Resistors are rated in ohms. Use this simple explanation to determine how much resistance a resistor has. If your resistor has four color bands, turn the resistor so that the gold or silver band is on the right hand side.
The first band is now on the left hand side. This represents the first digit.
Black = 0
Brown = 1
Red = 2
Orange = 3
Yellow = 4
Green = 5
Blue = 6
Violet = 7
Gray = 8
White = 9
Make a note of the first digit.
The second band represents the second digit. The colors represent the same numbers as did the first digit.
Black = 0
Brown = 1
Red = 2
Orange = 3
Yellow = 4
Green = 5
Blue = 6
Violet = 7
Gray = 8
White = 9
Now write the second digit to the right of your first digit.
The third band divulges how many zeros to add to the first two numbers.
Black = DO NOT ADD ANY ZEROS
Brown = Add 1 Zero
Red = Add 2 Zeros
Orange = Add 3 Zeros
Yellow = Add 4 Zeros
Green = Add 5 Zeros
Blue = Add 6 Zeros
Violet = Add 7 Zeros
Gray = Add 8 Zeros
White = Add 9 Zeros
But what if you need a resistor that is smaller than 1?
If the third band is
Gold - Divide by 10
Silver - Divide by 100
Now take two digit number from the first two steps and either add the appropriate number of zeros or divide as called for.
You now know the value of the resistor.
The fourth band only deals with tolerance.
The most common are
20% None
10% Silver
5% Gold
Some others are:
2% Red
1% Brown
0.5% Green
0.25% Blue
0.1% Violet
0.05% Gray
Again these lower colors are not used much.
If you need a mnemonic to help you remeber the order of the colors try these:
Bad Boys Rape Our Young Girls But Violet Gives Willingly.
If you need a tamer version try this:
Bad Boys Race Our Young Girls But Violet Generally Wins.