Resistors are one of the most widely used components in electronic circuits. The values of resistance can vary from few ohms to mega ohms. Carbon or film resistors used in low power electronic circuits are very small in size. Size of typical 0.25 W carbon resistor is approximately 6 mm in width and 2 mm in diameter. If we print the values of resistances on such small size, then practically it becomes very difficult to read those values. Lets say, if somehow we manage to read the resistance values printed on them, still there are chances of the markings getting rubbed off or smeared after long usage. To overcome this, a standard “Color code for resistors” was developed to identify the values of resistance and their tolerance values. As per the standards, the resistors are marked with different color bands. The values of resistance and tolerance can be decoded using resistor color chart. In this article, we discuss in detail how to find the resistance and tolerance from the color bands.

Number of color bands on resistors :

How many color bands are there on resistors? This is quite an obvious question. The number of color bands can range from three bands to six bands. Actually, the number of bands depends on the type of resistors on which marking is done. Standard precision resistors use four color bands while high precision resistors use five color bands. The tolerance of standard precision resistors can vary from 2% to a whooping 20% of the nominal value! In case f high precision resistors, the tolerance is approximately 1% of the nominal value. We shall discuss in detail how to find the resistance values for three to six color bands. But first, let us see what does color and position of the band indicate.

What does color and position of bands on resistors indicate?

Standard Precision Resistor. Image credit : Tudor Barker

The value of resistance is coded in the form of color bands. Hence color of the band gives an indication of the actual value of resistance. The color of the band represents a number and its position indicates how to use that number to decode the resistance value from the resistor color chart. Lets take a look at the color chart which tells us how to decode the resistance values from the color bands.

Color of the bandNumber associated with the colorTolerance valuesMnemonic (B B ROY Goes Bombay Via Gateway With Genelia & Susanne.
Black0± 20%B
Brown1± 1%B
Red2± 2%R
Orange3± 3%O
Yellow4± 4%Y
Green50.5%Goes
Blue60.25%Bombay
Violet70.1%Via
Grey80.05%Gateway
White9-With
Gold0.1± 5%Genelia &
Silver0.01 10%Susanne

The above color chart indicates the color of the band and the corresponding numerical values associated with the colors. A simple way to remember the sequence of colors is by the use of mnemonic ” B B ROY Goes Bombay Via Gateway With Genelia & Susanne”. Now that we know the colors of the bands and the associated numerical values, let us see how we can decode the values of resistance.

Decoding the resistance from the color bands.

For standard precision resistors (four bands) : As mentioned above, standard precision resistors use four color bands. An image of the standard precision resistor is also shown above. For standard precision resistors,

• The First band indicates the first digit of the resistance value.
• The second band indicates the second digit of the resistance value.
• The third band indicates the number of zeros to be added after the first two digits. Except when the color is silver or golden. Example 2 given below shows what should be done if the color is silver or golden.
• The fourth band indicates tolerance.

Lets take an example to understand how to determine the resistance value.

Example 1 :As shown in the above figure, consider a resistor with color bands as red-violet-red-golden. Red and violet has numerical values of 2 and 7 respectively (refer the color chart). Write down the two number side-by-side. This gives us number 27. Now the third band indicates the number  of zeros (In this case – red). The number associated with red color is 2. Hence we add two zeros after 27 which gives us the nominal value of resistance as 2700Ω. The fourth color – golden indicates a tolerance of ± 5%.

Example 2: Consider a resistor with color bands orange-white-gold-silver. First two bands – orange and white indicates first two digits. Orange and white has corresponding numerical values of 3 and 9 respectively. We write the numbers side-by-side and we get the number 39. The third band (golden) indicates the number 0.1 which means you have to multiply the first two digits with  0.1. Hence the value of resistance 3.9 Ω. The tolerance as indicated by the color silver is 10%.

Three band resistors : Resistors with three color bands can be decoded the same way as that of resistors with four color bands with the exception that the tolerance value is assumed to be 20%. Three bands indicates the nominal value of resistance and the tolerance is 20%.

For High precision resistors (five bands) : High  precision resistors uses five bands. The procedure to determine the resistance of high precision resistors (5 band) resistors is quite similar to that of four band resistors. In 5 band resistors, the first three bands indicates the value, fourth band indicates multiplier and the fifth band indicates tolerance (usually ± 1%). Consider a resistor with color bands red-violet-black-silver-brown. The first three colors red(2), violet(7) and black(0) indicates the number 270. The fourth band-silver (0.01) indicates that we should multiply 270 with 0.01, resulting in the nominal value of resistance to be 2.7 Ω. The fifth color (brown) indicates a tolerance of  ± 1%.

In this article we learned how to calculate the resistance from the color codes. If you faced any difficulty in understanding  or still have some doubts, or wanna give some feedback, feel free to use the comments section below.