E12 and E24 Resistor Values Explained

Why not just use round numbers?

Because a resistor is sold with a tolerance. A ±10% part marked 10 Ω is anything from 9 to 11 Ω. The next value up should start where that one ends — so it must be about 12 Ω, not 11 or 15.

Follow that logic around a decade and you get twelve values. Each is roughly 1.21 times the last, because the twelfth root of 10 is 1.212.

The general rule

• En steps by the nth root of 10.
• E12: ⁸²√10 ≈ 1.21 → 21% steps, matching ±10% parts.
• E24: ²⁴√10 ≈ 1.10 → 10% steps, matching ±5% parts.
• E96: ≈ 1.024 → 2.4% steps, matching ±1% parts.

The E12 series

Multiply any of these by 1, 10, 100, 1 k, 10 k, 100 k or 1 M. That is why 4.7 kΩ, 47 kΩ and 470 kΩ are all common, while 5 kΩ is not stocked anywhere.

The E24 series

E24 contains every E12 value plus twelve more between them:

Choosing a series for a design

Hitting a value that is not on the list

Suppose a divider calls for 3.3 kΩ but you need 3.4 kΩ:

• Series: 3.3 k + 100 Ω = 3.4 kΩ exactly.
• Parallel: 4.7 k ∥ 12 k = 3.38 kΩ.
• Move to E96, where 3.40 kΩ is a stocked value.
• Re-scale the whole divider so both legs land on preferred values — usually the cleanest fix, since only the ratio matters.

Check any combination with the series or parallel resistor calculator.

Preferred values beyond resistors

The same logarithmic spacing appears throughout electronics, because the reasoning about tolerance is identical wherever a component is manufactured to a percentage rather than an absolute figure.

• Capacitors follow E6 or E12 — which is why 100 nF, 220 nF and 470 nF are everywhere and 300 nF is nowhere.
• Inductors use E12 or E24 in the same way.
• Zener diodes follow a similar preferred set: 3.3 V, 3.9 V, 4.7 V, 5.6 V.

Designing so the exact value does not matter

The most robust designs are the ones where a 5% shift changes nothing important. Pull-up resistors, LED current limiters and decoupling networks all work across a wide range of values.

Save the tight-tolerance parts for the places where the ratio genuinely matters — feedback networks, references and measurement dividers — and use whatever is in the drawer everywhere else.

Reading values back

Knowing the preferred series makes colour codes self-checking: if your reading gives 43 kΩ you can trust it, but 44 kΩ almost certainly means a misread band. The resistor color code chart covers the decoding, and Ohms Law Wheel's calculator flags the nearest preferred value automatically.