Resistor values are marked on each resistor. Since these resistors are so small in size, it is very difficult to write values on them. Hence another interesting method of writing the resistor values was invented, which is called color coding. Value of a resistor is represented in terms of the colored rings around it. One can easily decode these color codes to find out actual value of resistor using the method below

1. The resistor shown above has four colored circles, red, green, black and silver.
2. The first three color bands are used to compute resistance value and last silver band is use to compute the tolerance.
3. Due to the manufacturing process all resistors cannot be manufactured of exact values (color coded value). Hence manufacturer provides error or tolerance percentage. The last band can be silver or gold color indicating 10% or 5% tolerance.
4. First three bands can have any color from "resistor color code chart". Each color has its own value as shown in the chart below.
5. Value of each color band from the chart is used in the formula to compute resistor value.

The formula is very simple.

resistor value = ((band1 x 10) + band2 ) x 10 ^band3

Third band gives multiplication factor in power of 10. For example, in above figure band1 is red, so band1 = 2. Band2 is green, so band2=5. And band3 is black, so band3=0.
Now the resistor value will be, ((2 x 10) + 5) x10⁰ => (20 + 5) x 1 => 25 Ohms.

Try Resistor Calculator tool present in the utilities section to test your understanding..