2.4 Colour coding and standard resistor values

Conventional fixed resistors have coloured bands showing their value. These can be decoded using Table 3. For example, a resistor with four bands where the coding is yellow (4), violet (7), red (× 100) and brown (1%) has a resistance of

 (47 Ω × 100) ± 1% = 4700 Ω ± 47 Ω

There is a similar five-band scheme where there are three instead of two digits in the resistance value, and a six-band scheme that also gives temperature information. Sometimes it can be hard to see these colour values on the resistors and a multimeter is used to check them.

Table 3  The four-band colour-coding scheme for resistors

Show description|Hide description

This table contains five columns. The first column gives a colour. The other four columns describe what that colour means in each of the four band positions.

Colour	Band 1 First digit	Band 2 Second digit	Band 3 Multiplier	Band 4 Tolerance
Black	0	0	× 1 (× 1)	–
Brown	1	1	× 10 (× 10)	1%
Red	2	2	× 100 (× 100)	2%
Orange	3	3	× 1 000 (× 1k)	not used
Yellow	4	4	× 10 000 (× 10k)	not used
Green	5	5	× 100 000 (× 100k)	not used
Blue	6	6	× 1 000 000 (× 1M)	not used
Violet	7	7	–	not used
Grey	8	8	–	not used
White	9	9	–	not used
Gold	–	–	–	5%
Silver	–	–	–	10%

Resistors are required across a range of millions of ohms. To keep costs low, manufacturers have defined ‘standard’ sets of resistor values such as those shown in Table 4 for resistors with a tolerance of ±5%.

Table 4  Standard resistor values in ohms (±5%)

Maximise

Show description|Hide description

This table lists standard resistor values in ohms. The first column contains the values 1.0, 1.1, 1.2, 1.3, 1.5, 1.6, 1.8, 2.0, 2.2, 2.4, 2.7, 3.0, 3.3, 3.6, 3.9, 4.3, 4.7, 5.1, 5.6, 6.2, 6.8, 7.5, 8.2 and 9.1. Then the values in each of the subsequent six columns are those in the previous column multiplied by 10. So the second column contains the values 10 to 91, the third column contains the values 100 to 910, and so on, until the last column contains the values 1 million to 9.1 million.

The standard or preferred values shown in Table 4 are chosen such that whatever value resistance is required, there is one within 5% of the specified value. Fixed resistors’ costs are usually very low.