Rules to follow in SI system of unit.

This is the manuscripts with proper SI usage and the basic principles concerning quantities and units. 

        (1) Only SI units 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 such as sec (for either s or second), cc (for either cm3 or cubic centimeter), or 

mps (for either m/s or meter per second), are avoided and only standard unit symbols, SI 

prefix symbols, unit names, and SI prefix names are used.

            (3) 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. The 

following forms, for example, are used instead: 2.0 µL/L or 2.0 × 10–6 V, 4.3 nm/m or 

4.3 × 10–9 l, 7 ps/s or 7 × 10−12 t, where V, l, and t are, respectively, the quantity symbols for volume, length, and time.

         (4) Unit symbols (or names) are not modified by the addition of subscripts or other information. 

The following forms, for example, are used instead.

Vmax = 1000 V but not: V = 1000 Vmax a mass fraction of 10 % but not: 10 % (m/m) or 10 % (by weight) 

           (5) Statements such as “the length l1 exceeds the length l2 by 0.2 %” are avoided because it is 

recognized that the symbol % represents simply the number 0.01. Instead, forms such as “l1 = l2 (1 + 0.2 %)” or “Δ = 0.2 %” are used, where Δ is defined by the relation 

Δ = (l1 − l2)/l2.

           (6) Information is not mixed with unit symbols (or names). For example, the form “the water 

content is 20 mL/kg” is used and not “20 mL H2O/kg” or “20 mL of water/kg.” 

           (7) It is clear to which unit symbol a numerical value belongs and which mathematical operation 

applies to the value of a quantity because forms such as the following are used. 

35 cm × 48 cm but not: 35 × 48 cm 

1MHz to 10 MHz or (1 to 10) MHz but not: 1 MHz – 10 MHz or 1 to 10 MHz 

20 ºC to 30 ºC or (20 to 30) ºC but not: 20 ºC – 30 ºC or 20 to 30 ºC 

123 g ± 2 g or (123 ± 2) g but not: 123 ± 2 g 

70 % ± 5 % or (70 ± 5) % but not: 70 ± 5 % 

240 × (1 ± 10 %) V but not: 240 V ± 10 % (one cannot add 

 240 V and 10 %)

         (8) Unit symbols and unit names are not mixed and mathematical operations are not applied to unit names. For example, only forms such as kg/m3

, kg · m−3, or kilogram per cubic meter are used and not forms such as kilogram/m3, kg/cubic meter, kilogram/cubic meter, kg per 

m3, or kilogram per meter3.

           (9) Values of quantities are  and the Symbols for the units. 

m = 5 kg but not: m = five kilograms or m = five kg the current was 15 A but not: the current was 15 amperes. 

           (10) There is a space between the numerical value and unit symbol, even when the value is used as an adjective, except in the case of superscript units for plane angle.

a 25 kg sphere but not: a 25-kg sphere 

an angle of 2º3'4" but not: an angle of 2 º3 '4 " 

 If the spelled-out name of a unit is used, the normal rules of English are applied: “a roll of 

35-millimeter film.” 

          (11) 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. For example, 15 739.012 53 is highly preferred to 15739.01253. Commas are not used to separate digits into groups of three. 

            (12) 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.

             (13) Standardized quantity symbols such as those given in are used, for example, R for resistance and Ar for relative atomic mass, and not words, acronyms, or ad hoc groups of letters. Similarly, standardized mathematical signs and symbols  are used, for example, “tan x” and not “tg x.” 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 x ), ln x (meaning loge x), or lg x (meaning log10 x ). 

          (14) Unit symbols are in roman type, and quantity symbols are in italic type with superscripts and subscripts in roman or italic type as appropriate.

            (15)  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.)

             (16) A quotient quantity, for example, mass density, is written “mass divided by volume” rather 

than “mass per unit volume.” 

             (17) 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.”) 

              (18) The obsolete term normality and the symbol N, and the obsolete term molarity and the 

symbol M, are not used, but the quantity amount-of-substance concentration of B and its symbol cB and SI unit mol/m3 (or a related 

acceptable unit), are used instead. Similarly, the obsolete term molal and the symbol m are 

not used, but the quantity molality of solute B, and its symbol bB or mB and SI unit mol/kg.