Inductor Code Calculator
Decode 3-, 4-, and 5-band THT colour codes and SMD markings. Supports µH power inductors and nH RF inductors. Result auto-scales nH / µH / mH / H.
10 × 1 µH = 10 µH±5%
Inductance
10 µH
±5%
= 10 µH
How inductor codes work
THT colour bands (through-hole)
The base unit for THT inductor colour codes is microhenries (µH) — the same digit/multiplier logic as resistors, but the value you read off the bands is in µH, not ohms.
4-band format: 2 digits + multiplier + tolerance
3-band format
Older parts use only 3 bands: two significant digits and a multiplier. The tolerance is not marked and defaults to ±20%.
5-band format
Precision inductors add a third significant digit before the multiplier, giving three digits of resolution. The last band is still tolerance. Read: D1 D2 D3 × Multiplier, Tolerance.
Colour band reference table
Tolerance column shows inductor standard values (IEC 62024-1). Green–Violet–Grey–White tolerance bands are for precision resistors only and do not appear on standard inductors.
| Color | Digit | Multiplier (µH) | Tolerance |
|---|---|---|---|
| Black | 0 | ×1 | ±20% |
| Brown | 1 | ×10 | ±1% |
| Red | 2 | ×100 | ±2% |
| Orange | 3 | ×1,000 | ±3% |
| Yellow | 4 | ×10,000 | ±4% |
| Green | 5 | ×100,000 | — |
| Blue | 6 | ×1,000,000 | — |
| Violet | 7 | ×10,000,000 | — |
| Grey | 8 | ×100,000,000 | — |
| White | 9 | ×1,000,000,000 | — |
| Gold | — | ×0.1 | ±5% |
| Silver | — | ×0.01 | ±10% |
SMD power inductors (µH codes)
Power inductors — used in buck/boost converters, filters, and energy storage — are marked with a 3-digit code where the base unit is µH (microhenries). An optional tolerance letter follows the digits.
3-digit code
471 → 47 × 10¹ µH = 470 µH102 → 10 × 10² µH = 1,000 µH = 1 mH330 → 33 × 1 µH = 33 µH
R-notation (µH)
4R7 = 4.7 µHR47 = 0.47 µH10R = 10 µH
Tolerance letters
B ±0.1%C ±0.25%D ±0.5%F ±1%G ±2%J ±5%K ±10%M ±20%SMD RF inductors (nH codes)
RF inductors operate in the nanohenry range (1–100 nH) and are used in impedance matching, LC filters, and antenna circuits. The key difference from power inductors:
- Values expressed in nH, not µH
- Smallest packages (0402, 0201, 01005) often carry no body marking at all — you must rely on the reel label or PCB BOM
- N-notation replaces the decimal point:
4N7= 4.7 nH
N-notation (nH)
4N7 = 4.7 nH12N = 12 nHN47 = 0.47 nH
3-digit nH code
100 → 10 × 1 nH = 10 nH101 → 10 × 10 nH = 100 nH220 → 22 × 1 nH = 22 nH
Self-resonant frequency (SRF)
Every inductor has parasitic capacitance between its windings. At the self-resonant frequency (SRF) this capacitance resonates with the inductance — above the SRF the component behaves as a capacitor, not an inductor. Always choose an inductor whose SRF is well above your operating frequency. RF inductors typically list SRF on the datasheet; for power inductors it is rarely relevant because they operate far below resonance.
Quick experiments
- Decode the R notation. Enter 4R7 and read 4.7 µH — the R marks the decimal point. Then try 101, which is 10 followed by one zero, so 100 µH. The two schemes look similar and are easy to confuse.
- Compare saturation against RMS current. Pick a part where the two ratings differ. Saturation is a magnetic limit that collapses inductance; RMS is a thermal limit from winding resistance. The design must respect whichever is lower.
- See what saturation costs you. Inductance typically falls 10–30 % at the rated saturation current. In a buck converter that means ripple current rises sharply, which is how a marginal inductor destroys the switching FET.
- Watch DCR turn into heat. Winding resistance times current squared is dissipated as heat. At 2 A, a 100 mΩ DCR burns 0.4 W — often the dominant loss in a small converter.
- Check the self-resonant frequency. Above self-resonance an inductor behaves capacitively. A part specified for 100 kHz switching is useless as an RF choke at 100 MHz, however tempting its inductance looks.
Formula reference
- Inductive reactance
100 µH at 100 kHz gives about 63 Ω.
- Energy stored
The energy the core must hold without saturating.
- Copper loss from winding resistance
2 A through 100 mΩ dissipates 0.4 W.
- Self-resonant frequency
The parasitic winding capacitance resonates with the inductance. Above that frequency the part looks capacitive.
| Symbol | Meaning | Unit |
|---|---|---|
| Inductance | H | |
| Saturation current — magnetic limit | A | |
| DC winding resistance | Ω | |
| Self-resonant frequency | Hz |
Common mistakes
Reading the code as microhenries when it marks nanohenries.
Most chip inductors code in µH, but small RF parts often mark nH. Check the series datasheet — being out by 1000× makes a filter useless.
Sizing only on inductance and current rating.
There are two current ratings. Saturation collapses inductance, RMS sets temperature rise. Meeting one and ignoring the other is how converters fail under load.
Assuming inductance is constant with current.
Ferrite cores lose inductance progressively as current rises, well before the quoted saturation point. Check the L-versus-I curve, not just the headline number.
Confusing an inductor's bands with a resistor's.
The colour scheme is nearly identical but the unit is microhenries. Check the silkscreen designator — L, not R — before decoding.
Ignoring shielding in a switching layout.
Unshielded inductors radiate their field into nearby traces. In a switching regulator that shows up as noise on adjacent analogue signals; a shielded part costs more but avoids the problem.
Frequently asked questions
How do I read an inductor's 3-digit code?
The first two digits are significant figures and the third is the number of zeros, giving microhenries. Code 101 means 10 followed by 1 zero, so 100 uH. An R marks a decimal point, so 4R7 is 4.7 uH.
What does an inductor's saturation current rating mean?
It is the current at which the core saturates and inductance collapses, typically defined as a 10 to 30 percent drop. Beyond it the inductor stops behaving like an inductor and current rises steeply, which is what destroys switching regulators.
What is the difference between saturation current and RMS current?
Saturation current is a magnetic limit that causes inductance loss. RMS current is a thermal limit set by winding resistance and allowable temperature rise. A design must stay below both, and which one binds first depends on the part.
Does an inductor's colour code work like a resistor's?
The band scheme is similar but the unit is microhenries rather than ohms, and a silver or gold band acts as a decimal multiplier. Because the two look alike, inductors are frequently misread as resistors.
What is self-resonant frequency?
Every inductor has parasitic winding capacitance that resonates with its inductance. Above that frequency the part behaves capacitively, so an inductor is only useful well below its self-resonant frequency.
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