Reading resistor colour codes

Small resistors are too small to print a number on, so their value is shown with coloured bands. Learning to read them takes a little practice, but there are only ten colours to remember. This page also explains two things that come up whenever you buy resistors: tolerance and standard values.

In the tool below you can tap a band and change its colour, or type a value and see which colours it has.

Reading four bands

1. Hold the resistor so that the gold or silver band is on the right.
2. The first two bands are the first two digits of the value.
3. The third band is the multiplier, which tells you how many zeros to add after the digits.
4. The last band is the tolerance.

For example, yellow, violet, red, gold. Yellow is 4 and violet is 7, which gives 47. Red as a multiplier means two zeros, so the value is 4,700 Ω, written 4.7 kΩ. Gold means a tolerance of ±5%.

The colours

• 0 black
• 1 brown
• 2 red
• 3 orange
• 4 yellow
• 5 green
• 6 blue
• 7 violet
• 8 grey
• 9 white

The middle of the list follows the rainbow, from red (2) to violet (7). Black and brown come before it, grey and white after it. Two extra colours are used only as multipliers for small values: gold divides by 10 and silver divides by 100, so red, red, gold is 2.2 Ω.

Five bands

More precise resistors use three digit bands instead of two, then the multiplier, then the tolerance. Brown, black, black, red, brown is 1, 0, 0, with two zeros added: 10,000 Ω, or 10 kΩ, with a tolerance of ±1%.

Tolerance

No resistor is made exactly to its value. The tolerance says how far the real value may be from the marked one. A 4.7 kΩ resistor with ±5% tolerance can be anything from about 4.47 kΩ to 4.94 kΩ. The tool shows that range for whatever you choose.

For most circuits, including LED resistors, 5% is perfectly fine. For circuits where the exact value matters, such as timers or voltage dividers that set a precise voltage, 1% resistors are the better choice.

Standard values

Because of tolerance, it would make no sense to manufacture every possible value. Resistors are made in standard series of values instead. The E12 series has twelve values in each power of ten: 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68 and 82, then the same again times 10 (100, 120, 150 and so on), times 100, and so on. The E24 series adds twelve more values in between. E12 values are the ones you will find in almost any shop, and the calculators in these guides round to them.

When a calculation gives a value that isn't in the series, you pick the nearest standard value, usually on the safe side. Which side is safe depends on the circuit, and each guide says which way to round.

Which end to start from

• The tolerance band is usually a little further from the others.
• Gold and silver are almost never the first band.
• If you're unsure, read it both ways. Usually only one direction gives a standard value.
• If it still isn't clear, measure it with a multimeter, as shown in the multimeter guide.

Common values

• 220 Ω: red, red, brown
• 330 Ω: orange, orange, brown
• 1 kΩ: brown, black, red
• 4.7 kΩ: yellow, violet, red
• 10 kΩ: brown, black, orange
• 100 kΩ: brown, black, yellow