SI Unit rules and style conventions

(A check List for Reviewing Manuscripts before publication)

Only units of the SI and those units recognized for use with the SI are used to express the values of quantities in published scientific work - that includes practical assessments in education, especially when at University.

Equivalent values in other units can be given in brackets following values in acceptable units, but only when deemed necessary for the intended audience.

This was extracted at a time when rigorous examination of experimental data was more commonplace than today, but read through the standards - you will find that your teachers often do not adhere to them!

Example of correct usage
Example of improper usage


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; cm3 or cubic centimeter; m s-1, m/s or metre per second sec; cc; mps


Unit symbols are unaltered in the plural.
l = 75 cm l = 75 cms


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 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 (brackets) are used.
The speed of the car is about 21 m·s-1 (metres 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 the car is about 21 ms-1 (reciprocal milliseconds)
The decay rate of 113Cs is about 21 m s-1 (metres per second)
m ÷ s, m/s/s, m·kg/s3/A


  • 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.



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 symbols (or names) are not to be modified by the addition of subscripts or other information. Vmax = 1000 V V= 1000 Vmax


The symbol % is used to represent simply the number 0.01 l1 = l2(1 + 0.2 %), or
D = 0.2 %, where D is defined by the relation D = (l1 - l2)/l2.
The length l1 exceeds the length l2 by 0.2 %

and units

Information should not be mixed with unit symbols or names.
the water content is 20 ml/kg 20 mL H2O/ kg
20 ml of water/ kg


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 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 %)

Unit symbols
and names

Unit symbols and unit names should not be mixed and mathematical operations should not be applied to unit names. kg/m3, kg · m-3, or kilogram per cubic metre kilogram/m3, kg/cubic metre, kilogram/cubic metre, kg per m3, or kilogram per metre3.

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


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 spelled-out 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 35-millimeter film."

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 15739.01253
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.


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 a of x), lb x (meaning log2 of x), ln x (meaning loge x), or lg x (meaning log10 x).
tan x
R for resistance
Ar for relative atomic mass

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 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.)  


A quotient quantity should be written explicitly. mass divided by volume mass per unit volume

Object and

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


The obsolete terms normality, molarity, and molal and their symbols N, M, and m are not to be 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)
normality and the symbol N, molarity and the symbol M
molal and the symbol m