What the Inductor Color Code Calculator does
This calculator reads moulded inductor colour bands and printed inductor codes into a value in microhenries, with the tolerance window, the value in nanohenries and millihenries, the reactance at a frequency you choose and the capacitance that would resonate with the part.
Inductor bands use the same colour-to-digit table as resistors, with one difference that catches people out: the value is counted in microhenries, so brown-black-brown is 100 uH, not 100 ohms. A gold band in the multiplier position means x0.1, which is how small values such as 4.7 uH are marked.
How to use it
- Choose whether the part has colour bands or a printed code. Axial moulded chokes use bands; surface-mount and shielded parts are nearly always printed.
- For bands, set the two digit colours, the multiplier and the tolerance. A three-band part with no tolerance band is read as +/-20%.
- For a printed code, type it exactly: 101 is 100 uH, 4R7 is 4.7 uH, 2N2 is 2.2 nH, and a trailing J, K or M is the tolerance.
- Enter the frequency you will run the part at. The reactance and the resonating capacitance appear with it.
- Check the reactance table. If the reactance you need is only a few ohms at your frequency, the part's DC resistance and its self-resonance will matter more than the inductance.
Reading the results
Reactance, not inductance, is what the circuit sees. 100 uH is 0.63 ohms at 1 kHz and 628 ohms at 1 MHz - the same part is almost a short circuit in one place and almost an open circuit in the other.
The tolerance band is a manufacturing window on the inductance measured at the maker's test frequency, usually well below the part's self-resonant frequency. Measured somewhere else, the same part reads differently.
The resonating capacitance shown is what you would deliberately add to make a tank at your frequency. Every inductor also has its own stray capacitance, which sets a self-resonant frequency above which the part behaves as a capacitor.
Worked example: brown, black, brown, gold on an axial choke
Brown is 1 and black is 0, giving 10; the brown multiplier band is x10, so the value is 100 uH. The gold band is +/-5%, so the part is between 95 and 105 uH.
At 1 MHz its reactance is 2 pi x 1,000,000 x 100e-6 = 628.3 ohms. At 1 kHz the same choke is only 0.63 ohms, which is why a choke chosen for a mains filter does nothing at all at audio frequencies.
To resonate it at 1 MHz you would need C = 1 / ((2 pi f)^2 L) = 253 pF. Swap the gold multiplier band for a gold band in the third position on a smaller part - yellow, violet, gold, silver - and the value drops to 47 x 0.1 = 4.7 uH with a 10% tolerance.
Formulas and scoring rules
- Value from bands
L[uH] = (two digits) x multiplierCounted in microhenries. Gold in the multiplier position is x0.1 and silver is x0.01.- Printed code
L[uH] = (two digits) x 10^(third digit)101 = 10 x 10 = 100 uH. R and N mark a decimal point in microhenries and nanohenries: 4R7 = 4.7 uH, 2N2 = 2.2 nH.- Inductive reactance
XL = 2 pi f LIn ohms, with L in henries. It doubles every time the frequency doubles.- Resonating capacitance
C = 1 / ((2 pi f)^2 L)The capacitance that puts an LC pair's resonance at f.- Tolerance letters
F +/-1%, G +/-2%, J +/-5%, K +/-10%, M +/-20%
The three ratings a marking never gives you
Saturation current is where the core stops behaving like a core: past it, the inductance collapses, often by 30% or more, and a switching converter that was stable becomes a current source into its own switch. It is a property of the core material and gap, not of the inductance value.
RMS or heating current is a separate, usually lower, limit set by the winding resistance and the permitted temperature rise. Self-resonant frequency is the third: above it the part is a capacitor. All three come from the data sheet, and all three are as likely to decide your choice as the inductance itself.
Reading a part that has no bands at all
Many surface-mount inductors are printed with a bare code, some shielded parts carry only a batch number, and some ferrite beads are unmarked because their useful specification is an impedance at 100 MHz rather than an inductance at all.
If a part is unmarked, an LCR meter at a stated frequency is the only honest answer; a multimeter cannot measure inductance. Beware of measuring a bead as an inductor - its data sheet gives an impedance curve, and a single inductance number misrepresents it.
Limitations: what the result does not prove
- It reads the marking, not the part. Inductance varies with frequency, with DC bias current and with temperature, and a saturated core reads far below its marked value.
- Nothing in the marking gives saturation current, RMS current rating, DC resistance, Q factor or self-resonant frequency - the specifications that usually decide whether a part is suitable.
- Ferrite beads are specified as an impedance at a frequency, not as an inductance, and decoding them as inductors is misleading.
- Military and some older parts use additional bands or a body-colour convention that this calculator does not decode.
Privacy: where your data goes
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Standards and sources
- IEC 60062 - Marking codes for resistors and capacitors - checked 19 Sep 2026
- Coilcraft - Selecting the right inductor, including saturation and self-resonance
- Wurth Elektronik - Trilogy of magnetic components (design reference)
Frequently asked questions
Are inductor colour codes the same as resistor colour codes?
The colour-to-digit table is the same, but the unit is microhenries rather than ohms and the tolerance colours differ: silver is 10% and gold 5%, while gold in the third position is a multiplier of 0.1. So brown-black-brown is 100 uH on an inductor and 100 ohms on a resistor.
What does an inductor marked 101 mean?
100 microhenries: 10 followed by one zero, counted in microhenries. As with resistors, the last digit is the number of zeros, so 220 is 22 uH and not 220 uH.
What does 4R7 mean on an inductor?
4.7 microhenries - the R stands in for the decimal point. An N is used the same way for nanohenries, so 2N2 is 2.2 nH, a value you see on RF parts.
Why does a gold band sometimes mean 5% and sometimes x0.1?
Its position decides. As the third band it is a multiplier of 0.1, which is how values under 10 uH are marked; as the fourth band it is a 5% tolerance. Count the bands from the end without the tolerance band to tell them apart.
How do I find an inductor's current rating from the code?
You cannot - the marking gives inductance and tolerance only. Saturation current, RMS current and DC resistance come from the manufacturer's data sheet for that series and package, and they are usually the limits that matter.
What is self-resonant frequency and why does it matter?
Every inductor has stray capacitance between its turns. At the frequency where that resonates with the inductance, the part's impedance peaks; above it the winding behaves as a capacitor and the inductance is no longer useful. Choose a part whose self-resonant frequency is comfortably above your working frequency.
Can I measure inductance with a multimeter?
No. A multimeter measures the winding's DC resistance, which tells you the part is not open circuit and nothing more. Inductance needs an LCR meter, and the reading depends on the test frequency and any DC bias applied.
My inductor reads well below its marked value in circuit. Why?
Usually core saturation from DC bias current, or a measurement taken near or above the self-resonant frequency. Check the data sheet's inductance-versus-current curve for the part, and measure at the frequency the maker specifies.
Last reviewed by the A2Z.Tools team against the sources listed above.
