The direct answer for through-hole axial and radial inductors is that color bands follow the EIA and MIL-PRF-15305 standards, and the values are read in microhenries (µH). The first two bands represent significant digits, the third band is the multiplier, and the fourth band indicates tolerance. Unlike resistors, where the base unit is Ohms, the base unit for standard molded chokes is the microhenry.
The Standard 4-Band Inductor Color Code Chart (EIA / MIL)
The table below maps the standard 4-band color code used by major manufacturers like Vishay, Bourns, and Fastron for axial molded inductors. Read the bands starting from the end closest to the lead wire, moving toward the center of the component body.
| Band Color | Digit 1 (Band 1) | Digit 2 (Band 2) | Multiplier (Band 3) | Tolerance (Band 4) |
|---|---|---|---|---|
| Black | 0 | 0 | ×1 µH | — |
| Brown | 1 | 1 | ×10 µH | ±1% |
| Red | 2 | 2 | ×100 µH | ±2% |
| Orange | 3 | 3 | ×1,000 µH (1mH) | ±3% |
| Yellow | 4 | 4 | ×10,000 µH (10mH) | ±4% |
| Green | 5 | 5 | ×100,000 µH (100mH) | — |
| Blue | 6 | 6 | — | — |
| Violet | 7 | 7 | — | — |
| Gray | 8 | 8 | — | — |
| White | 9 | 9 | — | — |
| Gold | — | — | ×0.1 µH | ±5% |
| Silver | — | — | ×0.01 µH | ±10% |
| No Band | — | — | — | ±20% |
Rows and Bands People Get Wrong
Even experienced technicians misread inductor bands because they default to resistor logic. Here are the specific traps to avoid:
- The Gold and Silver Multipliers: In resistors, gold and silver multipliers are rarely used outside of precision low-value current shunts. In inductors, they are extremely common. A Gold multiplier means ×0.1 µH (100 nH), and Silver means ×0.01 µH (10 nH). If you see Red-Red-Gold-Silver, it is 2.2 µH ±10%, not 22 Ohms.
- The "No Band" Tolerance: Many cheap, unbranded molded chokes only have three bands. If the fourth band is missing, the tolerance defaults to ±20%. This is standard for basic RF chokes and power supply filters where exact inductance is less critical than current handling.
- Reading Direction: Unlike resistors, the tolerance band on an inductor is not always a distinct gold or silver color. On 3-band inductors, or when all bands are the same width, the first band is always positioned closer to the lead wire. If the bands are perfectly centered, look for a slightly wider band or a thicker epoxy shoulder on one end to indicate the starting side.
- Assuming the Base Unit is Nanohenries: While SMD chip inductors are often marked in nH, through-hole axial inductors almost universally use µH as the base unit for the color code. Do not apply nH logic to an axial molded choke.
Standard Variants: MIL-Spec, EIA, and SMD Markings
The 4-band EIA system is the baseline, but you will encounter variants depending on the component's origin and package type.
MIL-PRF-15305 (5-Band Military Spec)
Military and aerospace-grade inductors (often manufactured by companies like API Delevan or Vishay's Mil-Spec lines) use a 5-band system. The first four bands match the EIA standard (Digit 1, Digit 2, Multiplier, Tolerance). The fifth band indicates the failure rate per 1,000 hours of operation:
- Brown: 1.0% failure rate
- Red: 0.1% failure rate
- Orange: 0.01% failure rate
- Yellow: 0.001% failure rate
SMD Inductors (No Color Bands)
Surface mount inductors do not use color bands. Instead, they rely on:
- Printed Text: e.g., "100" means 10 µH (10 × 10^0), "471" means 470 µH (47 × 10^1). Note that SMD text usually defaults to microhenries for power inductors, but nanohenries for tiny RF chip inductors (like the Murata LQG series). Always check the physical size; a 0402 package marked "100" is 10 nH, while a 6x6mm power shielded inductor marked "100" is 10 µH.
- EIA-96 Code: High-precision SMD inductors may use a two-digit/one-letter code, requiring an EIA-96 lookup table, though this is far more common on resistors.
For a deeper look at component identification standards, the Electronics Club inductor guide provides excellent visual references for physical package variations, and the Bourns inductor catalog details how modern manufacturers are moving away from color bands in favor of laser-etched markings on conformal coated parts.
Decision Path: Identifying Faded or Unmarked Inductors
Molded inductors are coated in phenolic resin or epoxy. When subjected to high ripple currents or thermal cycling, this resin yellows, cracks, or chars, obliterating the color bands. Never guess the value of a thermally damaged inductor. The heat that faded the paint also altered the magnetic permeability of the core, meaning the component is likely out of spec even if you guess the original value correctly.
Follow this decision tree to resolve an unknown or faded inductor on a PCB:
| Symptom / Condition | Action to Take | Concrete Next Step |
|---|---|---|
| Bands are visible but faded/ambiguous | Desolder one leg and measure with an LCR meter. | Measure at 1kHz for power chokes (>10µH) or 100kHz for RF chokes (<10µH). Match the reading to the nearest standard E12 value. |
| Bands are completely missing / charred | Trace the circuit to determine the inductor's function. | If in a buck converter feedback loop, check the controller IC datasheet (e.g., LM2596) for the exact required µH and saturation current (Isat). |
| Inductor measures open circuit (OL) on DMM | The internal wire has snapped. Component is dead. | Replace with a new part matching the original schematic BOM, ensuring the replacement has an equal or higher Isat rating. |
| Inductor reads correct µH but circuit still fails | Check Self-Resonant Frequency (SRF) and Q-factor. | The replacement has the wrong core material (e.g., using an iron powder core instead of ferrite). Buy a specialized RF choke like the Coilcraft 1812CS series. |
Safe Interpretation and Replacement Protocol
When replacing an inductor where the color code has been successfully read, matching the inductance (µH) and tolerance is only the first step. To ensure the replacement does not overheat or cause EMI failures, you must match three hidden parameters that color codes do not display:
- DC Resistance (DCR): The internal resistance of the copper wire. If your original 10µH inductor had a DCR of 0.5Ω and you replace it with a smaller physical package that has a DCR of 2.0Ω, the new inductor will overheat and drop excessive voltage in a power supply rail.
- Saturation Current (Isat): The current level at which the inductance drops by 20% to 30%. In switching power supplies, using an inductor with an Isat lower than the peak switch current will cause the core to saturate, turning the inductor into a dead short and instantly destroying the switching MOSFET.
- Shielding: If the original inductor was a shielded drum core (enclosed in a magnetic sleeve) and you replace it with an unshielded bobbin core, the radiated magnetic field will couple into nearby high-impedance analog traces, causing noise.
The Default Replacement Pick: If you are repairing general-purpose power supply or filtering circuits and need a reliable, widely available axial replacement that uses standard color codes, specify the Bourns 78F series or the Vishay IM series. For example, if you need a 100µH ±5% choke, order the Bourns 78F101J-RC. It features a robust epoxy coating that resists thermal fading, a clearly documented Isat curve in the datasheet, and strict adherence to the EIA color code standard, eliminating the guesswork on your workbench.
