The Common Parts Library is a curated collection of commonly used components for connected devices. Our goal is to provide a selection of components large enough to accommodate many designs, but small enough to make purchasing and organizing simple. To select resistors for the Common Parts Library, we looked at the history and engineering behind the standard resistor values. In addition, we analyzed our broad and deep database of supply chain information from over 200 component distributors. We found some intriguing differences, which I would like to share with you today. Resistors are manufactured and rated with specific resistance values. But because of variations in manufacturing, a resistor is never exactly its rated value. It's engineering: everything has a tolerance! Typical resistor tolerances are ±1%, ±5%, ±10%. As an example, when you pull a 100 Ω ±10% resistor out of the bag, its actual value is between 90 Ω and 110 Ω. Therefore, it doesn't make sense to manufacture or buy a 105 Ω ±10% resistor – they overlap. The next value practical value in 10% resistors is roughly 120 Ω. After that comes 150 Ω, then 180 Ω, then 220 Ω, and so on. The values are chosen so that when your circuit requires any arbitrary resistance value, you can select the nearest standard value and the maximum error will be 1%, 5%, or 10%, depending on the resistor tolerance. This practice is an example of a preferred values system . In electronics, these numbers are called the E series and are defined by the Electronic Industries Association and the IEC 60063 standard: "Preferred number series for resistors and capacitors." Here is the chart for the hundreds decade. You move the decimal point to get your 1k, 10k, or 100k resistors. (Click the table to view it larger.) Table of standard resistor values – click the table to view it larger. From this chart, we can see that 330 Ω ±10% resistor is standard, but a 330 Ω ±1% resistor is not. For 1% tolerance resistors, the E96 series tells us that 324 Ω or 332 Ω are standard.The E12 series is entirely contained within the E24 series. The E48 series is entirely contained within E96. The E24 series intersects the E96 series. This preferred system was developed a long time ago – 1963 is the original publication date on the IEC standard. Resistor technology has changed a lot since then. Through-hole, carbon-composition resistors have largely been replaced by surface-mount, film-type resistors. It has become much cheaper to make higher-quality resistors with tight tolerances. As engineers shift to 1% resistors instead of 5% or 10%, they look for familiar values, like 330 Ω and 470 Ω. Many manufacturers have responded to that by producing these "non-standard" resistors: 330 Ω ±1% or 470 Ω ±1%, for example. So, with the established preferred value system getting blurry, what are the most common resistor values? Which ones should be in the Common Parts Library? We decided to investigate. Today, I'm going focus exclusively on size 0603 surface-mount, 100 mW, 1% tolerance resistors. Are you pumped? I am. The chart below compares total available stock across all authorized distributors for different resistance values. It turns out, the "non-standard" 330 Ω ±1% resistor has much higher availability than the preferred value ±1% resistors. This holds for different decades: And different values: Here's a chart of all the values in the E12, E24, and E96 series, colored by the series in which they appear. Note that both axes are log scale! (View as a larger image or the interactive version ) Notice that the values that exist in all series (multiples of 10 and 15) have generally the highest availability.The values in E24 and E96 but not E12 are multiples of 11, 13, 20, and 75, and have middle availability. Finally, the values only in E96, come in at the bottom with just a few exceptions, even though E96 is the series of 1% tolerance resistors. The curve is also interesting: availability is higher for the middle-valued resistors, between 100 Ω and 100 kΩ, as we might expect. Multiples of 10 have significantly higher availability and 10 kΩ is the most available resistor. That agrees with our common sense. We've curated the CPL with common resistors based on this analysis. If you have any thoughts or questions, we'd love to hear from you!