Decode colours to a value or go the other way, and decode SMD markings including EIA-96. The E-series check is the useful part: if the result is not a standard value, you almost certainly read the bands from the wrong end.
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Most colour-code calculators only decode. In practice you need the other direction just as often — you have a value from a schematic and you are hunting through a drawer, or checking that what you fitted is what the BOM said. This does both, plus the three SMD marking systems, which are where people actually get stuck now that through-hole parts are rare.
Resistors are not made in arbitrary values. They come from the E-series — E24 for 5% parts, E96 for 1% — which divide each decade into logarithmically even steps. So if your decode produces a value that is not in any standard series, you almost certainly read the bands backwards.
That is by far the most common mistake, because there is no universal marking for which end to start from. The convention is that the tolerance band sits nearer one end, with a wider gap before it, but on a small part with a gold band at each end the cue can be genuinely ambiguous. The tool flags a non-standard result rather than confidently reporting it, which is the check that catches it.
A 4.7 kΩ 5% resistor is anything from 4.465 to 4.935 kΩ, and every value in that window is in specification. Any circuit whose behaviour depends on it being 4.7 is not a design, it is a coincidence — which is the same argument the divider calculator makes at more length, with the worst-case stack worked out.
Note too that the colours encode tolerance but say nothing about temperature coefficient, which on a 5% carbon film part can be several hundred ppm/°C. The sixth band on a six-band part is tempco, and it is the only place that appears.
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