Why components use codes
A through-hole resistor is a few millimetres long, and a 0603 SMD is smaller than a grain of rice. Printing the value in full is impossible, so the industry marks parts with a short code.
The IEC 60062 standard gathers these codes for resistors and capacitors. The 2016 edition specifies the marking codes, methods for value and tolerance, and date code systems for small components.
In practice you only need three skills: reading bands, reading three or four-digit numbers, and converting picofarads into nanofarads and microfarads.
Banded resistors: the color code
The table below follows Digi-Key’s resistor color code calculator. The colors from black to white stand for 0 to 9 as a digit; the same color changes role depending on the band’s position.
| Color | Digit | Multiplier | Tolerance | Temp. coefficient |
|---|---|---|---|---|
| Black | 0 | x1 Ω | n/a | n/a |
| Brown | 1 | x10 Ω | ±1% | 100 ppm |
| Red | 2 | x100 Ω | ±2% | 50 ppm |
| Orange | 3 | x1 kΩ | n/a | 15 ppm |
| Yellow | 4 | x10 kΩ | n/a | 25 ppm |
| Green | 5 | x100 kΩ | ±0.5% | n/a |
| Blue | 6 | x1 MΩ | ±0.25% | 10 ppm |
| Violet | 7 | x10 MΩ | ±0.1% | 5 ppm |
| Grey | 8 | x100 MΩ | ±0.05% | n/a |
| White | 9 | x1 GΩ | n/a | n/a |
| Gold | n/a | x0.1 Ω | ±5% | n/a |
| Silver | n/a | x0.01 Ω | ±10% | n/a |
The most common format has four bands: two digits, multiplier and tolerance. With five bands, a third digit adds precision. In calculators such as Digi-Key’s, the temperature coefficient (ppm) shows up as an extra band, in six-band versions.
- Find the right endThe tolerance band (gold or silver) usually sits apart from the others. Start reading from the opposite end.
- Read the digitsConvert the first two or three colors into numbers and join them.
- Apply the multiplierMultiply the number by the power given by the next band.
- Note the toleranceThe last band says how much the real value may vary.
Example 1: brown, black, red, gold is 1, 0, x100, ±5%, that is, 10 x 100 = 1 kΩ with 5% tolerance. Example 2: yellow, violet, orange, silver is 47 x 1 kΩ = 47 kΩ ±10%.
Example 3, with five bands: brown, black, black, red, brown is 100 x 100 = 10 kΩ ±1%. The color code decoder works both ways, from colors to value and from value to colors.
SMD resistors: numbers instead of colors
According to Digi-Key, standard-tolerance SMD resistors use a three-digit code: the first two are the significant digits and the third is the multiplier. The four-digit version has three significant digits and allows higher precision.
When an “R” appears, it marks the position of the decimal point. The EIA-96 system, used on 1% parts, has three characters: two numbers that give the value’s position in the E96 series and a letter that gives the multiplier.
| Marking | Rule | Value |
|---|---|---|
| 472 | 47 x 10^2 | 4.7 kΩ |
| 103 | 10 x 10^3 | 10 kΩ |
| 4R7 | R is the decimal point | 4.7 Ω |
| 1002 | 100 x 10^2 (four digits) | 10 kΩ |
Capacitors: the three-digit code and the 104
On ceramic and film capacitors, the three-digit code follows the same logic as SMD resistors, but the result comes in picofarads (pF). The first two digits are the value and the third is the power of 10.
C (pF) = AB x 10 ^ N (código ABN)- AB
- The first two digits
- N
- The third digit, the power of 10
Applying it to 104: 10 x 10^4 pF = 100,000 pF. Since 1,000 pF = 1 nF and 1,000,000 pF = 1 µF, that is 100 nF or 0.1 µF. It is a value widely used in decoupling capacitors.
| Code | Calculation | Value |
|---|---|---|
| 2R2 | R is the decimal point | 2.2 pF |
| 223 | 22 x 10^3 pF | 22 nF (0.022 µF) |
| 473 | 47 x 10^3 pF | 47 nF (0.047 µF) |
| 104 | 10 x 10^4 pF | 100 nF (0.1 µF) |
| 474 | 47 x 10^4 pF | 470 nF (0.47 µF) |
| 106 | 10 x 10^6 pF | 10 µF |
A letter after the digits indicates tolerance: J is ±5%, K is ±10% and M is ±20%. A letter before the digits indicates the temperature coefficient, as in “Y5V106”. So “106J” is a 10 µF capacitor at ±5%.
Careful: electrolytic and large capacitors usually have the value printed in full. The three-digit code is typical of small parts.
To convert without losing zeros, use the capacitor code tool. The idea of reading a short code and checking a digit also appears in barcodes and check digits.
Putting it to work: an LED resistor
The classic calculation is the resistor in series with an LED. By Ohm’s law, the resistor must absorb the difference between the supply voltage and the LED drop, at the desired current.
R = (Vfonte - Vf) / I
P = (Vfonte - Vf) x I- Vfonte
- Supply voltage, in volts
- Vf
- LED forward voltage, in volts
- I
- Desired current, in amperes
Example 1: 5 V supply, 2 V LED and 20 mA. R = (5 - 2) / 0.020 = 150 Ω and P = 3 x 0.020 = 0.06 W. Digi-Key recommends a resistor rated between 2 and 10 times the calculated value, that is, 0.12 to 0.6 W.
Example 2: a 9 V supply, 2 V LED and 20 mA give 350 Ω, a value that does not exist in the E12 series. With 330 Ω the current would be 7 / 330 = 21.2 mA, above target; with 390 Ω it is 17.9 mA. The safest choice is the next standard value up, 390 Ω.
The LED resistor calculator does this calculation for one or several LEDs, in series or parallel, and shows the standard value and the band colors.
Frequently asked questions
What does 104 mean on a capacitor?
How do I know which end to start reading resistor bands from?
What does the letter R mean in 4R7?
How do I read an SMD resistor marked 103?
What is the difference between J, K and M on a capacitor?
Resistors and capacitors speak in short codes, and the logic is similar: significant digits, a multiplier and, at the end, tolerance. Remember that a color or digit becomes a power of 10, that “104” is 100 nF, and that R marks the decimal point. When the value is critical, the multimeter and the datasheet have the final word.