Decode a 4-band resistor's color code into its resistance value and tolerance percentage.
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Resistors use a standardized color-band system to print their resistance value directly on their tiny bodies without requiring readable text. On a common 4-band resistor, the first two bands represent significant digits, the third band is a multiplier that determines how many zeros to add (or, for gold and silver, a fractional multiplier below 1), and the fourth band indicates tolerance — how far the actual resistance may vary from the printed nominal value. This calculator combines your selected band colors using the standard EIA color code table to compute the exact resistance and its tolerance range. The color code system was standardized decades ago specifically because printed text would be impractical to read on such small components, and colored bands remain readable from any angle at a glance. Learning to read a handful of common color combinations quickly becomes second nature for anyone who regularly works with electronic components on a breadboard.
Printing a number like "4,700" clearly on a component roughly the size of a grain of rice is, in practice, nearly impossible — which is exactly the problem the resistor color code was invented to solve. Rather than fighting to fit readable text onto a tiny cylindrical body, manufacturers print a sequence of colored bands that encode the same information far more legibly at a glance, and the system has remained essentially unchanged since it was standardized decades ago.
The logic behind the code is genuinely elegant once you see the pattern: each color from black through white is assigned a single digit from 0 to 9, in a sequence that (not coincidentally) roughly follows the visible light spectrum from low to high frequency — black and brown at the "cool" end representing 0 and 1, climbing through the rainbow to white representing 9. The first two color bands on a standard resistor simply spell out the first two significant digits of the resistance value using this same digit-to-color mapping.
The third band, called the multiplier, tells you how many zeros to add after those first two digits — or, uniquely for the colors gold and silver, to instead multiply by a fraction less than one, allowing the same limited set of colors to represent both very large and very small resistance values without needing additional bands. A resistor reading brown-black-red, for instance, gives digits 1 and 0, followed by a red multiplier band meaning ×100 — yielding 10 × 100 = 1,000 ohms.
The fourth and final band breaks from the digit pattern entirely and instead indicates manufacturing tolerance: how far the resistor's true resistance is guaranteed to fall from its printed nominal value. No manufacturing process produces a component with perfectly exact resistance every time, so this tolerance band sets honest expectations — a gold tolerance band means the true value falls within 5% of nominal, a fair compromise between manufacturing cost and precision for most general-purpose electronics.
Reading direction matters and is usually easy to determine: the tolerance band is conventionally spaced slightly further from the other bands than they are from each other, which serves as a visual cue for which end to start reading from, ensuring the same physical resistor is read identically regardless of which way it happens to be oriented on a workbench or breadboard.
Start from the band closest to one edge of the resistor; the tolerance band (commonly gold or silver) is usually spaced slightly further from the others and is read last, which also helps confirm you're reading in the correct direction.
5 and 6-band resistors add an extra significant digit (and sometimes a temperature coefficient band) for higher precision; this calculator covers the standard 4-band case, the most common for general-purpose resistors.
It's the maximum expected deviation between the resistor's printed nominal value and its true manufactured resistance — a 5% tolerance on a 1,000 Ω resistor means the actual value could be anywhere from 950 to 1,050 Ω.