Those Numbers Are Not Random
22 pF, 33 uF, 47 uF, 68 uF, 220 uF, 470 uF. None of these are round numbers. Why? The answer goes back to early 20th-century manufacturing.
The E-Series Standard
Capacitor values follow the E-series: E6, E12, E24, E48, E96. The number after E tells you how many values exist per decade (a 10x range). E12 gives 12 values between 1 and 10: 1.0, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, 8.2. Multiply or divide by 10 to get any decade.
Each value is spaced roughly 20% apart — matching the manufacturing tolerance of early components. A 10 uF capacitor with +/-20% tolerance has a range of 8 to 12 uF. The next E12 value, 12 uF, ranges from 9.6 to 14.4 uF. These ranges touch without too much overlap.
Why Not Equal Spacing?
If values were equally spaced (10, 20, 30, 40), at higher values the tolerance ranges would overlap. At 100, a +/-20% part could be 80 to 120 — overlapping with both 80 and 120 nominal values. The E-series uses logarithmic progression: each step is the Nth root of 10. For E12, each step is 10 raised to 1/12 = approximately 1.212, or about 21%. This guarantees adjacent tolerance bands just barely touch.
Which E-Series to Use
- E6 (20% tolerance): Old electrolytics, general-purpose. 6 values per decade
- E12 (10% tolerance): Standard aluminum electrolytics, common ceramics
- E24 (5% tolerance): Film capacitors, better ceramics
- E48/E96 (2%/1%): Precision resistors mainly. Rarely needed for capacitors
Does This Still Matter?
Modern manufacturing can hit tighter tolerances. But the E-series persists because it is baked into every component database, CAD library, and BOM tool. Inventory is simpler with standard values. And circuit designs rarely need an exact capacitance — you pick the nearest E-series value meeting your minimum requirement.
Next time you see 47 uF on a schematic, that is not a random number. It is E12 value number 10, and the system has been working for nearly a century.
