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SI Unit rules and style conventions 
Check List for Reviewing Manuscripts

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.
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
Unit symbols are unaltered in the plural.
l = 75 cm
l = 75 cms
Unit symbols are not followed by a period unless at the end of a sentence.
The length of the bar is 75 cm.
The bar is 75 cm long.
The bar is 75 cm. long.
& 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.
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
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
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
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
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
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
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
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)
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 %
& 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

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) %
x (1 ± 10 %) V
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 %)
& 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.
Numerals &
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
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 "
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.
15 739.012 53
15,739.012 53
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"
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
r for relative atomic mass
improper: tg x for tangent of x
words, acronyms, or ad hoc groups of letters
Weight vs.
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.)
A quotient quantity is written explicitly.
proper: mass divided by volume
improper: mass per unit volume
Object &
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
The obsolete terms normality, molarity, and molal and their symbols N, M, and m are not used.

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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Online: February 1998   -   Last update: September 2004