Guidelines for Use of SI Units

Following is a quick summary of guidelines for correct usage of SI (metric) units, taken from the leading SI usage references worldwide.  A reference list for the document numbers cited below appears at the end of this page.  For further details, consult one or more of the reference list documents.

1. Insider's tip for mechanical/structural design and analysis:  Whenever possible, convert to and keep units in N, mm, and MPa during all calculations, because this utilizes the very convenient axiom 1 MPa = 1 N/mm^2.  (Memorize this definition; i.e., MPa = 1 N/mm².)  This will create much cleaner, simpler, more readable notation, almost eliminates the need for all the scientific notation and trickier units, and eliminates almost all conversions.  Don't convert away from these units midstream during the calculations; convert only the final answer if it's required and has a real purpose.  There's no requirement in SI to convert everything to meters.  Quite the contrary.  The standard unit in mechanical design is millimeter (SAE TSB-003 [1], Sect. 7.6.4).  Centimeter is not used.  Use meters only for very large dimensions, such as the length of a building, etc.

2. Stress is normally reported in MPa, not fundamental units (SAE TSB-003, Table B1; ASTM SI-10 [5], Sect. 3.3.4.1; NAS10000 [6], Sect. 4.2.5.2).  If derived units have a special name in SI, the SI special name should be used instead of fundamental units.  E.g., use MPa, not N/mm^2 nor MN*m^-2.

3. Whenever a numerical value is less than 1, a zero must always precede the decimal point (ASTM SI-10, Sect. 3.5.4.1; ASAE EP285.7 [2], Sect. 3.6; NAS10000, Sect. 4.2.4.7).  This avoids possible misinterpretation due to poor legibility of screens, fonts, prints, or photocopies.

4. "Kilogram-force" and other such arbitrarily contrived, physically incoherent, nonstandard units are not allowed in SI and shall not be used (SAE TSB-003, Sects. 3.11, 6.1, 7.4.1; ASTM SI-10, Sects. 3.3.4.1, C.7.1).  SI has eliminated considerable confusion by using (a) physically coherent units for force and mass and (b) distinctly different unit nomenclature for these two quantities.  The unit of force in SI is newton (N).  And the name kilogram is restricted to the unit of mass.

   
   

International Style Standards for Expressing and Writing SI Units in Text

5. Always put a space between the numeric value and its following unit symbol (SAE TSB-003, Sect. 7.3.10; ISO 1000 [4], Sect. 6.1; ASTM SI-10, Sect. 3.5.1.e).

6. Unit symbols are not abbreviations, and therefore must never be pluralized, nor ended with a period (except at end of sentence), nor replaced by abbreviations (ISO 1000, Sect. 6.1; ASAE EP285.7, Sect. 3.6).

7. No alterations nor changes in capitalization (case) of unit symbols are allowed (ISO 1000, Sect. 6.1).  E.g., in SI, M does not equal m, K is not k, and there's no such thing as Kg nor MM.  To learn the correct spelling and capitalization of unit names and symbols, see any document in the reference list, or see capitalization tip for a brief summary.

8. Don't run two or more unit symbols together.  Adjacent unit symbols should be separated by a space (N mm), half-high dot (N·mm), or asterisk (N*mm) multiplication symbol (ISO 1000, Sect. 6.2).  A space character always implies multiplication (as in all mathematics).  Back-to-back parentheses always imply multiplication (as in all mathematics); e.g., (N m)(s).

9. Only one solidus (/) is allowed in compound units, unless parentheses are used (ISO 1000, Sect. 6.2).  E.g., use N/(m/s^2), not N/m/s^2.

10. The unit symbol for gram is g, not gm.  The symbol for earth gravitational acceleration constant (9.80665 m/s^2) is italic g.  In situations where you think these two symbols could get confused, you have one other (generally nonpreferred) option:  writing out full unit names.  So that would be gram, not gm.  Luckily, in mechanics, kg is used, whereas gram is rarely or never used.

11. For a short list of structural design and analysis conversion factors, see handy conversion factors.

Reference List

The above items are explained very well in

[1]  SAE TSB-003, "Rules for Use of SI Units," 1999 (excellent but not free);

[2]  ASAE EP285.7, "Use of SI Units" (pdf, 89 KB), 2001 (excellent); or

[3]  NIST SP-811, Guide for Use of SI Units, Barry N. Taylor, 1995.

Other references (not free) are

[4]  ISO 1000, "SI Units," 1992 (too succinct);

[5]  IEEE/ASTM SI-10, "Standard for Use of SI Units," 1997;

[6]  NAS10000, "Documents Preparation in SI Units," 1997;

[7]  BS 5555, "Specification for SI Units," 1993.