Practical Resistor value calculation

Resistor is a passive electrical component that reduce flow of electrical current in an electronic or Electronics circuit. resister has a linear IV ( V=IR ) characteristic. meaning that the voltage across the resistor is directly proportional to the current through the resistor.
Resisters's 10³ Symbol

Resistor In Ohms	Unit	Symbol
1	Ohm	Ω
1,000	Kilo Ohm	KΩ
1,000,000	Mega Ohm	MΩ

Resistors practical value

Resistors only come in specified values. These values are spread across multiples of ten.

1

1.2

1.5

1.8

2.2

2.7

3.3

3.91

4.7

5.6

6.8

8.2

After that, the cycle repeats, but a power of ten higher.

Resistor Color code

Color	Significant figures	Multiplier	Tolerance
Black	–	×100	-
Brown	1	×101	±1%
Red	2	×102	±2%
Orange	3	×103	-
Yellow	4	×104	-
Green	5	×105	±0.5%
Blue	6	×106	±0.25%
Violet	7	×107	±0.1%
Gray	8	×108	±0.05%
White	9	×109	-
Gold	–	×10-1	±5%
Silver	–	×10-2	±10%
None	-	-	±20%

Example Calculation

Eg 1

Red      Red    Orange   Gold    

  2               2            000        ±5% 

  22000 Ω  ±5%    Or    22 KΩ

Eg 2
Blue      Gray   Brown    Silver
  6                8            0            ±10% 
  680 Ω  ±10% 

Power rating Of Resistor 
  power rating of a resistor is the specification given with a resistor that serves to tell the maximum amount of power that the resistor can withstand. When current passes through Resistor, it normally generates heat. there for we have to find suitable wattage resistor for the Electronics circuit.
 Let's look at examples of resistors and the power ratings we would need for them for you to get a practical idea:

Resister Value                    820 Ω
Resister between Voltage     12V

P = V² / R

P =12 x 12 / 820
p = 0.1756

1/4 watt resistor would suffice and be suitable for the circuit

The common standard power ratings of resistors are 0.25W, 0.5W, 1W, 2W, 5W, 10W and 25W. So the circuit designer must choose accordingly for the circuit.

Resistor Watt range with physical size