

Example
of correct usage

Example
of improper usage

Abbreviations

Abbreviations
such as sec, cc, or mps are to be avoided and only standard unit
symbols, prefix symbols, unit names, and prefix names are to be
used. 
s or second;
cm^{3} or cubic centimeter; m s^{1}, m/s or metre per second 
sec; cc; mps 
Plurals 
Unit symbols are unaltered in the plural.

l
= 75 cm 
l
= 75 cms 
Punctuation 
Unit symbols
are not followed by a period (full stop) 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. 
Multiplication
and division 
A space or halfhigh 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 (brackets) are used.

The speed
of the car is about 21 m·s^{1} (metres per second)
The decay rate of ^{113}Cs is about 21 ms^{1}
(reciprocal milliseconds)
m/s, m·s^{2}, m·kg/(s^{3}·A),
m·kg·s^{3}·A^{1}
m/s, m s^{2,} m kg/(s^{3} A), m kg s^{3} A^{1}

The
speed of the car is about 21 ms^{1} (reciprocal milliseconds)
The decay rate of ^{113}Cs is about 21 m s^{1} (metres per
second)
m ÷ s, m/s/s, m·kg/s^{3}/A 
Typeface

 Variables
and quantity symbols should be in italic type.
 Unit symbols
should be in roman type.
 Numbers
should generally be written in roman type.
These rules
apply irrespective of the typeface used in the surrounding text
or when submitting hand written work your handwriting style!
This also
applies to sub and superscripts.



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.

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. 
"ppm," "ppb," and "ppt,"
and the terms part per million, part per billion, and part per trillion,
and suchlike 
Unit
modifications 
Unit symbols
(or names) are not to be modified by the addition of subscripts
or other information. 
Vmax = 1000
V 
V=
1000 Vmax 
Percent 
The symbol
% is used to represent simply the number 0.01 
l_{1}
= l_{2}(1 + 0.2 %), or
D = 0.2 %, where D is defined by the relation D = (l_{1}
 l_{2})/l_{2}. 
The
length l_{1} exceeds the length l_{2} by 0.2 % 
Information
and units 
Information
should not be mixed with unit symbols or names.

the water content
is 20 ml/kg 
20
mL H_{2}O/ kg
20 ml of water/ kg 
Mathematical
notation 
It must be clear to which unit symbol a numerical value belongs
and which mathematical operation applies to the value of a quantity.

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 
35
x 48 cm
1 MHz10 MHz or 1 to 10 MHz
20 °C30 °C or 20 to 30 °C
123 ± 2 g
70 ± 5 %
240 V ± 10 % (one cannot add 240 V and 10 %) 
Unit symbols
and names 
Unit symbols and unit names should not be mixed and mathematical
operations should not be applied to unit names. 
kg/m^{3},
kg · m^{3}, or kilogram per cubic metre 
kilogram/m^{3},
kg/cubic metre, kilogram/cubic metre, kg per m^{3}, or kilogram
per metre^{3.} 
Numerals
and
unit symbols 
Values of quantities
are expressed in acceptable units using Arabic numerals and symbols
for units. 
m = 5 kg
the current was 15 A 
m
= five kilograms
m = five kg
the current was 15 amperes 
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.
If the spelledout
name of a unit is used, the normal rules of English apply:

a 25 kg sphere
an angle of 2° 3' 4"
"a roll
of 35millimeter film."

a
25kg sphere
an angle of 2 ° 3 ' 4 " 
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.

15
739.012 53 
15739.01253
15,739.012 53 
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 numericalvalue
equation is used, it is properly written and the corresponding quantity
equation is given where possible.



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 a of x), l_{b} x (meaning log2
of x), ln x (meaning log_{e} x), or lg x (meaning log10
x).

tan x
R for resistance
A_{r }for relative atomic mass 
tg
x for tangent of x
words,
acronyms, or ad hoc groups of letters

Weight vs.
mass 
When the word "weight" is used, the intended meaning must be 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.) 


Quotient
quantity 
A quotient
quantity should be written explicitly. 
mass divided
by volume 
mass
per unit volume 
Object and
quantity 
An object
and any quantity describing the object are supposed to be distinguished
clearly. (Note the difference between "surface" and "area,"
"body" and "mass," "resistor" and
"resistance," "coil" and "inductance.")

A body of mass
5 g 
A
mass of 5 g 
Obsolete
Terms 
The obsolete
terms normality, molarity, and molal and their symbols N, M, and
m are not to be used.

amountofsubstance
concentration of B (more commonly called concentration of B), and
its symbol c_{B} and SI unit mol/m^{3} (or a related
acceptable unit)
molality of solute B, and its symbol b_{B} or m_{B}
and SI unit mol/kg (or a related unit of the SI) 
normality
and the symbol N, molarity and the symbol M
molal and the symbol m 