Why do we need standard values

Standardization of the values and other parameters of passive electric and electronic components such as resistors, capacitors and inductors is necessary because:

(1) They are industrial products and excessive proliferation of values would imply an equally excessive proliferation of product types with consequent excessive packaging, storage and distribution costs.
(2) They often need to be carried around by engineers (for example during field maintenance of electric and electronic devices) which is best done when a limited number of standard values is collected into compact kits.
(3) Standardization of values, tolerances, power limits and labels represents the first step towards product quality control.

The standard values are maintained by the International Electrotechnical Commission and described in the IEC 60063 publication of January 1, 1963 and its 1967 and 1977 amendments (the document is better known just as IEC 63).

Requirements on the standard values

Passive components notoriously cover an enormous range of values (for example, resistor values from a fraction of Ohm up to tens of maga-ohms are quite common). It is therefore convenient to distribute the standard values in a regular way along a logarithmic scale.

In order to reduce the number of possible mantissas to the bare minimum, each decade is divided in exactly the same manner. This means, for example, that when there is a standard value of say 150 Ω, then values such as 1.50 mΩ, 1.50 Ω or 1.50 MΩ are standard as well and, of course, the same applies also to capacitors and inductors.

With this convention in mind, it is sufficient to take a single reference decade and specify the distribution of the standard values in its interior; values in other decades then simply mimic those in the reference one. The IEC-63 standard uses as reference the decade which covers all values greater than 100 but smaller than 1000. This choice is related to the fact that the standard values mantissas are required to contain just three valid digits, which implies that
(i) the regular logarithmic-scale distributions of standard values are approximate and rounded and,
(ii) within the reference decade they can be conveniently written as integer numbers.

Tolerance classes

How many values standard values should there be in each decade? The answer to this question depends upon the required relative precision of the values which, in turn, has to do with the quality of the components, their calibration and their stability in time at various temperatures (in ultimate analysis, all these factors have also a direct impact on their cost).

IEC-63 specifies six tolerance classes corresponding to relative precision of 20%, 10%, 5%, 2%, 1% and 0.5%. These lead to six distinct series of standard values denoted respectively as E6, E12, E24, E48, E96 and E192, where the values after the E denote the number of standard values per decade.

The most popular among the six series are E12 (10% precision, 12 values/decade) and E96 (1% precision, 96 values/decade). The standard values of the E6 and E12 series can be written in a compact manner as:

E6:   100, 150, 220, 330, 470, 680
E12: 100, 120, 150, 180, 220, 270, 330, 390, 470, 560, 680, 820

Table of values