Definitions of the SI units: Non-SI units

Units outside the SI Certain units are not part of the International System of Units, that is, they are outside the SI, but are important and widely used. Consistent with the recommendations of the International Committee for Weights and Measures (CIPM, Comité International des Poids et Mesures), the units in this category that are accepted for use with the SI are given in Table 6. Table 6.  Units outside the SI that are accepted for use with the SI Name Symbol  Value in SI units minute (time) min 1 min = 60 s hour h 1 h = 60 min = 3600 s day d 1 d = 24 h = 86 400 s degree (angle) ° 1° = ( /180) rad minute (angle) 1 = (1/60)° = (/10 800) rad second (angle) 1 = (1/60) = (/648 000) rad liter L 1 L = 1 dm3 = 10-3 m3 metric ton (a) t 1 t = 103 kg neper Np 1 Np = 1 bel (b) B 1 B = (1/2) ln 10 Np (c) electronvolt (d) eV 1 eV = 1.602 18 x 10-19 J, approximately unified atomic mass unit (e) u 1 u = 1.660 54 x 10-27 kg, approximately astronomical unit (f) ua 1 ua = 1.495 98 x 1011 m, approximately (a) In many countries, this unit is called "tonne.'' (b) The bel is most commonly used with the SI prefix deci: 1 dB = 0.1 B. (c) Although the neper is coherent with SI units and is accepted by the CIPM, it has not been adopted by the General Conference on Weights and Measures (CGPM, Conférence Générale des Poids et Mesures) and is thus not an SI unit. (d) The electronvolt is the kinetic energy acquired by an electron passing through a potential difference of 1 V in vacuum. The value must be obtained by experiment, and is therefore not known exactly. (e) The unified atomic mass unit is equal to 1/12 of the mass of an unbound atom of the nuclide 12C, at rest and in its ground state. The value must be obtained by experiment, and is therefore not known exactly. (f) The astronomical unit is a unit of length. Its value is such that, when used to describe the motion of bodies in the solar system, the heliocentric gravitation constant is (0.017 202 098 95)2 ua3·d-2. The value must be obtained by experiment, and is therefore not known exactly.

The liter in Table 6 deserves comment. This unit and its symbol l were adopted by the CIPM in 1879. The alternative symbol for the liter, L, was adopted by the CGPM in 1979 in order to avoid the risk of confusion between the letter l and the number 1. Thus, although both l and L are internationally accepted symbols for the liter, to avoid this risk the preferred symbol for use in the United States is L. Neither a lowercase script letter l nor an uppercase script letter L are approved symbols for the liter. Other units outside the SI that are currently accepted for use with the SI by NIST are given in Table 7. These units, which are subject to future review, should be defined in relation to the SI in every document in which they are used; their continued use is not encouraged. The CIPM currently accepts the use of all of the units given in Table 7 with the SI except for the curie, roentgen, rad, and rem. Because of the continued wide use of these units in the United States, NIST still accepts their use with the SI. Table 7.  Other units outside the SI that are currently accepted for use with the SI, subject to further review Name Symbol Value in SI units nautical mile 1 nautical mile = 1852 m knot 1 nautical mile per hour = (1852/3600) m/s are a 1 a = 1 dam2 = 102 m2 hectare ha 1 ha = 1 hm2 = 104 m2 bar bar 1 bar = 0.1 MPa = 100 kPa = 1000 hPa = 105 Pa ångström Å 1 Å = 0.1 nm = 10-10 m barn b 1 b = 100 fm2 = 10-28 m2 curie Ci 1 Ci = 3.7 x 1010 Bq roentgen R 1 R = 2.58 x 10-4 C/kg rad rad 1 rad = 1 cGy = 10-2 Gy rem rem 1 rem = 1 cSv = 10-2 Sv

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SI Unit rules and style conventions

Check List for Reviewing Manuscripts

#1 General	Only units of the SI and those units recognized for use with the SI are used to express the values of quantities. Equivalent values in other units are given in parentheses following values in acceptable units only when deemed necessary for the intended audience.
#2 Abbreviations	Abbreviations such as sec, cc, or mps are avoided and only standard unit symbols, prefix symbols, unit names, and prefix names are used.
	proper:	s or second; cm3 or cubic centimeter; m/s or meter per second
	improper:	sec; cc; mps
#3 Plurals	Unit symbols are unaltered in the plural.
	proper:	l = 75 cm
	improper:	l = 75 cms
#4 Punctuation	Unit symbols are not followed by a period unless at the end of a sentence.
	proper:	The length of the bar is 75 cm. The bar is 75 cm long.
	improper:	The bar is 75 cm. long.
#5 Multiplication & division	A space or half-high dot is used to signify the multiplication of units. A solidus (i.e., slash), horizontal line, or negative exponent is used to signify the division of units. The solidus must not be repeated on the same line unless parentheses are used.
	proper:	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·kg/(s3·A),  m·kg·s-3·A-1 m/s,  m s-2,  m kg/(s3 A),  m kg s-3 A-1
	improper:	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/s/s,  m·kg/s3/A
#6 Typeface	Variables and quantity symbols are in italic type. Unit symbols are in roman type. Numbers should generally be written in roman type. These rules apply irrespective of the typeface used in the surrounding text.  For more details, see Typefaces for symbols in scientific manuscripts
	proper:	She 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
	improper:	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 Typeface	Superscripts and subscripts are in italic type if they represent variables, quantities, or running numbers. They are in roman type if they are descriptive.
	subscript category typeface proper usage  quantity italic cp, specific heat capacity at constant pressure descriptive roman mp, mass of a proton running number italic
#8 Abbreviations	The combinations of letters "ppm," "ppb," and "ppt," and the terms part per million, part per billion, and part per trillion, and the like, are not used to express the values of quantities.
	proper:	2.0 µL/L; 2.0 x 10-6 V; 4.3 nm/m; 4.3 x 10-9 l; 7 ps/s; 7 x 10-12 t, where V, l, and t are the quantity symbols for volume, length, and time.
	improper:	"ppm," "ppb," and "ppt," and the terms part per million, part per billion, and part per trillion, and the like
#9 Unit modifications	Unit symbols (or names) are not modified by the addition of subscripts or other information. The following forms, for example, are used instead.
	proper:	Vmax = 1000 V a mass fraction of 10 %
	improper:	V= 1000 Vmax 10 % (m/m) or 10 % (by weight)

#10 Percent	The symbol % is used to represent simply the number 0.01.
	proper:	l1 = l2(1 + 0.2 %), or D = 0.2 %, where D is defined by the relation D = (l1 - l2)/l2.
	improper:	the length l1 exceeds the length l2 by 0.2 %
#11 Information & units	Information is not mixed with unit symbols or names.
	proper:	the water content is 20 mL/kg
	improper:	20 mL H2O/ kg 20 mL of water/ kg
#12 Math notation	It is clear to which unit symbol a numerical value belongs and which mathematical operation applies to the value of a quantity.
	proper:	35 cm x 48 cm 1 MHz to 10 MHz or (1 to 10) MHz 20 °C to 30 °C or (20 to 30) °C 123 g ± 2 g or (123 ± 2) g 70 % ± 5 % or (70 ± 5) % 240 x (1 ± 10 %) V
	improper:	35 x 48 cm 1 MHz-10 MHz or 1 to 10 MHz 20 °C-30 °C or 20 to 30 °C 123 ± 2 g 70 ± 5 % 240 V ± 10 % (one cannot add 240 V and 10 %)
#13 Unit symbols & names	Unit symbols and unit names are not mixed and mathematical operations are not applied to unit names.
	proper:	kg/m3, kg · m-3, or kilogram per cubic meter
	improper:	kilogram/m3, kg/cubic meter, kilogram/cubic meter, kg per m3, or kilogram per meter3.
#14 Numerals & unit symbols	Values of quantities are expressed in acceptable units using Arabic numerals and symbols for units.
	proper:	m = 5 kg the current was 15 A
	improper:	m = five kilograms m = five kg the current was 15 amperes
#15 Unit spacing	There is a space between the numerical value and unit symbol, even when the value is used in an adjectival sense, except in the case of superscript units for plane angle.
	proper:	a 25 kg sphere an angle of 2° 3'  4" If the spelled-out name of a unit is used, the normal rules of English apply: "a roll of 35-millimeter film."
	improper:	a 25-kg sphere an angle of 2 ° 3 ' 4 "
#16 Digit spacing	The digits of numerical values having more than four digits on either side of the decimal marker are separated into groups of three using a thin, fixed space counting from both the left and right of the decimal marker. Commas are not used to separate digits into groups of three.
	proper:	15 739.012 53
	improper:	15739.01253 15,739.012 53
#17 Quantity equations	Equations between quantities are used in preference to equations between numerical values, and symbols representing numerical values are different from symbols representing the corresponding quantities. When a numerical-value equation is used, it is properly written and the corresponding quantity equation is given where possible.
	proper:	(l/m) = 3.6-1 [v/(km/h)](t/s)
	improper:	l = 3.6-1 vt, accompanied by text saying, "where l is in meters, v is in kilometers per hour, and t is in seconds"
#18 Standard symbols	Standardized quantity symbols are used. Similarly, standardized mathematical signs and symbols are used. More specifically, the base of "log" in equations is specified when required by writing loga x (meaning log to the base aof x), lb x (meaning log2 x), ln x (meaning loge x), or lg x (meaning log10 x).
	proper:	tan x R for resistance Ar for relative atomic mass
	improper:	tg x for tangent of x words, acronyms, or ad hoc groups of letters
#19 Weight vs. mass	When the word "weight" is used, the intended meaning is clear. (In science and technology, weight is a force, for which the SI unit is the newton; in commerce and everyday use, weight is usually a synonym for mass, for which the SI unit is the kilogram.)
#20 Quotient quantity	A quotient quantity is written explicitly.
	proper:	mass divided by volume
	improper:	mass per unit volume
#21 Object & quantity	An object and any quantity describing the object are distinguished. (Note the difference between "surface" and "area," "body" and "mass," "resistor" and "resistance," "coil" and "inductance.")
	proper:	A body of mass 5 g
	improper:	A mass of 5 g
#22 Obsolete Terms	The obsolete terms normality, molarity, and molal and their symbols N, M, and m are not used.
	proper:	amount-of-substance concentration of B (more commonly called concentration of B), and its symbol cB and SI unit mol/m3 (or a related acceptable unit) molality of solute B, and its symbol bB or mB and SI unit mol/kg (or a related unit of the SI)
	improper:	normality and the symbol N, molarity and the symbol M molal and the symbol m

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