When we learn the theory behind electronic circuits we often calculate the exact value a certain resistor, capacitor or inductance should have.

However, in reality there is no 23.94 Ohms resistor available as standard off-the-shelf (SOTS) electronic part. Go and check it if you don’t believe me.

In reality only a selected range of resistor values is available. The specific resistor values are standardized in the EIA “E range” of standards, with EIA E12 and EIA E24 being the most common. This way manufacturers only need to produce and store a limited variety of different resistors and consumers can rely on the fact that resistors of different manufacturers are interchangeable.

The following graphic shows the values and color codings for the E24 series. The E12-series is obtained by skipping even row numbers. Knowing the common resistor values can help a lot when trying to decode the colored stripes on a resistor 🙂

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As an example, for the E12 set of resistor values every decade of resistance values is divided into 12 equal parts. This is done in a way that each part is equally spaced on a logarithmic scale, i.e. R_i = round(10 * 10^(i/12)) for i = 1..12. So for the range from 10 Ω to 100 Ω the resistor values become 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82, 100 Ω.

It works analogous for the other E-series. The higher the number of the E-series the more divisions per decade exist and thus the precision of the resistors values increases. The typical tolerances for the E12 series resistor values are 10%, for the E24 series it’s 5% and for the E96 series tolerances are down to 1%.

That’s the basic idea behind standard resistor series. In practice some deviations may occur, e.g. it is not uncommon to find resistors of the E24 series offered at 1% tolerance.

References:

http://www.resistorguide.com/resistor-values/
http://www.logwell.com/tech/components/resistor_values.html