resistor color code

Resistors will have either 3 or 4 bands on them. The first three bands tell you how to compute the resistance in ohms. The fourth, if present, indicates the tolerance (accuracy).

Color   Band 1 Band 2   Band 3
gold      X      X    x   .01
silver    X      X    x   .1
Black     0      0    x   1
Brown     1      1    x   10
red       2      2    x   100
orange    3      3    x   1,000
yellow    4      4    x   10,000
green     5      5    x   100,000
blue      6      6    x   1,000,000
violet    7      7    x   10,000,000
gray      8      8    x   100,000,000
white     9      9    x    ---------

Band four(tolerance band code):

Gold = ±5%
Silver = ±10%
None = ±20%
Red = ±2%
Brown = ±1%

EXAMPLE
You have a resistor with bands: brown-black-red, what is it's resistance in ohms?
band one(brown) = 1
band two(black) = 0
band three(red) = 100

(band 1 & 2) * (band 3) = resistance
10 * 100 = 1000 or 1K
Therefor a resistor with bands brown-black-red would be a 1K ohm resistor. Since the fourth band is not present, it has a ±20% accuracy.

Couple of small additions. First, each colour has a single value. So the numbers in band 3 are not independent of the numbers in band 1 or band 2. The third band simply tells you how many zeroes to put in front. So brown is one, and a brown in the third place, tells you to put one zero in front of the number obtained using the other two bands.
So for example if you have Red,Red, Orange -- you would interpret it by writing 22 and following it with 3 zeroes. Thus the final value would be 22,000 ohms.

Second,the following sentence is often helpful in remembering the sequence of colours involved here.

B.B. ROY of Great Britain had a Very Good Wife

Of course, we find this only in Indian textbooks, because Roy is a Bengali name. You could always change the surname to something you are more comfortable with!

Standard Color Code System

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In the standard resistor color code system, four bands are painted on the resistor. The color of the first band indicates the value of the first significant digit. The color of the second band indicates the value of the second significant digit. The third color band represents a decimal multiplier by which the first two digits must be multiplied in order to obtain the resistance value of the resistor. The colors for the bands and their cooresponding values is shown below:

------------------------------------------------------------------------------------
| COLOR   |   SIGNIFICANT  |   DECIMAL          |   TOLERANCE   |  RELIABILITY     |
|         |   FIGURE       |   MULTIPLIER       |   PERCENT     |  LEVEL PER 1,000 |
|         |                |                    |               |  HOURS           |
------------------------------------------------------------------------------------
| BLACK   |   0            |   1                |   PERCENT     |  1.0%            |
------------------------------------------------------------------------------------
| BROWN   |   1            |   10               |   1           |  .1%             |
------------------------------------------------------------------------------------
| RED     |   2            |   100              |   2           |  .01%            |
------------------------------------------------------------------------------------
| ORANGE  |   3            |   1,000            |               |  .001%           |
------------------------------------------------------------------------------------
| 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    |                |   .1               |   5           |                  |
------------------------------------------------------------------------------------
| SILVER  |                |   .01              |   10          |                  |
------------------------------------------------------------------------------------
| NONE    |                |                    |   20          |                  |
------------------------------------------------------------------------------------

By example, take a given resistor with bands of red, violet, orange and silver. Since the first color is red, the first significant digit is 2. With the second band being violet, the second digit must be 7. The third band is orange, which indicates that the number formed by the first two digits is multiplied by 1000. In this case, 27 * 1000 = 27000 ohms. The fourth band on the resistor indicates the tolerance; that is, the manufacturer's allowable ohmic deviation above and below the numerical value indicated by the first three bands. In this case, silver indicates a tolerance of 10 percent. So we have a 10% allowable deviation on either side of 27000 ohms, or 2700 ohms each way. Therefore, we have an actual value is somewhere between 24300 and 29700 ohms.

Resistors that conform to military specifications have a fifth band. This band indicates the reliability level per 1000 hours of operation. For example, in a piece of equipment containing 10000 orange fifth-band resistors, no more than one resistor will fail during 1000 hours of operation.

The color code is usually only used on carbon resistors. Resistors, such as wirebound and variable type, will not use the color code. These resistors will have their ohmic value and tolerance imprinted on the resistor itself.