Physics
Resistor colour code
Pick the bands you can see on the component and read the value, or go the other way starting from the ohms. Works with 4, 5 and 6 bands.
How the colour code is read
The bands are read left to right, with the tolerance band at the far end. On a four-band resistor the first two are digits, the third is the multiplier — the power of ten to multiply by — and the fourth is the tolerance. Red, red, brown, gold is therefore 22 × 10 = 220 ohms at 5%.
Five-band resistors add a third significant digit and are the precision ones: the structure is identical, with the multiplier fourth and the tolerance fifth. A sixth band, where present, gives the temperature coefficient in parts per million per kelvin — how far the value drifts as the temperature changes.
Working out which end to start from is the real practical problem, because the component has no orientation. Two clues nearly always work: the tolerance band is usually gold or silver, colours that never appear as a first digit, and it is typically separated from the rest by a wider gap. If both ends look plausible, read it both ways and pick the value that belongs to a commercial series.
Common mistakes
- Reading the resistor from the wrong end: red-brown-brown-gold is 210 ohms, but read backwards it gives a value that does not exist. Keep the gold or silver on the right.
- Mistaking the third band of a five-band resistor for the multiplier: on five bands there are three digits and the multiplier is fourth.
- Ignoring the tolerance: a 220-ohm resistor at 5% may legitimately measure between 209 and 231 ohms. A reading inside that band does not mean a faulty component.
Frequently asked questions
How do you read a 4-band resistor?
The first two bands are the digits, the third is the multiplier and the fourth the tolerance. Brown-black-red-gold is 10 × 100 = 1,000 ohms, that is 1 kΩ, at 5%.
Which end do you start from?
The end away from the tolerance band, which is nearly always gold or silver and usually set apart by a wider gap. Gold and silver never appear as a first digit.
What do gold and silver mean as a multiplier?
Multipliers below one: gold is 0.1 and silver is 0.01. They are used for very low-value resistors, under ten ohms.
What is the sixth band for?
It gives the temperature coefficient in ppm/K, how many parts per million the resistance changes per degree. It appears only on precision resistors meant for circuits sensitive to thermal drift.
How this calculation works
Value with 4 bands: R = (c₁·10 + c₂) × multiplier. With 5 or 6 bands: R = (c₁·100 + c₂·10 + c₃) × multiplier. Digits: black 0, brown 1, red 2, orange 3, yellow 4, green 5, blue 6, violet 7, grey 8, white 9. Multiplier: the same scale as a power of ten, plus gold ×0.1 and silver ×0.01. Tolerance: brown ±1%, red ±2%, green ±0.5%, blue ±0.25%, violet ±0.1%, grey ±0.05%, gold ±5%, silver ±10%. Permitted range: from R·(1 − t) to R·(1 + t). The sixth band gives the temperature coefficient in ppm/K.