When you calculate a theoretical resistance of 4,123 ohms for a bias network, you cannot simply buy a 4,123Ω resistor. You must map your math to the standardized resistor ohm chart (the IEC 60063 E-series) and then verify the component will survive your enclosure's ambient heat using a power derating matrix. This reference provides the complete E24 base decade, the standard temperature derating table, and the bench-level context needed to select the right physical part.
How to Read the Resistor Ohm and Derating Charts
A complete resistor reference actually consists of two distinct charts: the value chart (which dictates available ohmic steps) and the derating chart (which dictates safe power dissipation).
Which Column Applies to Your Installation?
When looking at the power derating chart below, you must choose the column that matches your physical mounting method. Free Air applies to resistors suspended by their leads with at least 10mm of clearance from any surface. PCB Mounted applies to standard through-hole or SMD parts soldered flat to a standard FR4 board (where the board acts as a mild thermal insulator but also a spreader). Heatsink/Chassis Mounted applies specifically to wirewound or thick-film power resistors bolted to an aluminum extrusion or metal chassis with thermal paste.
How Derating Rows Modify the Base Value
Resistors are rated for their nominal power (e.g., 0.25W) at a specific baseline ambient temperature, usually 25°C or 70°C depending on the manufacturer. As the ambient temperature rises past that baseline knee, the derating rows dictate a linear reduction in allowable power. For example, if the 85°C row shows 60%, a 1/4W (250mW) resistor can only safely dissipate 150mW at that temperature.
What the Table Cannot Tell You
Standard ohm and derating charts assume continuous DC or low-frequency AC loads. They cannot tell you:
- Pulse Load Handling: A 1/4W carbon film resistor can survive a 5W pulse for 10 milliseconds, but the derating chart won't show this. You need the manufacturer's specific pulse-energy curve.
- Parasitics: At RF frequencies (above 10MHz), a standard axial resistor behaves like an inductor due to its helical film cut. The chart won't warn you to use a thin-film SMD instead.
- Voltage Coefficient of Resistance (VCR): High-voltage applications (>500V) can cause the actual ohmic value to drop as voltage increases, an effect absent from standard value charts.
Standard Resistor Ohm Chart (IEC 60063 E-Series)
The IEC 60063 standard defines preferred numbers for resistors. The E24 series (5% tolerance) is the most common for general-purpose through-hole and SMD designs. The table below shows the complete base decade. To find your exact value, multiply the base number by powers of 10 (e.g., base 4.7 becomes 4.7Ω, 47Ω, 470Ω, 4.7kΩ, 47kΩ).
Bookmark Quick-Jumps for Most Queried Values:
1.0 / 10 / 100 / 1k / 10k / 100k / 1M •
2.2 / 22 / 220 / 2.2k / 22k / 220k / 2.2M •
3.3 / 33 / 330 / 3.3k / 33k / 330k / 3.3M •
4.7 / 47 / 470 / 4.7k / 47k / 470k / 4.7M
| Base Value (Multiplier x10^n) | E24 Series (5% Tol.) | E12 Series (10% Tol.) | Common 5-Band Color Code (Base) |
|---|---|---|---|
| 1.0 | Yes | Yes | Brown - Black - Black |
| 1.1 | Yes | No | Brown - Brown - Black |
| 1.2 | Yes | Yes | Brown - Red - Black |
| 1.3 | Yes | No | Brown - Orange - Black |
| 1.5 | Yes | Yes | Brown - Green - Black |
| 1.6 | Yes | No | Brown - Blue - Black |
| 1.8 | Yes | Yes | Brown - Grey - Black |
| 2.0 | Yes | Yes | Red - Black - Black |
| 2.2 | Yes | Yes | Red - Red - Black |
| 2.4 | Yes | No | Red - Yellow - Black |
| 2.7 | Yes | Yes | Red - Violet - Black |
| 3.0 | Yes | Yes | Orange - Black - Black |
| 3.3 | Yes | Yes | Orange - Orange - Black |
| 3.6 | Yes | No | Orange - Blue - Black |
| 3.9 | Yes | Yes | Orange - White - Black |
| 4.3 | Yes | No | Yellow - Orange - Black |
| 4.7 | Yes | Yes | Yellow - Violet - Black |
| 5.1 | Yes | No | Green - Brown - Black |
| 5.6 | Yes | Yes | Green - Blue - Black |
| 6.2 | Yes | Yes | Blue - Red - Black |
| 6.8 | Yes | Yes | Blue - Grey - Black |
| 7.5 | Yes | No | Violet - Green - Black |
| 8.2 | Yes | Yes | Grey - Red - Black |
| 9.1 | Yes | Yes | White - Brown - Black |
Note: For 1% tolerance designs, engineers use the E96 series (96 values per decade). The E96 chart is too extensive for this page, but standard E24 values are a subset of E96, meaning a 4.7kΩ 5% resistor is also a valid E96 value.
Power Derating Chart by Ambient Temperature
The following matrix is based on IEC 60115-1 and MIL-PRF-55342 generalized derating curves for standard thick-film and carbon-film resistors. Always check the specific manufacturer's datasheet (e.g., Vishay, Yageo, KOA Speer) for exact part-number deviations, but this table serves as the universal baseline for practical circuit design.
| Ambient Temp (°C) | Free Air Mount | PCB Mounted (FR4) | Chassis / Heatsink Mount |
|---|---|---|---|
| 25°C | 100% | 100% | 100% |
| 50°C | 100% | 95% | 100% |
| 70°C (Typical Knee) | 85% | 75% | 100% |
| 85°C | 65% | 55% | 90% |
| 100°C | 45% | 35% | 75% |
| 125°C | 20% | 10% | 45% |
| 155°C (Max Limit) | 0% | 0% | 10% |
Math: P = V² / R = 25 / 1000 = 25mW (0.025W).
Derating: At 85°C PCB mounted, a resistor can only handle 55% of its rating. If you use a standard 1/16W (62.5mW) 0402 SMD resistor, its derated limit is 34.3mW. Your 25mW load is safe, but only with a 9mW margin. Step up to a 1/10W (100mW) 0603 package to ensure long-term reliability.
Resistor Ohm Chart FAQ
Why isn't my exact calculated ohm value on the resistor chart?
The IEC 60063 standard uses a logarithmic scale (preferred numbers) rather than a linear one. This ensures that the tolerance bands of adjacent values overlap perfectly without gaps. For example, in the E12 (10%) series, the value 1.0 covers 0.9 to 1.1, and the next value, 1.2, covers 1.08 to 1.32. Because the tolerances overlap, there is no mathematical need for a '1.1' value in the 10% series. If your SPICE simulation demands exactly 4,123Ω, you must either select the closest E96 value (4.12kΩ) or use a series/parallel combination of two standard E24 resistors.
How do I read the 5-band color code using this ohm chart?
The 5-band color code is used for 1% (E96) or precision resistors. The first three bands represent the base value digits, the fourth band is the multiplier, and the fifth is the tolerance (usually Brown for 1%). To use the ohm chart with a 5-band resistor, read the first three bands to get a 3-digit number (e.g., Yellow-Violet-Black = 470), then use the chart's multiplier logic. If the fourth band is Red (x100), your value is 470 x 100 = 47,000Ω (47kΩ). The chart confirms 47 is a valid base number in both E24 and E96 series.
Does the resistor ohm chart apply to SMD (surface mount) components?
Yes, the underlying IEC 60063 E-series values apply universally to SMD resistors. However, the physical marking system changes. Instead of color bands, SMD resistors use a 3-digit or 4-digit numeric code. A 3-digit code (e.g., '472') means 47 x 10² = 4,700Ω (4.7kΩ). A 4-digit code (e.g., '4701') means 470 x 10¹ = 4,700Ω, used for 1% tolerance E96 parts. Additionally, the EIA-96 SMD marking system uses a two-character code (like '68C') for ultra-small 0201 and 0402 packages, which requires a separate EIA-96 lookup table to map back to the standard ohm chart base values.
