Resistor colour code

Both directions, and the reverse is the useful one

Reading bands into a value is what every calculator does. The direction people standing at a parts drawer actually need is the other one: I want 4.7 kΩ — which bands?

That direction has to confront something the forward direction never does: not every value exists. Resistors are manufactured in the E-series, a geometric progression chosen so that consecutive values overlap within their tolerance. E12 has twelve values per decade, E24 twenty-four.

There is no 4.3 kΩ resistor in E12. Asking for one here snaps to the nearest value that is made — and says so — rather than quietly emitting bands for a component nobody sells.

“Nearest” is measured as a ratio rather than a difference, because the E-series is geometric. 4.3k sits exactly between 3.9k and 4.7k in absolute terms; as a ratio, 4.7k is the closer step, which is the answer an engineer expects.

Reading the bands

Four bands: two digits, a multiplier, a tolerance. Five bands: three digits, a multiplier, a tolerance. Six bands: as five, plus a temperature coefficient.

The tolerance band is the one set slightly apart from the others, and it is usually gold or silver — which is also how you tell which end to read from. Reading a resistor backwards is the single most common mistake, and it is easy to catch: brown-black-red is 1 kΩ, and red-black-brown is 200 Ω, so if the value looks implausible for the circuit, turn it around.

Gold and silver as multipliers mean ×0.1 and ×0.01, which is how values below 10 Ω are expressed.

Tolerance is a range, not a decoration

A 1 kΩ ±5% resistor is anywhere from 950 Ω to 1050 Ω, and the manufacturer makes no promise beyond that. Two resistors from the same tape can differ by 100 Ω and both be in specification.

That is fine for a pull-up and not fine for a divider setting a reference voltage — which is why 1% parts exist and why the tolerance band is worth reading rather than ignoring.

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