Reading the Colour Code
Resistors are too small to print a number on reliably, so their value is encoded in coloured bands. Read them with the tolerance band — usually gold or silver, and set slightly apart — on the right.
| Bands | Meaning |
|---|---|
| 4 band | Digit, digit, multiplier, tolerance |
| 5 band | Digit, digit, digit, multiplier, tolerance |
| 6 band | As 5 band, plus a temperature coefficient |
A four-band resistor coloured brown-black-red-gold reads 1, 0, ×100, ±5% — that is 1,000 Ω, or 1 kΩ, with a tolerance of 5%.
The Colour Chart
| Colour | Digit | Multiplier | Tolerance |
|---|---|---|---|
| Black | 0 | ×1 | — |
| Brown | 1 | ×10 | ±1% |
| Red | 2 | ×100 | ±2% |
| Orange | 3 | ×1 k | — |
| Yellow | 4 | ×10 k | — |
| Green | 5 | ×100 k | ±0.5% |
| Blue | 6 | ×1 M | ±0.25% |
| Violet | 7 | ×10 M | ±0.1% |
| Grey | 8 | ×100 M | ±0.05% |
| White | 9 | ×1 G | — |
| Gold | — | ×0.1 | ±5% |
| Silver | — | ×0.01 | ±10% |
| None | — | — | ±20% |
Which End to Start From
Getting the direction wrong is the classic mistake — brown-black-red is 1 kΩ, but read backwards, red-black-brown is 200 Ω. Three clues resolve it:
- The tolerance band is gold or silver on most resistors, and those colours never appear as the first digit.
- The tolerance band usually sits with a wider gap before it.
- A resistor never starts with black — a leading zero would be meaningless.
Series and Parallel
Series: Rtotal = R1 + R2 + R3 + …
Parallel: 1 ÷ Rtotal = 1/R1 + 1/R2 + 1/R3 + …
Series always increases total resistance; parallel always decreases it, and the total is always lower than the smallest resistor in the group. Two equal resistors in parallel give exactly half the value — a useful trick when you need a value you do not have.
| Combination | Series | Parallel |
|---|---|---|
| Two 100 Ω | 200 Ω | 50 Ω |
| Two 1 kΩ | 2 kΩ | 500 Ω |
| 100 Ω and 200 Ω | 300 Ω | 66.7 Ω |
| Three 330 Ω | 990 Ω | 110 Ω |
Standard Values: the E Series
Resistors are not made in every value. They follow logarithmic series chosen so that, at a given tolerance, every possible value falls within one part's range.
| Series | Values per decade | Tolerance |
|---|---|---|
| E6 | 6 | ±20% |
| E12 | 12 | ±10% |
| E24 | 24 | ±5% |
| E96 | 96 | ±1% |
The E24 series runs 1.0, 1.1, 1.2, 1.3, 1.5, 1.6, 1.8, 2.0, 2.2, 2.4, 2.7, 3.0, 3.3, 3.6, 3.9, 4.3, 4.7, 5.1, 5.6, 6.2, 6.8, 7.5, 8.2, 9.1 — which is why 4.7 kΩ and 2.2 kΩ appear everywhere while 5 kΩ is unusual.
Power Rating
Colour bands encode resistance, not power. Power capacity is set by physical size, and exceeding it burns the part.
| Rating | Typical body length | Common use |
|---|---|---|
| 1/8 W | 3 mm | Dense signal circuits |
| 1/4 W | 6 mm | The general-purpose default |
| 1/2 W | 9 mm | Moderate loads |
| 1 W | 12 mm | Power supplies |
| 5 W and above | Ceramic bodied | Current sensing, dummy loads |
Check with P = I2R, and derate: run a resistor at no more than about half its rating for reliability, and less still in an enclosed or hot space.
Frequently Asked Questions
What does a resistor with no fourth band mean?
±20% tolerance. These are rare in modern equipment; almost everything is now 5% or 1%.
Why is the body of some resistors blue rather than beige?
Blue usually indicates a metal film type, typically 1% tolerance and five bands. Beige or tan is normally carbon film at 5% with four bands.
How do I get a value I do not have?
Combine what you have. Two 1 kΩ in series give 2 kΩ; two in parallel give 500 Ω. Check the power rating of the combination as well as the value.
Do SMD resistors use colour codes?
No, they use printed numbers. A three-digit code such as 472 means 47 × 102 = 4.7 kΩ; four digits work the same way with three significant figures.