Nearest E-Series Value
Snap a target resistance to the nearest E12 / E24 / E96 standard value.
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The engineering
Standard values are logarithmically spaced so that adjacent values' tolerance bands tile the decade: E12 steps are ~21% apart for ±10% parts, E24 ~10% for ±5%, E96 ~2.4% for ±1%. The worst-case snap error is therefore about half a step — ~1.2% for E96 — which is why chasing an exact computed resistance is usually pointless past the second significant figure.
A wrinkle worth knowing: E12 and E24 keep pre-standard historical roundings (2.7, 3.3, 4.7, 8.2) that don't match the pure 10^(i/N) formula, while E96 follows the formula exactly to three digits. If the snapped error still hurts, put two E96 values in series and split the difference — cheaper than a 0.1% part.
Where this math comes from
Preferred numbers are French military engineering: Charles Renard cut balloon-rope inventory in 1877 by spacing sizes geometrically, and the ISO 'R-series' still bears his initial. Component makers borrowed the idea when radio parts became mass goods — tolerance bands, not round numbers, should decide the catalog.
The electronics-specific series was standardized as IEC publication 63 in 1952 (today IEC 60063), blessing the E6/E12/E24 values already common in 1930s–40s practice and later extending to E48/E96/E192 as metal-film precision made ±1% ordinary in the 1970s.
- 1877Charles RenardPreferred-number series — geometric sizing of standard parts.
- 1952IECIEC 63 standardizes the E-series, circa.
- 1975Metal-film resistor era±1% parts make E96 the working series, circa.
See the full timeline of the math behind every calculator →
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