Basic Electronic Components: Resistor Color Code

Colour Code | Colour Code Calculator | Tolerance | Real Values (E6 & E12 series) | Power Rating

Also see: Resistance | Ohm's Law | Variable Resistors

Example:

Circuit symbol:

Function

Resistors restrict the flow of electric current, for example a resistor is placed in series with a light-emitting diode (LED) to limit the current passing through the LED.

Connecting and soldering

Resistors may be connected either way round. They are not damaged by heat when soldering.

The Resistor Colour Code
Colour	Number
Black	0
Brown	1
Red	2
Orange	3
Yellow	4
Green	5
Blue	6
Violet	7
Grey	8
White	9

Resistor values - the resistor colour code

Resistance is measured in ohms, the symbol for ohm is an omega

.
1

is quite small so resistor values are often given in k

and M

.
1 k

= 1000

1 M

= 1000000

Resistor values are normally shown using coloured bands.
Each colour represents a number as shown in the table.

Most resistors have 4 bands:

The first band gives the first digit.
The second band gives the second digit.
The third band indicates the number of zeros.
The fourth band is used to shows the tolerance (precision) of the resistor, this may be ignored for almost all circuits but further details are given below.

This resistor has red (2), violet (7), yellow (4 zeros) and gold bands.
So its value is 270000

= 270 k

.
On circuit diagrams the

is usually omitted and the value is written 270K.

Find out how to make your own Colour Code Calculator.

Small value resistors (less than 10 ohm)

The standard colour code cannot show values of less than 10

. To show these small values two special colours are used for the third band: gold which means × 0.1 and silver which means × 0.01. The first and second bands represent the digits as normal.

For example:
red, violet, gold bands represent 27 × 0.1 = 2.7

green, blue, silver bands represent 56 × 0.01 = 0.56

Resistor Colour Code Calculator

The Resistor Colour Code Calculator can be used to identify resistors. It consists of three card discs showing the colours and values, these are fastened together so you can simply turn the discs to select the value or colour code required. Simple but effective!

There are two pdf versions to download and print on A4 white card (two per sheet):

Colour (for a colour printer)
Black and White (for a black only printer)
This version must be coloured manually, it is easiest to do this before cutting out.

To make the calculator, carefully cut out the three discs and fasten them together with a small brass paper fastener.

The calculator design is copyright but it may be copied for educational purposes.

Tolerance of resistors (fourth band of colour code)

The tolerance of a resistor is shown by the fourth band of the colour code. Tolerance is the precision of the resistor and it is given as a percentage. For example a 390

resistor with a tolerance of ±10% will have a value within 10% of 390

, between 390 - 39 = 351

and 390 + 39 = 429

(39 is 10% of 390).

A special colour code is used for the fourth band tolerance:
silver ±10%, gold ±5%, red ±2%, brown ±1%.
If no fourth band is shown the tolerance is ±20%.

Tolerance may be ignored for almost all circuits because precise resistor values are rarely required.

Resistor shorthand

Resistor values are often written on circuit diagrams using a code system which avoids using a decimal point because it is easy to miss the small dot. Instead the letters R, K and M are used in place of the decimal point. To read the code: replace the letter with a decimal point, then multiply the value by 1000 if the letter was K, or 1000000 if the letter was M. The letter R means multiply by 1.

For example:

560R

2K7

39K

1M0

Real resistor values (the E6 and E12 series)

You may have noticed that resistors are not available with every possible value, for example 22k

and 47k

are readily available, but 25k

and 50k

are not!

Why is this? Imagine that you decided to make resistors every 10

giving 10, 20, 30, 40, 50 and so on. That seems fine, but what happens when you reach 1000? It would be pointless to make 1000, 1010, 1020, 1030 and so on because for these values 10 is a very small difference, too small to be noticeable in most circuits. In fact it would be difficult to make resistors sufficiently accurate.

To produce a sensible range of resistor values you need to increase the size of the 'step' as the value increases. The standard resistor values are based on this idea and they form a series which follows the same pattern for every multiple of ten.

The E6 series (6 values for each multiple of ten, for resistors with 20% tolerance)
10, 15, 22, 33, 47, 68, ... then it continues 100, 150, 220, 330, 470, 680, 1000 etc.
Notice how the step size increases as the value increases. For this series the step (to the next value) is roughly half the value.

The E12 series (12 values for each multiple of ten, for resistors with 10% tolerance)
10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82, ... then it continues 100, 120, 150 etc.
Notice how this is the E6 series with an extra value in the gaps.

The E12 series is the one most frequently used for resistors. It allows you to choose a value within 10% of the precise value you need. This is sufficiently accurate for almost all projects and it is sensible because most resistors are only accurate to ±10% (called their 'tolerance'). For example a resistor marked 390

could vary by ±10% × 390

= ±39

, so it could be any value between 351

and 429

Resistors in Series and Parallel

For information on resistors connected in series and parallel please see the Resistance page,

Power Ratings of Resistors

High power resistors
(5W top, 25W bottom)Photographs © Rapid Electronics

Electrical energy is converted to heat when current flows through a resistor. Usually the effect is negligible, but if the resistance is low (or the voltage across the resistor high) a large current may pass making the resistor become noticeably warm. The resistor must be able to withstand the heating effect and resistors have power ratings to show this.

Power ratings of resistors are rarely quoted in parts lists because for most circuits the standard power ratings of 0.25W or 0.5W are suitable. For the rare cases where a higher power is required it should be clearly specified in the parts list, these will be circuits using low value resistors (less than about 300

) or high voltages (more than 15V).

The power, P, developed in a resistor is given by:

P = I² × R
or
P = V² / R

where:

P = power developed in the resistor in watts (W)
I = current through the resistor in amps (A)
R = resistance of the resistor in ohms (

)
V = voltage across the resistor in volts (V)

Examples:

A 470 resistor with 10V across it, needs a power rating P = V²/R = 10²/470 = 0.21W.
In this case a standard 0.25W resistor would be suitable.
A 27 resistor with 10V across it, needs a power rating P = V²/R = 10²/27 = 3.7W.
A high power resistor with a rating of 5W would be suitable.

Fixed resistors are marked in several ways. These are:

Color coding
Straight numerical value
Certain numerical codes that can be easily translated

Because carbon resistors are small physically, they are color coded to mark their R value in ohms. In memorizing the colors, note that the darkest color, black and brown are the for the lowest numbers, zero and one, through lighter colors to white for nine.

Typical Example on How to Calculate the Resistor Color Code

Reading form left to right, the first band close to the edge gives the first digit in the numerical value of R. the next band marks the second digit. The third band is the decimal multiplier, which gives the number of zeros after the two digits.

Example 1:

The first band is red for 2 and the next band is violet for 7. The red multiplier in the third band means add two zeroes to 27. The result can be illustrated as follows:

Therefore, this R value is 2700Ω with tolerance ±5%.

The resistor tolerance means the amount by which the actual R can be different form the color-coded value. For instance, the alone value 2700Ω resistor with ±5% tolerance can have resistance 5 percent above or below the coded value.

This R, therefore, is between 2565Ω and 2835Ω. the calculations are as follows:

5 percent of 2700 is 0.05 x 2700 = 135

For +5 percent, the value is 2700 + 135=2835Ω

For -5 percent, the value is 2700 – 135 = 2566Ω

Example 2:

Therefore, the R value is 56000Ω or 56KΩ with tolerance ±10%.

Example 3:

The example 3 illustrates that black for the third band means “do not add zeroes to the first two digits”. Since the resistor has orange, orange and black band, the R value is 33Ω with tolerance ±5%.

Example 4:

18 x 1 = 18Ω

Therefore, the R value is 18Ω with tolerance ±10%.

Example 5:

For these values, the third band is gold, indicates a fractional decimal multiplier. When the third band is gold, multiply the first two digits by 0.1. The R value is