Resistor colour code calculator, reading and lookup

The colour code both ways round: from the bands of a resistor to its value and tolerance interval, or from a wanted value to the bands to look for in the drawer. The requested value is projected onto the normalised series of its tolerance (E24 for 5%, E96 for 1%): for 5,000 Ω, the value proposed is 5.1 kΩ, with the gap. When both reading directions are plausible, the ambiguity is flagged.

Each colour carries a digit from 0 (black) to 9 (white); the second-to-last band is a power of ten, the last one the tolerance. Gold and silver carry no digit: they only occupy the multiplier (dividing by 10 or 100) or the tolerance, never the first position.

Tolerance

the gap the manufacturer guarantees: a 1 kΩ at 5% really sits between 950 and 1,050 Ω.

E24 series

24 normalised values per decade (1.0, 1.1, 1.2… 9.1), about 10% apart, those of 5% resistors.

E96 series

96 values per decade, three significant figures, about 2.4% apart, those of the 1%.

Decade

a factor of 10: the series repeat identically from 1 Ω to 10 Ω, 10 to 100, and so on.

The code covers through-hole resistors with 4 and 5 bands per IEC 60062, from 0.1 Ω to 9.99 GΩ. Six-band codes add a temperature coefficient, not handled; surface-mount parts carry numeric marking. The tolerance band is read on the right. When both directions each give a plausible value, both are shown; the value outside the normalised series is almost always the backwards reading.

The IEC 60063 series are geometric: the tolerance intervals of two neighbouring values just overlap. A requested value outside the series is brought to the closest one in relative terms.