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The resistor color code is the scheme of coloured bands painted on a resistor to show its value, used because the parts are too small to print numbers on. Reading left to right, the first bands are significant digits, the next band is a power-of-ten multiplier and the last band is the tolerance. For a 4-band part the value is (10 × digit1 + digit2) × 10^multiplier ohms; 5- and 6-band parts add a third significant digit for tighter precision (and, on 6-band parts, a temperature-coefficient band). Each colour maps to a fixed number: black 0, brown 1, red 2, orange 3, yellow 4, green 5, blue 6, violet 7, grey 8, white 9, with gold and silver serving as ×0.1 and ×0.01 multipliers or ±5% and ±10% tolerances.

As a worked example, the classic brown-black-red-gold band pattern reads as digits 1 and 0, a ×100 multiplier and ±5% tolerance: (10 + 0) × 100 = 1,000 Ω, i.e. a 1 kΩ ±5% resistor whose true value lies anywhere from 950 Ω to 1,050 Ω. Decode any part by selecting the colour of each band from the drop-downs and the calculator returns the nominal resistance, the tolerance and the guaranteed value range.

Switch to reverse (encode) mode to go the other way: type a target resistance and the tool rounds it to the nearest value the digit bands can express, chooses the multiplier to match, and lists the band colours you would read off a real resistor of that value — handy when you have a schematic value and need to find or verify the physical part on your bench.