Standard Resistor Value Finder (E24, E96)

Preferred-number series from IEC 60063. Each divides a decade into equal steps on a logarithmic scale — 12, 24 or 96 of them — so that with the matching tolerance (10 %, 5 %, 1 %) the ranges of neighbouring values roughly touch. Any resistance can then be approximated within about half a tolerance band.

E3 to E24 predate the formula and were standardised as the values already in use. Several differ from rounded powers of ten — 2.7, 3.0, 3.3, 3.6, 3.9, 4.3, 4.7 and 8.2 among them — while E48 to E192 follow the formula exactly apart from E192's 9.20. The tool uses the standard's tables, not the formula.

When the nearest single value is off by more than your circuit tolerates and a tighter series is not available. Two 5 % parts in series or parallel can land within a fraction of a percent of the target, though each still carries its own tolerance, so the combination is only as accurate as its parts allow.

For each standard value in a window around the target, the tool works out the partner that would make the pair exact — the remainder for series, or the value that completes the parallel combination — and tries the standard values on either side of it. That window is wide enough to include pairs such as 68 k ∥ 68 k for 33.3 kΩ, where the smaller part is more than twice the target. The best result has been checked against a brute-force search over every pair of standard values from 0.1 Ω to 100 MΩ.

The series are the same — capacitors are commonly made in E6 or E12, sometimes E24 — so the nearest-value results apply. Remember that capacitors combine the other way round: in parallel they add, and in series they combine like resistors in parallel.