Understanding E-Series Standard Values: E12, E24, and E96 Explained
Why are resistor values strange numbers like 4.7k or 6.8k instead of round numbers? Discover the logarithmic geometry behind IEC 60063 preferred numbers.
Have you ever wondered why you cannot easily buy a 5.0 kΩ or 7.0 kΩ standard resistor, but you can find 4.7 kΩ and 6.8 kΩ in almost any parts bin?
The explanation lies in the international standard IEC 60063, which defines preferred number series known as E-series.
The Geometry of Logarithmic Decades
If resistors were manufactured at linear steps (e.g. 10 Ω, 20 Ω, 30 Ω …), the relative percentage gap between successive values would change wildly. A step from 10 Ω to 20 Ω would be a 100% jump, while a step from 90 Ω to 100 Ω would only be 11%.
To maintain a consistent relative overlap based on manufacturing tolerance, preferred numbers follow a logarithmic scale where each decade is divided into N logarithmic intervals:
Interval Ratio = 10^(1 / N) where N is 12, 24, 48, 96, or 192Comparing the Primary E-Series
The E12 Series (±10% Tolerance)
E12 divides each decade (e.g. 1 Ω to 10 Ω, 100 Ω to 1 kΩ) into 12 logarithmic steps:
1.0, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, 8.2
Notice that with a ±10% manufacturing spread, the upper tolerance limit of 1.0 Ω (1.1 Ω) nearly meets the lower tolerance limit of 1.2 Ω (1.08 Ω). This guarantees that any manufactured resistance falls within an acceptable standard bucket with minimal factory scrap.
The E24 Series (±5% Tolerance)
E24 doubles the density to 24 steps per decade:
1.0, 1.1, 1.2, 1.3, 1.5, 1.6, 1.8, 2.0, 2.2, 2.4, 2.7, 3.0, 3.3, 3.6, 3.9, 4.3, 4.7, 5.1, 5.6, 6.2, 6.8, 7.5, 8.2, 9.1
This remains the worldwide standard for general-purpose surface mount (SMD) and through-hole resistors.
The E96 Series (±1% Tolerance)
High-precision analog circuits use the E96 series, which has 96 values per decade. These 3-digit significant values allow engineers to achieve extremely precise gain settings in instrumentation amplifiers, audio filters, and ADC input networks.
Consult our interactive Standard Values Reference Table on ResistCalc to explore the entire decade matrix across all commercial series.