Introduction

Resistors are marked with colored bands to indicate their value. Translating these color bands to numbers is easy, as long as you orient the resistor and read it in the proper order. Generally, there are four bands. Orient the resistor such that the side with the color bands closest to the edge, are on the left. The first two color bands convert to a two digit number, the third color band converts to a multiplier and the fourth color band indicates the tolerance. The tolerance is a measure of the accuracy of the marked resistance value. For example, a 1,000 Ohm, 10% tolerance, resistor can have an actual resistence that is anywhere between 900 Ohms and 1,100 Ohms. If that same resistor was 5% tolerance the actual resistance would be limited to between 950 Ohms and 1,050 Ohms.

The most common resistors are of the 5% and 10% tolerance variety. For them, the simplest method for decoding their value is to, first, locate the Gold, 5%, or Silver, 10%, tolerance band and place it facing to the Right. Then, reading from Left to Right, you should have four bands of color. The first three determine the value and the fourth is the tolerance.

Note that some resistors may have a Red, 2%, tolerance band or no fourth band at all. In these cases look for the band that is closest to an edge. This will be the left-most, or first, color band.

Color Chart Decoding

With the resistor oriented properly, and starting from the left side of the resistor, write down the number from the Value column, in the table below, that corresponds to the first color band. Next to it write down the number from the Value column, in the table below, that corresponds to the second color band.

For the third color band, take the number from the Multiplier column, in the table below, that corresponds to the color and multiply the previous two digit number by it.

The fourth band is the tolerance. Normally this band will be either Gold (5%) or Silver (10%) but can be Red (2%) or Clear (20%).

As an example, suppose you had a resistor with colors bands that read, from Left to Right, as Red, Violet, Yellow, and Gold. The first two color bands yield the number 27 and the third color band yields a multiplier of 10,000. This would decode to a value of 27 x 10,000 = 270,000 Ohms. The fourth color band indicates that this resistor has a 5% tolerance.

Color		Value	Multiplier	Tolerance
	Black	0	1	-
	Brown	1	10	-
	Red	2	100	2%
	Orange	3	1,000	-
	Yellow	4	10,000	-
	Green	5	100,000	-
	Blue	6	1,000,000	-
	Violet	7	10,000,000	-
	Gray	8	100,000,000	-
	White	9	1,000,000,000	-
	Gold	-	0.1	5%
	Silver	-	0.01	10%

Resistor Decoder

To the right is a handy little calculator that will allow you to easily decode a resistor's color code without writing anything down. Just select the colors for the bands and read the resistance value below.

Note: Not all resistor color combinations represent valid or 'standard' resistor values.

Resistors in Series

Resistors in Series are pretty straight. The total resistance is equal to the sum of the resistances.

Enter your resistors values in the spaces below and then click outside the box to find R Total for the resistors in Series.

Resistors in Parallel

There are two equations that cover resistors in parallel. A simplified one when you are only dealing with two resistors, and a more complex one when there are more than 2 resistors. In the case of only 2 resistors in Parallel, the equation is relatively simple.

Enter your resistors values in the spaces below and then click outside the box to find R Total for the resistors in Parallel. If you only wish to calculate the resultant value of two resistors in parallel, leave one of the boxes empty or enter 0.

You may not even have to deal with the equations listed above, under certain circumstances. Listed below are some quick things to remember about resistors in parallel that will give you a better insight into the usage.

• The resultant value, of two or more resistors in parallel, is always less than the lowest resistor.

• If the resistors are of equal value, the resultant value is equal to the value of one of the resistors, divided by the number of resistors. For example, if there are three 100 Ohm resistors in parallel the resultant value will be 100/3 = 33.333 Ohms.

• If a very large resistor and a very small resistor are in parallel, the resultant value will only be a little bit less then the smaller resistor. For example, if there are a 10K Ohm resistor in parallel with a 100 Ohm resistor the resultant value will be 99 Ohms. Just 1 Ohm less than the lower value resistor.