Resistors are physically quite small most of the time and so it is inconvenient to simply write the value and tolerance on the resistor. A clear but compact code is needed!

On circuit diagrams it is important not to miss the decimal place and use a 47kΩ resistor instead of a 4.7kΩ so clarity is required.

The Resistor Colour Code uses coloured bands to make it easy to identify resistor values and tolerances and the Resistor Printed Code (BS1852) gives a standard for printed markings on resistors and on circuit diagrams.

The resistor colour code uses 10 different colours to represent the value of the resistor. A different set of colours is used to represent other information such as the tolerance or thermal stability. It is worth knowing what the colours represent as it makes life easier when building circuits.

For the bands representing the value, the colour code is:

Black = 0

Brown = 1

Red = 2

Orange = 3

Yellow = 4

Green = 5

Blue = 6

Violet = 7

Gray = 8

White = 9

For the tolerance band, the colour code is:

Brown = ± 1%

Red = ± 2%

Gold = ± 5%

Silver = ± 10%

Blank = ± 20%

In many cases, such as in schools, the resistors used are all ± 5% or worse. Therefore there is no point in giving the value to a high level of precision using many significant figures. Resistor values are usually given to 2 significant figures. The value is given as the 2 most significant digits and then the relevant number of zeros. For example, a resistance of 127.5Ω is simple given as 130Ω to keep things simple.

- Band 1 = Value of Resistor (First significant figure)
- Band 2 = Value of Resistor (Second significant figure)
- Band 3 = Number of following zeros
- Band 4 = Tolerance

- Band 1 - Red - 2 - The first digit is 2
- Band 2 - Red - 2 - The second digit is also 2
- Band 3 - Brown - 1 - There is 1 zero after the first two digits
- Band 4 - Gold - ± 5% - The tolerance is ± 5%

- Band 1 - Brown - 1 - The first digit is 1
- Band 2 - Black - 0 - The second digit is 0
- Band 3 - Orange - 3 - There are 3 zeros after the first two digits
- Band 4 - Gold - ± 5% - The tolerance is ± 5%

Note: 10,000Ω = 10kΩ

- Band 1 - Yellow - 4 - The first digit is 4
- Band 2 - Violet - 7 - The second digit is 7
- Band 3 - Red - 2 - There are 2 zeros after the first two digits
- Band 4 - Gold - ± 5% - The tolerance is ± 5%

Note: 4700Ω = 4.7kΩ but the decimal place is easy to miss so the decimal place is represented by the position of the âkâ in the value. Therefore, 4.7kΩ is written as 4k7Ω

The BS1852 Printed Resistor Code uses letters and numbers to signify the **value** of the resistor.

- R means x1
- K means x1000
- M means x1000,000

Tolerance is indicated by adding a letter at the end:

- J = ± 5%
- K = ± 10%
- M = ± 20%

A **220Ω** resistor with a tolerance of **± 5%** is written as **220 R J**

A **4700Ω** resistor with a tolerance of **± 5%** is written as **4 K 7 J**

A **10,000Ω** resistor with a tolerance of **± 5%** is written as **10 K J**

**3R9K**has a value of 3.9Ω with a tolerance of ± 10%**39RJ**has a value of 39Ω with a tolerance of ± 5%**390RM**has a value of 390Ω with a tolerance of ± 20%**3K9K**has a value of 3900Ω ± 10%**39KJ**has a value of 39000Ω ± 5%**390KM**has a value of 390000Ω ± 20%**3M9K**has a value of 3900000Ω ± 10%**39MM**has a value of 39000000Ω ± 20%

Small value resistors - between 10Ω and 100Ω - are often identified wrongly. The third band in this case is always **black** which means that there are **NO zeros** following the value. It is all too easy to read the third band as zero and assume that the zero follows the value ... it does not! Black means there are zero zeros after the value!

- Band 1 - Brown - 1 - The first digit is 1
- Band 2 - Black - 0 - The second digit is 0
- Band 3 - Black - 0 - There are NO (zero) zeros after the first two digits
- Band 4 - Gold - ± 5% - The tolerance is ± 5%

- Band 1 - Green - 5 - The first digit is 5
- Band 2 - Blue - 6 - The second digit is 6
- Band 3 - Black - 0 - There are NO (zero) zeros after the first two digits
- Band 4 - Gold - ± 5% - The tolerance is ± 5%

- Band 1 - Brown - 1 - The first digit is 1
- Band 2 - Black - 0 - The second digit is 0
- Band 3 - Black - 0 - There are NO (zero) zeros after the first two digits
- Band 4 - Gold - ± 5% - The tolerance is ± 5%

Low value resistors - those less than 10Ω - use the **THIRD** coloured band to **reduce** the value of the resistor by **dividing** by either 10 or 100. The first two coloured bands give the value of the resistor and the third coloured band divides the value to make it smaller.

Gold ÷ 10

Silver ÷ 100

For example, a 4.7Ω ± 5% resistor is 47Ω ÷ 10 and would therefore be Yellow, Violet, Gold, Gold

Similarly, a 0.56Ω ± 1% resistor is 56Ω ÷ 100 and would therefore be Green, Blue, Silver, Brown

- Band 1 - Brown - 1 - The first digit is 1
- Band 2 - Black - 0 - The second digit is 0
- Band 3 - Gold - ÷10 - Divide the value given by the first two digits by 10
- Band 4 - Gold - ± 5% - The tolerance is ± 5%

A resistor with a value of 0Ω is useful as a wire link. The reason why a zero ohm resistor is better than a simple piece of wire is that the same machines that fit resistors into circuit boards on production lines can also be used to fit the "wire links" to the circuit boards. A zero ohm resistor has a **single black band**.

For resistors with a tolerance of better than ± 5%, more than two significant figures are needed to represent the value.

High precision resistors have **THREE** bands for the value and then one band for the number of zeros - four bands in total to represent the value. The colour code otherwise works in exactly the same way. Note: 5th band gives tolerance where Brown = ± 1%

- Band 1 - Brown - 1 - The first digit is 1
- Band 2 - Orange - 3 - The second digit is 3
- Band 3 - Black - 0 - The third digit is 0
- Band 4 - Orange - 3 - There are 3 zeros after the value
- Band 5 - Brown - ± 1% - The tolerance is ± 1%

An entirely different approach is adopted for Surface Mounted Device (SMD) resistors - and, as it happens, low value capacitors

The idea of 2 significant figures and then a 3rd digit representing the multiplying factor (number of zeros) is retained but the numbers are simply printed on the device / resistor and there is no tolerance indicated. The first two numbers represent the value and the 3rd number represents the number of zeros. Therefore, if the 3rd digit is 4, multiply by 10,000 (4 zeros) but if the 3rd digit is 1 just multiply by 10. If the 3rd digit is 0, multiply by 1 - there are no extra zeros.

- 102 = 1 0 x 100 = 1000Ω
- 334 = 3 3 x 10,000 = 330kΩ
- 221 = 2 2 x 10 = 220Ω
- 560 = 5 6 x 1 = 56Ω

Resistors with a tolerance of ± 10% (Silver) form a series of 12 values, each approximately 20% bigger than the last. **This series is called the E12 series**.

The E12 series is:

**10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82**

... and all the subsequent decades / powers of 10

Examples of E12 values include: 100Ω, 15kΩ, 220Ω, 330kΩ, 4.7Ω, 6.8MΩ, 10kΩ, 1kΩ

Resistors with a tolerance of ± 5% (Gold) form a series of 24 values, each approximately 10% bigger than the last. **This series is called the E24 series**.

The E24 series is:

**10, 11, 12, 13, 15, 16, 18, 20, 22, 24, 27, 30, 33, 36, 39, 43, 47, 51, 56, 62, 68, 75, 82, 91**

... and all the subsequent decades / powers of 10

Examples of E24 values include: 110Ω, 13kΩ, 200Ω, 360kΩ, 4.3Ω, 6.2MΩ, 11kΩ, 1k3Ω

© Paul Nicholls

January 2016

Electronics Resources by Paul Nicholls is licensed under a Creative Commons Attribution 4.0 International License.