SI Unit rules and style conventions checklist

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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 c_p, specific heat capacity at constant pressure

descriptive roman m_p, 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: V_max = 1000 V
a mass fraction of 10 %

improper: V= 1000 V_max10 % (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 l₁exceeds the length l₂by 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 log_a x (meaning log to the base aof x), lb x (meaning log₂ x), ln x (meaning log_e x), or lg x (meaning log₁₀ x).

proper: tan x
R for resistance
A_r 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 c_Band SI unit mol/m³ (or a related acceptable unit)
molality of solute B, and its symbol b_Bor m_Band 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


#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 l₁exceeds the length l₂by 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 log_a x (meaning log to the base aof x), lb x (meaning log₂ x), ln x (meaning log_e x), or lg x (meaning log₁₀ x).
	proper:	tan x R for resistance A_r 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 c_Band SI unit mol/m³ (or a related acceptable unit) molality of solute B, and its symbol b_Bor m_Band 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