with few exceptions
- Express values in acceptable units first, followed by other units in parentheses if needed
- Avoid using non-SI units except when required by law/regulation
B. Writing unit symbols:
- Use lowercase for symbols, except those derived from proper names (e.g. N for newton)
- Do not use periods after unit symbols
- Leave a space between the number and unit symbol (e.g. 5.0 m, not 5.0m)
- Do not mix unit symbols and unit names (e.g. m/s, not meters/second)
- Do not use abbreviations for units (e.g. s, not sec)
C. Using prefixes:
- Attach prefixes to unit symbols without spaces (e.g. km, not k m)
- Do not use compound prefixes (e.g. μm, not mμm)
- Use a single prefix per unit (e.g. GW, not kMW)
D. Expressing values:
- Use Arabic numerals, not spelled-out numbers (e.g. 5 m, not five meters)
- Use decimal points, not commas (e.g. 3.14, not 3,14)
- Use spaces to separate long numbers into groups of three digits (e.g. 12 345, not 12,345)
- Express ranges with "to" or parentheses, not dashes (e.g. 5 m to 10 m or (5 to 10) m)
E. Equations and mathematical expressions:
- Use quantity equations rather than numerical value equations when possible
- Use standard symbols for quantities and mathematical operations
- Use italic type for quantities and variables, roman type for units
F. Formatting:
- Use roman (upright) type for unit symbols and names
- Use italic type for quantity symbols
- Use bold italic for vectors, sans-serif bold italic for tensors
G. Conversions:
- When converting units, round to appropriate significant figures
- Provide the original unit value when reporting converted values
H. Special cases:
- For the kilogram, attach prefixes to "gram" not "kilogram" (e.g. mg, not μkg)
- Use degree Celsius (°C) for Celsius temperature
- Use percent (%) as a symbol, not the spelled-out word.
Note: A space should be left between the number and the % symbol.
All of these and more examples are in the document.
In addition, there are several keyboard shortcuts that are not in the document that I have found very useful over the years.
Some metrology keyboard shortcuts
+/- should always be written ± = Alt + 241
Half Dot · = Alt + 0183
Degree Symbol ° = Alt + 0176
µ = Alt + 0181
Conclusion:
Some of the most common mistakes I tend to always notice is someone writing 5% and not 5 %.
Another one is k=2 instead of k = 2 which is not in NIST SP-811.
The NIST website has a lot more guidance in how to properly write Constants, Units, and Uncertainty.
Properly expressing SI units and adherence to standardized scientific writing conventions are crucial for clarity, precision, and consistency in technical and scientific communication. The NIST Guide for the Use of the International System of Units (SI) provides comprehensive guidance on this topic that all authors of technical content should familiarize themselves with.
Some key points to remember include expressing SI units whenever possible, leaving spaces between numbers and unit symbols, using proper formatting for unit symbols and prefixes, and expressing values and equations correctly. Common mistakes to avoid include improper spacing with percentages, inconsistent use of uncertainty notation, and mixing unit symbols with unit names.
While mastering all the nuances of expressing SI units can seem daunting, resources like the NIST guide and its handy manuscript checklist provide invaluable reference. Taking the time to ensure proper SI usage demonstrates attention to detail and adherence to international standards. For scientific and technical professionals, it is an important aspect of clear and credible communication.
Ultimately, correctly expressing SI units is about more than just following arbitrary rules - it enables precise and unambiguous expression of quantitative information that is critical for scientific understanding and technological progress. As such, it should be a priority for anyone engaged in technical writing and editing across scientific and engineering fields.
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