A resistor is too small to print a number on, so its value is painted on in bands. Once you can read them the code is quicker than a meter, and unlike a meter it works on a resistor still sealed in its tape.
Every band is one of ten colours standing for a digit from 0 to 9. The first bands are digits, the next band is a multiplier — how many zeros to write after those digits — and the last band is the tolerance, how far the real part is allowed to sit from the printed value.
Which end do you read from?
Getting this backwards is the commonest mistake there is, and it turns 220 into 22. Three things tell you which end is the start:
- Gold and silver are never a first band. They only ever appear as a multiplier or a tolerance. If you can see one, it is at the far end — turn the resistor round.
- The tolerance band sits slightly apart, with a wider gap before it. The digit bands are evenly spaced and grouped together.
- Start from the band nearest the end of the body. The first band is close to one lead; the tolerance band is set in from the other.
The digit and multiplier table
| Colour | Digit | Multiplier | Tolerance |
|---|---|---|---|
| Black | 0 | x 1 | - |
| Brown | 1 | x 10 | +/- 1 % |
| Red | 2 | x 100 | +/- 2 % |
| Orange | 3 | x 1 000 | - |
| Yellow | 4 | x 10 000 | - |
| Green | 5 | x 100 000 | +/- 0.5 % |
| Blue | 6 | x 1 000 000 | +/- 0.25 % |
| Violet | 7 | x 10 000 000 | +/- 0.1 % |
| Grey | 8 | - | +/- 0.05 % |
| White | 9 | - | - |
| Gold | - | x 0.1 | +/- 5 % |
| Silver | - | x 0.01 | +/- 10 % |
| No band | - | - | +/- 20 % |
The digit column is the one worth memorising. The multiplier is the same ten colours counting zeros, so learning it twice is wasted effort.
Resistor colour code calculator
Open on its own pageClick the bands you can actually see on the part in your hand.
Gold and silver are never a first band — if you can see one, you are reading from the wrong end. Turn the resistor round.
Four bands: the common case
Two digits, a multiplier, a tolerance. This is nearly every carbon film resistor you will be handed in a kit.
- Yellow = 4, the first digit.
- Violet = 7, the second digit. So far, 47.
- Red as a multiplier = x 100, so 47 x 100 = 4 700 ohms.
- Gold = +/- 5 %, so the real part is somewhere between 4 465 and 4 935 ohms.
Written down that is 4700R, 4.7k, or on a schematic 4k7. The letter stands in for the decimal point, so a smudge or a bad photocopy cannot turn 4.7k into 47k.
| Bands | Value | Where it turns up |
|---|---|---|
| Brown, Black, Brown, Gold | 100R | In series with a bright LED on 5 V |
| Red, Red, Brown, Gold | 220R | The default LED resistor on 5 V |
| Brown, Black, Red, Gold | 1k | Transistor base drive, pull-ups |
| Yellow, Violet, Red, Gold | 4k7 | I2C pull-ups, sensor dividers |
| Brown, Black, Orange, Gold | 10k | Pull-up and pull-down, everywhere |
| Brown, Black, Green, Gold | 1M | High-impedance inputs, slow RC timers |
Five and six bands: precision parts
Metal film resistors carry three digit bands instead of two, because quoting +/- 1 % is meaningless if you can only state the value to two figures. The body is usually pale blue rather than beige.
Brown, Black, Black, Red, Brown reads as 1, 0, 0 — so 100 — then x 100, giving 10k at +/- 1 %.
A sixth band, set further apart again, is the temperature coefficient in ppm per degree C: brown 100, red 50, orange 15, yellow 25, blue 10, violet 5. You can ignore it on a project kit. It matters when a circuit has to hold its calibration across a temperature range.
Why 4.7k, and never 5k
Resistors come in preferred values rather than round numbers, and the reasoning is neat. If a part is only accurate to +/- 10 %, then values spaced closer than about 20 % apart would overlap each other, so there is no point manufacturing them. The E12 series is twelve values per decade, each roughly 20 % above the last — no gaps, no duplication.
| 10 | 12 | 15 | 18 | 22 | 27 |
| 33 | 39 | 47 | 56 | 68 | 82 |
E24, used for +/- 5 % and +/- 2 % parts, interleaves 11, 13, 16, 20, 24, 30, 36, 43, 51, 62, 75 and 91.
This is why a calculation that asks for 313 ohms gets built with 330, and why the right answer in a viva is a preferred value plus one sentence saying why rounding that way is safe.
Surface-mount resistors have no bands
An SMD resistor is a small black chip with a number printed on it, and the number is the same idea in digits: the last figure is the count of zeros.
| Printed | Reads as | Value |
|---|---|---|
472 | 47, then 2 zeros | 4 700 = 4k7 |
103 | 10, then 3 zeros | 10 000 = 10k |
1002 | 100, then 2 zeros (4-digit, +/- 1 %) | 10 000 = 10k |
4R7 | R marks the decimal point | 4.7 ohms |
R022 | R leads, so 0.022 | 0.022 ohms, a current-sense shunt |
000 or 0 | A zero-ohm link, not a resistor | A wire in resistor clothing |
Very small parts use EIA-96, a two-digit code plus a letter, where 01C means 100 x 100 = 10k. It needs a lookup table and is not worth memorising — measure it instead.
Checking it with a meter
The code tells you what the part is meant to be. A meter tells you what it is. Both are worth having and they answer different questions.
- 1
Take it out of circuit, or lift one leg
Everything else on the board sits in parallel with the resistor and drags the reading down. This is why a measured resistor reads wrong nine times out of ten, and the resistor is fine.
- 2
Set the meter to ohms, one range above the expected value
An auto-ranging meter does this for you. On a manual one: the 2k range for anything under 2 000 ohms, 20k above that.
- 3
Do not pinch both leads between your fingers
Your body is a few hundred k across two hands. On a 1M resistor you are measuring yourself in parallel with the part, and the reading falls.
- 4
Judge it against the tolerance, not the printed value
A 4k7 +/- 5 % part reading 4.61k is in spec and perfectly good. Reading 3.9k is not, and that one is damaged or misread.