Physics reference
SI rules and conventions
A list of the rules and conventions for the use of SI.

These are the official "rules and style conventions" for using SI, to ensure that there is no confusion introduced by peculiar usage, as dictated by NIST Special Publication 811 [reference 1] .
  1. Only the SI units and those units recognized for use with SI are used to express quantities. Equivalent values in other units, given in parentheses following the SI units are allowed only when necessary for the reader.

  2. Abbreviations not approved for SI for SI units are not allowed; only standard symbols, unit names, and quantities are used.

    Correct: s. cm3. m/s. Incorrect: sec. cc. mps.

  3. Symbols are unaltered when expressing plural quantities.

    Correct: l = 75 cm. Incorrect: l = 75 cms.

  4. Unit symbols are not abbreviations, and are not followed with a period, unless actually ending a sentence (as with normal grammar).

    Correct: The length of the bar is 75 cm. The bar is 75 cm long. Incorrect: The bar is 75 cm. long.

  5. A space or centered dot is used to represent multiplication of units; a slash, horizontal line, or negative exponent is used for division. A slash, if used, must not appear more than once in the same expression unless parentheses are used, to avoid ambiguities.

    Correct: The speed of sound is about 344 m s-1 (meters per second). The decay rate of 113Cs is about 21 ms-1 (reciprocal milliseconds). m/s. m.s-2. m s-2. m kg/(s3 A). m kg s-3 A-1. Incorrect: The speed of sound is about 344 ms-1 (reciprocal milliseconds). The decay rate of 113Cs is about 21 m s-1 (meters per second). m/s. m kg/s3/A.

  6. Variables and quantity symbols are in italics. Unit symbols are in straight, Roman type. Numbers are usually written in Roman.

    Correct: She exclaimed, "The dog weighs 10 kg!" t = 3 s, where t is time and s is second. T = 22 K, where T is thermodynamic temperature, and K is kelvin. Incorrect: He exclaimed, "That dog weighs 10 kg!" t = 3 s, where t is time and s is second. T = 22 K, where T is thermodynamic temperature and K is kelvin.

  7. Superscripts and subscripts are in italic if they signify variables or quantities. If they are merely descriptive, they are in Roman.

    Correct: cp, specific heat capacity and constant pressure p. X = sumi xi. mp, mass of the proton.

  8. Abbreviations such as ppm (parts per million), ppb (parts per billion), ppt (parts per trillion), etc. are not used.

    Correct: 2.0 uL/L. 2.0 x 106 V. 4.3 nm/m. 4.3 x 10-9 l. 7 ps/s. 7 x 10-12 t. Incorrect: 3.2 ppm. 7.2 ppm. 32.6 ppt.

  9. Unit symbols and names are not modified by the addition of subscripts or other information.

    Correct: A maximum of 1000 V. A mass fraction of 10 %. Incorrect: 1000 Vmax. 10 % (m/m). 10 % (by weight).

  10. The symbol % is used to represent the number 0.01.

    Correct: l1 = l2 (1 + 0.2 %). Incorrect: The length l1 exceeds the length l2 by 0.2 % (% does not mean 0.01).

  11. Information is not mixed with unit symbols or names.

    Correct: The water content is 20 mL/kg. Incorrect: 20 mL H2O/kg. 20 mL of water/kg.

  12. It is clear to which unit symbol a numerical value is associated and which mathematical operations apply to which quantities.

    Correct: 25 cm x 48 cm. 1 MHz to 10 MHz or (1 to 10) MHz. 20 oC to 30 oC or (20 to 30) oC. 135 g +- 2 g or (123 +- 2) g. 70 % +- 5 % or (70 +- 5) %. 240 x (1 +- 10 %) V. Incorrect: 25 x 48 cm. 1 MHz-10 MHz or 1 to 10 MHz. 20 oC-30 oC or 20 to 30 oC. 123 +- 2 g. 70 +- 5 %. 240 V +- 10 % (incompatible units).

  13. Unit symbols and unit names are not mixed. Mathematical operations are not applied to unit names.

    Correct: kg/m3. kg m-3. Kilogram per cubic metre. Incorrect: kilogram/m3. kg/cubic metre. kilogram/cubic metre. kg per m3. Kilogram per metre3.

  14. Values of quantities are expressed in appropriate units using Arabic numerals (not names) and symbols for units (not unit names).

    Correct: m = 5 kg. The current was 15 A. Incorrect: m = five kilograms. m = five kg. The current was 15 amperes.

  15. There is a space between the numerical value and the unit symbol, except for superscripts to represent plane angles (degrees, arcminutes, arcseconds).

    Correct: A 25 kg sphere. An angle of 2o3'4". Incorrect: A 25-kg sphere. A 25kg sphere. An angle of 2 o 3 ' 4 ".

  16. Digits of numerical values having more than four digits on either side of the decimal points are separated into groups of threes by a space. Commas are not used to separate groups of three.

    Correct: 15 739.012 53. 1200. Incorrect: 15739.01253. 1,200.

  17. Equations between quantities are used in preference to equations between numerical values. Symbols representing numerical values are different from symbols representing the corresponding quantities. When a numerical equation is used, it is formally written to indicate the units being used in the equation itself.

    Correct: l/m = 3.6 [v/(m/s)] (t/s). Incorrect: l = 3.6 v t, where l is in metres, v is in metres per second, and t is in seconds.

  18. Standardized quantity symbols are used. Standardized mathematical signs and symbols are used. The base of "log" in equations is specified by writing logb x, lb x (or log2 x), ln x (or loge x), or lg x (or log10 x).

    Correct: tan x. R for resistance. Ar for relative atomic mass. Incorrect: tg x for tangent of x.

  19. A quotient quantity is written explicitly.

    Correct: Mass divided by volume. Incorrect: Mass per unit volume.

  20. Distinguish between an object and a quantity describing the object.

    Correct: A body of mass 5 g. Incorrect: A mass of 5 g.

  21. The obsolete terms molarity, normality, and molal are not used.

  22. When the word weight is used, the intended meaning is clear. In science, weight is a force; but in everyday use, weight represents mass.

NIST Special Publication 811
reference 1
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