The Designer’s Guide to Standard Resistance: Decoding the E12 & E24 Resistor Series

If you’ve ever spent an afternoon digging through a component bin looking for a 140Ω resistor only to realize it doesn’t exist, you’ve encountered the “Preferred Number” system. As PCB engineers, we don’t just pick arbitrary numbers for our designs. We operate within a world defined by the International Electrotechnical Commission (IEC), specifically the IEC 60063 standard.

Understanding the E12 series resistors and the E24 resistor series isn’t just about memorizing numbers; it’s about understanding tolerance stack-up, manufacturing efficiency, and logarithmic spacing. When you’re laying out a board, choosing a value from these standard series ensures that your BOM (Bill of Materials) is cost-effective and that your components are actually in stock at distributors like Mouser or DigiKey.

In this guide, we’ll look at the mathematical logic behind these series, why we use them, and how to choose the right series for your specific circuit requirements.

The Mathematical Logic: Why These Specific Numbers?

It seems chaotic at first. Why is there a 4.7, but not a 5.0? Why 2.2 instead of 2.5? The answer lies in Renard numbers and logarithmic scales.

The “E” in E-series stands for “Exponential.” These values are calculated so that each successive value is a fixed percentage higher than the previous one. This ensures that the tolerance of one resistor “overlaps” or meets the tolerance of the next one in the series. This way, a manufacturer can produce a range of resistors and, regardless of the actual value measured, it will fit into one of the standard slots.

For any series $E_n$, the values are calculated using the $n$-th root of 10:

$$\sqrt[n]{10}$$

The E12 Logic

In the E12 series resistors, there are 12 steps in a decade (e.g., between 10Ω and 100Ω).

The multiplier is $\sqrt[12]{10} \approx 1.21$.

Each value is approximately 21% larger than the one before it. This series is designed for resistors with a 10% tolerance.

The E24 Logic

The E24 resistor series has 24 steps per decade.

The multiplier is $\sqrt[24]{10} \approx 1.10$.

Each value is about 10% larger than the previous one. This series is the industry standard for 5% tolerance resistors, though it is frequently used for 1% resistors today because of its widespread availability.

The E12 Series Resistors: The “General Purpose” Workhorse

The E12 series is the classic choice for non-critical applications. If you are designing a pull-up resistor for a digital input or a simple LED current limiter, E12 is your best friend.

Why use E12?

Cost: Historically, these were cheaper because the tolerances were looser (10%).

Inventory Simplicity: You only need to stock 12 values per decade to cover every possible resistance requirement within a 10% margin of error.

Prototyping: Most “resistor kits” sold for breadboarding are based on the E12 series.

E12 Standard Values Table

These base values are multiplied by powers of ten (0.1, 1, 10, 100, etc.).

Step	Base Value	Step	Base Value
1	1.0	7	3.3
2	1.2	8	3.9
3	1.5	9	4.7
4	1.8	10	5.6
5	2.2	11	6.8
6	2.7	12	8.2

The E24 Resistor Series: The Precision Standard

As a PCB engineer, the E24 resistor series is likely the most common series you will specify in your EDA tool (like Altium, KiCad, or OrCAD). It offers much finer control over circuit parameters.

Engineering Applications for E24

Voltage Dividers: When you need a specific output voltage for a DC-DC converter feedback loop.

Active Filters: Where the cutoff frequency depends heavily on the $RC$ constant.

Biasing Transistors: Getting the Q-point exactly where it needs to be for low-distortion amplification.

E24 Standard Values Table

The E24 series includes all E12 values plus an additional 12 intermediate values.

Value 1-6	Value 7-12	Value 13-18	Value 19-24
1.0	1.8	3.3	5.6
1.1	2.0	3.6	6.2
1.2	2.2	3.9	6.8
1.3	2.4	4.3	7.5
1.5	2.7	4.7	8.2
1.6	3.0	5.1	9.1
(Note: Bold values are the ones added to the E12 list to create the E24 series.)

Tolerance Overlap: Why 4.7 isn’t 5.0

One of the most common questions from junior engineers is: “Why can’t I just use a 500Ω resistor?”

Let’s look at the E12 series resistors (10% tolerance) around the 4.7 value:

A 4.7Ω resistor at +10% is 5.17Ω.

The next value is 5.6Ω. At -10%, it is 5.04Ω.

Notice how the upper limit of 4.7Ω nearly meets the lower limit of 5.6Ω. If we added a 5.0Ω resistor in the middle, it would be redundant because a “sloppy” 4.7Ω resistor might actually be 5.0Ω, and a “sloppy” 5.6Ω might also be 5.0Ω. The E-series eliminates this manufacturing ambiguity.

Selecting Between E12, E24, and Beyond (E48, E96)

In modern PCB design, the lines are blurring. Surface Mount Technology (SMT) has made 1% resistors so cheap that many firms just stock the E96 series as their baseline. However, there are still rules for when to jump up in complexity.

1. The Cost of Over-Engineering

While a 1% E96 resistor is cheap, using a highly specific value when an E12 value would work makes your BOM harder to manage. If you can use a 10kΩ (E12/E24) instead of a 10.2kΩ (E96), do it. It increases the “commonality” of parts on your board, which can lead to volume discounts during assembly.

2. Tolerance Stack-up

If you are designing a precision current sense circuit, a 5% E24 resistor series component might not be enough. Even if the value is correct, the drift might kill your accuracy. In these cases, you move to E96 or E192, not just for the values, but for the 1% or 0.1% tolerance that usually accompanies those series.

3. Power and Voltage Ratings

Remember that series like E12 and E24 apply to the resistance value, but not the physical package. You can find a 10k resistor in an 0402 package or a 2512 package. Always cross-reference your E-series choice with the required power dissipation ($P=V^2/R$).

Practical PCB Design Tips for Resistor Series

Standardize Your “House Values”

Most engineering firms maintain a “House Stock.” This is a list of resistors they keep in the pick-and-place machine at all times. Usually, this is a subset of the E24 resistor series. Before you pick an obscure E96 value, check if your assembly house has a similar E24 value already loaded. It can save you a $50 “custom setup” fee per reel.

Use Series/Parallel Combinations

If you absolutely need a value that isn’t in the E24 series (like a 140Ω), you can create it using two E12 resistors.

120Ω + 22Ω (Series) = 142Ω

270Ω || 300Ω (Parallel) = 142.1Ω

This is often better than ordering a single reel of a rare E96 value for a small production run.

EDA Libraries

When setting up your CAD library, I recommend naming your components by their E-series value and footprint (e.g., RES_10K_0603_1%). This makes it much easier to swap between the E12 series resistors and E24 equivalents during the layout phase.

Useful Resources for Resistance Selection

To help you navigate these standard values, here are some of the tools I use daily:

Standard Value Calculators:

E-Series Resistor Calculator – Input a custom value, and it tells you the closest E12, E24, or E96 match.

Resistor Color Code Tool – Essential for those of us still working with through-hole components in the lab.

Datasheet Databases:

Vishay Resistor Series Overview – A great look at how different materials (thin film, thick film, wirewound) utilize these series.

Yageo Standard Resistance Table – A clear PDF download for your workbench wall.

Engineering Books:

The Art of Electronics (Horowitz & Hill) – Chapter 1 covers the philosophy of component selection beautifully.

Frequently Asked Questions (FAQs)

1. Can I use an E24 resistor in a circuit designed for E12?

Absolutely. The E24 series is a superset of E12. Every E12 value is also an E24 value. Using an E24 resistor usually just means you are using a component with a tighter tolerance (5% vs 10%).

2. Why is the 5.1 value in E24 but not E12?

The 5.1 value exists to fill the gap between 4.7 and 5.6. In a 10% (E12) world, that gap is already covered by the tolerances of the neighbors. In a 5% (E24) world, the gap is large enough that a new standard value (5.1) is needed to ensure every resistance is reachable.

3. Are 1% resistors always E96?

No. You can buy 1% resistors in E24 values. In fact, many “1% Resistor Kits” are actually just the E24 series manufactured to a tighter 1% tolerance. This is great for engineers because it keeps the numbers “round” and easy to calculate.

4. What happens if I use a non-standard value?

If you write “145 Ohms” on your schematic, the purchasing department will likely flag it as a “non-stock” item. You will end up paying a premium for a “precision” or “custom” resistor, or the lead time will be 12 weeks instead of 2 days. Always try to fit your design into the E24 series.

5. How do I identify an E24 resistor by its color bands?

E24 resistors usually have four bands (Value, Value, Multiplier, Tolerance). E96 resistors have five bands to accommodate the extra digit of precision. If you see a resistor with a gold tolerance band, it’s a 5% E24 part.

Conclusion: Stick to the Standards

Whether you are a hobbyist or a professional PCB engineer, the E12 series resistors and E24 resistor series are the boundaries within which we play. These series weren’t created to limit us, but to provide a reliable, repeatable framework for manufacturing and design.

By sticking to these standard values, you ensure your designs are robust, your parts are replaceable, and your production costs stay low. Next time you’re tempted to use a weird, calculated value, take a second to see if a standard E24 value will get the job done. Nine times out of ten, it will.

Would you like me to generate a PDF cheat sheet of these E-series tables that you can print out for your lab or office?