Resistor Color Code Calculator

Through-hole resistors encode their value in colored bands. 4-band: two significant digits + multiplier + tolerance. 5-band: three significant digits + multiplier + tolerance (precision parts). 6-band: 5-band plus a temperature coefficient. This tool lets you pick each band's color from a visual selector and shows the computed resistance, tolerance range, and a live preview of the resistor body. Useful when you've pulled a part from a bin and can't read the markings off a datasheet.

Which end you start from is the first problem, because the bands are not symmetric but a resistor is. The tolerance band is conventionally separated by a wider gap and is usually gold or silver, neither of which appears as a digit, so finding it tells you which end is last. On a part where the tolerance is brown or red that cue is gone, and a 4-band resistor read backwards gives a completely different and entirely plausible value.

Colour is a poor medium for a code, which is the deeper problem. Brown against red, and red against orange, are hard to separate under warm light or on an aged body, and roughly one man in twelve has a colour vision deficiency that makes exactly those pairs ambiguous. A reading that matters should be confirmed with a meter rather than trusted to a glance, and that is true of good eyes as well.

The value you compute is a nominal one, and the tolerance band says how far the real part may sit from it. A 100 Ω resistor at 5 percent is anywhere from 95 to 105 and is not defective at either end. Circuits are designed around that, which is why a divider built from two nominal values can be several percent off the ratio you calculated.

The bands carry no power rating, and that is the specification most likely to matter next. Dissipation is set by the physical size of the body, not by anything printed on it, so a quarter-watt and a one-watt part of the same value carry identical bands and only look different.