Resistor color code

How to read it

On 4-band resistors, the first two bands are digits, the third is the multiplier (power of 10) and the fourth the tolerance. On 5-band (precision) resistors there are three digit bands. Classic example: yellow-violet-red-gold = 47 × 100 = 4,700 Ω = 4.7 kΩ ±5%.

Which way round

Keep the tolerance band (gold or silver, or the one set apart) on the right. If the value you get doesn't exist commercially (E12/E24 series: 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82…), you're probably reading it backwards.

Why colours instead of numbers

The code was born in the 1920s for a practical reason: on a component a few millimetres long a printed number would be illegible and, above all, readable from one side only. Bands instead run around the body and are visible anyway, even with the component soldered in and turned. It is the same reason they survived the arrival of laser printing.

The colours follow the order of the spectrum with black and brown in front: black 0, brown 1, red 2, orange 3, yellow 4, green 5, blue 6, violet 7, grey 8, white 9. Remembering the sequence of the six in the middle is enough to read nearly everything, because black and brown come first and grey and white are rare.

The third band is not a digit, it is a zero

This is the mistake everybody makes at first: reading «yellow violet red» as 472 instead of 4,700. The third band (or the fourth on precision resistors) is a multiplier, that is it says how many zeros to add. Red is not worth 2, it is worth «×100».

Two special multipliers handle small values: gold divides by ten and silver divides by a hundred. A brown-black-gold resistor is not a misreading, it is 1 ohm; and those are common values, because resistors of a few ohms are used to measure currents.

How to tell which way round it goes

The tolerance band is further from the others and sits at one end, usually gold (±5%) or silver (±10%): that one goes on the right. If the resistor has a single separated colour, that is it. If instead the bands are evenly spaced and none is gold, apply the standard-value check: commercial resistors follow the E12 and E24 series, and values like 43 or 68 exist while 44 or 67 do not.

On five bands the extra band is a digit, not a multiplier, and on six bands the last one is the temperature coefficient, which only matters in measurement work. A special case is a four-band resistor whose fourth band is black: black means ±20% tolerance, and that is an old resistor.

Before fitting it, look at the power too

The colour code gives the value in ohms, not how many watts the resistor can dissipate: that is given by its physical size, and a resistor too small for the current through it heats up, drifts in value and eventually burns. Common sizes run from an eighth of a watt to two watts, and the practical rule is to stay at twice the calculated power at least.

When in doubt it is still worth measuring with a multimeter: bands fade with heat, red and orange are frequently confused, and on a blue or green body the colours read worse than on beige. The code is for orientation, the instrument for deciding.

Nearby tools

The link between voltage, current and resistance is given by Ohm's law. For household consumption there are Electricity cost and How many lumens you need, and for sums in scientific notation Scientific calculator.