How to read resistor colour codes (4-band and 5-band)

Most through-hole resistors are too small to print a value on, so they carry coloured bands instead. Each colour stands for a digit from 0 to 9: the first bands give the significant digits, the next one a power-of-ten multiplier and the last the tolerance. Yellow-violet-red-gold, for example, is 4, 7, × 100, ±5%: 4,700 Ω, or 4.7 kΩ. The resistor colour code calculator decodes any combination with a live drawing; this guide explains the system so you can read most resistors at a glance.

The ten colours

The digit colours are standardised internationally, in IEC 60062 and the older American EIA standards:

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

The middle of the sequence, red to violet, follows the order of the rainbow, which makes it easier to remember than it looks: black and brown come before the rainbow, grey and white after it. Many people learn the order with a mnemonic sentence whose initials match the colours, and one you make up yourself works as well as any classic. Two metallic colours appear only in the later bands: gold and silver.

Reading a 4-band resistor

On a 4-band resistor the first two bands are digits, the third is the multiplier (a power of ten, or simply the number of zeros to add) and the fourth is the tolerance:

• Brown-black-red-gold: 1, 0, × 100 = 1,000 Ω (1 kΩ) ±5%.
• Yellow-violet-red-gold: 4, 7, × 100 = 4,700 Ω (4.7 kΩ) ±5%.
• Red-red-brown-gold: 2, 2, × 10 = 220 Ω ±5%.
• Brown-black-orange-gold: 1, 0, × 1,000 = 10 kΩ ±5%.

Gold and silver in the multiplier position mean × 0.1 and × 0.01, which is how values below 10 Ω are marked: yellow-violet-gold-gold is 47 × 0.1 = 4.7 Ω ±5%. The multiplier works just like the exponent in Scientific notation explained, with e-notation and engineering notation: a red third band means × 10², an orange one × 10³.

Tolerance and the 5-band code

Tolerance is how far the real resistance may differ from the marked value. Gold means ±5% and silver ±10%, and on old parts a missing tolerance band means ±20%. Precision colours include brown ±1%, red ±2%, green ±0.5%, blue ±0.25% and violet ±0.1%. A 4.7 kΩ ±5% resistor can measure anywhere from 4,465 to 4,935 Ω and still be within specification, so a multimeter reading of 4.62 kΩ does not mean it is faulty.

Precision resistors, usually ±1% or better, have five bands: three digits, a multiplier and a tolerance. Brown-black-black-brown-brown is 1, 0, 0, × 10 = 1 kΩ ±1%, and yellow-violet-black-brown-brown is 470 × 10 = 4.7 kΩ ±1%. The third digit allows values such as 4.99 kΩ that two digits cannot express. Some parts add a sixth band for the temperature coefficient: how much the resistance drifts as the part warms up.

Which end to start from

Read from the end where the bands begin, with the tolerance band on the right. The clues: the tolerance band is often gold or silver, it is usually set slightly apart from the others, and the first band sits closer to the end of the body. Gold and silver never appear as the first band, so a metallic band at one end marks the right-hand side. If you still get an absurd result, such as 82 MΩ in a circuit that obviously wants a few kilohms, turn the resistor round and try again.

Age and heat play tricks too. Bands discolour on hot-running parts, and brown, red and orange can look alike under warm indoor light. When in doubt, measure with a multimeter, ideally with one leg lifted out of the circuit so that parallel components do not skew the reading.

Preferred values and SMD codes

You will not find a 50 kΩ resistor in most kits, because values come from the E series of preferred numbers. The E12 series, historically paired with ±10% parts, has twelve values per decade: 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68 and 82, each about 21% above the last, so the tolerance ranges of neighbouring values roughly meet. E24 fills in steps for ±5% parts and E96 for ±1% parts. If you need a value in between, combine two: the series and parallel resistor calculator finds the closest standard pair.

Surface-mount resistors are too small even for bands and print a short code instead. "472" means 47 × 10² = 4.7 kΩ; "4701" is 470 × 10¹, also 4.7 kΩ; "4R7" uses R as the decimal point for 4.7 Ω; and tiny 1% parts use EIA-96 codes such as "01C", meaning 100 × 100 = 10 kΩ. The SMD resistor code calculator decodes all four formats, and the capacitor code calculator handles the similar three-digit codes on ceramic capacitors. With a value in hand, Ohm’s law explained: voltage, current, resistance and power and the Ohm's law calculator tell you what current it will pass and how much power it must handle.