Resistors are available in a wide range of values, but if you observe carefully you will realize that certain values of these electronic components like 15k ohm and 33k ohm are easily available where as some values like 20k ohm and 40k ohm are hard to find. Letâ€™s try and understand the logical reason behind this.

Take a hypothetical situation, where you make resistors every 10 ohm, thus giving you 10 ohm, 20 ohm, 30 ohm, etc. But once you reach the value of 1000 ohm, a difference of 10 ohm would hardly be noticeable as it is a very small value in comparison and making 1000 ohm, 1010 ohm, 1020 ohm and so on, would prove to be futile. In fact making such accurate resistors might prove to be very difficult.

Thus a acceptable range for these electronic components is one in which the (amount of the) step increases with the value. This is the logic that the resistor values are based upon, and they form a series following the exact pattern for every (multiple of) 10. There are two such series based on the above logic â€“ the E6 series and the E12 series.

E6 series: Has six values per every multiple of ten with 20% tolerance. So the series goes like:10 ohm, 15 ohm, 22 ohm, 33 ohm, 47 ohm and so on, continuing to 100 ohm ,150 ohm, 220 ohm, 330 ohm with each step size (to the higher value) being higher than the last step size, and approximately half of the value.

E12 series: Has twelve values per every multiple of ten (10% tolerance). So the series goes like:10 ohm, 12 ohm, 15 ohm, 18 ohm, 22 ohm, 33 ohm, 39 ohm and so on, continuing to 100 ohm, 120 ohm, 150 ohm etc, thus it is nothing but the E6 series with an additional value in each gap.

E12 series is in common use for resistors and lets you choose values with 10% error margin, and proves to be accurate enough for most projects.