Inductor Guide
How to Read Inductor Codes
Learn how to read inductor codes, SMD markings, color bands, and inductance values. Includes practical examples, conversion charts, and common mistakes.
- Reading Time
- 12 min
- Difficulty
- Beginner
- Last Updated
- July 18, 2026
Introduction
Inductors are used in switching converters, EMI filters, RF networks, LC tanks, relays, sensors, and many current-shaping circuits. Many parts are too small to print a full inductance value, so manufacturers use compact SMD markings, decimal codes, or color bands.
Understanding these markings helps engineers identify parts on a board, check substitutions, review prototypes, and avoid confusing nanohenry, microhenry, and millihenry values. The code is still only the starting point: current rating, saturation current, DCR, tolerance, shielding, and self-resonant frequency must be verified from the datasheet.
After decoding a value, related tools such as the Inductive Reactance Calculator and Inductor Energy Calculator can help estimate circuit behavior.
Inductor Value Basics
Inductance is measured in henries. Practical electronics usually works in millihenries, microhenries, or nanohenries. SMD inductor codes are often interpreted in µH for power inductors and in nH for small RF inductors, so the part family matters.
Formula reference
Inductance unit conversion
1 H = 1,000 mH1 mH = 1,000 µH1 µH = 1,000 nHVariable definitions
- H
- henry, the SI unit of inductance
- mH
- millihenry, common for larger coils and filters
- µH
- microhenry, common for power inductors and many filters
- nH
- nanohenry, common for RF and high-frequency chip inductors
| Base value | H | mH | µH | nH |
|---|---|---|---|---|
| 1 H | 1 H | 1,000 mH | 1,000,000 µH | 1,000,000,000 nH |
| 1 mH | 0.001 H | 1 mH | 1,000 µH | 1,000,000 nH |
| 1 µH | 0.000001 H | 0.001 mH | 1 µH | 1,000 nH |
| 1 nH | 0.000000001 H | 0.000001 mH | 0.001 µH | 1 nH |
SMD Inductor Marking
Common SMD markings include direct two-digit values, three-digit significant-figure codes, four-character markings, and decimal markers such as R. A three-digit code is often decoded as the first two digits multiplied by 10 raised to the third digit, with the result in microhenries for many power inductor families.
| Marking | Calculation | Final inductance | Unit | Notes |
|---|---|---|---|---|
| 100 | 10 × 10^0 µH | 10 µH | µH | The first two digits are 10 and the multiplier digit is 0. |
| 220 | 22 × 10^0 µH | 22 µH | µH | A common power inductor value for filters and converters. |
| 331 | 33 × 10^1 µH | 330 µH | µH | This follows the same three-digit µH marking pattern. |
| 471 | 47 × 10^1 µH | 470 µH | µH | The code identifies nominal inductance, not current rating or DCR. |
| 101 | 10 × 10^1 µH | 100 µH | µH | A common marking on radial, axial, and molded inductors. |
| 102 | 10 × 10^2 µH | 1 mH | mH | 1000 µH is usually written as 1 mH. |
| 4R7 | R replaces the decimal point | 4.7 µH | µH | Decimal R markings are common on SMD power inductors. |
| R22 | 0.22 µH | 220 nH | nH | R at the start commonly indicates a value below 1 µH. |
| Marking | Type | Decoded value | nH | µH | mH | Tolerance notes |
|---|---|---|---|---|---|---|
| R22 | alphanumeric | 0.22 uH | 220 | 0.22 | 0 | Usually defined by part family |
| 100 | 3-digit | 10 uH | 10,000 | 10 | 0.01 | Common suffixes: K ±10%, M ±20% |
| 220 | 3-digit | 22 uH | 22,000 | 22 | 0.022 | Common suffixes: K ±10%, M ±20% |
| 101 | 3-digit | 100 uH | 100,000 | 100 | 0.1 | Common suffixes: K ±10%, M ±20% |
| 471 | 3-digit | 470 uH | 470,000 | 470 | 0.47 | Common suffixes: K ±10%, M ±20% |
| 102 | 3-digit | 1 mH | 1,000,000 | 1,000 | 1 | Common suffixes: K ±10%, M ±20% |
Color Band Inductor Codes
Axial inductors and some older molded parts may use colored bands. The basic idea is similar to resistor color coding: significant digits are followed by a multiplier and sometimes a tolerance band. Inductor color-band conventions can vary, so treat color decoding as a value estimate until the datasheet or measurement confirms it.
| Color | Digit | Multiplier | Tolerance meaning |
|---|---|---|---|
| Black | 0 | ×1 | Often no precision tolerance |
| Brown | 1 | ×10 | ±1% when used as tolerance band |
| Red | 2 | ×100 | ±2% when used as tolerance band |
| Orange | 3 | ×1,000 | Usually digit or multiplier only |
| Yellow | 4 | ×10,000 | Usually digit or multiplier only |
| Green | 5 | ×100,000 | ±0.5% in some precision conventions |
| Blue | 6 | ×1,000,000 | ±0.25% in some precision conventions |
| Violet | 7 | ×10,000,000 | Usually digit or multiplier only |
| Gray | 8 | ×100,000,000 | Usually digit or multiplier only |
| White | 9 | ×1,000,000,000 | Usually digit or multiplier only |
| Gold | — | ×0.1 | ±5% when used as tolerance band |
| Silver | — | ×0.01 | ±10% when used as tolerance band |
Reading Examples
Marking 100
- Original Marking
- 100
- Calculation
- 10 × 10^0 µH
- Final Inductance
- 10 µH
- Unit
- µH
Marking 220
- Original Marking
- 220
- Calculation
- 22 × 10^0 µH
- Final Inductance
- 22 µH
- Unit
- µH
Marking 331
- Original Marking
- 331
- Calculation
- 33 × 10^1 µH
- Final Inductance
- 330 µH
- Unit
- µH
Marking 471
- Original Marking
- 471
- Calculation
- 47 × 10^1 µH
- Final Inductance
- 470 µH
- Unit
- µH
Marking 101
- Original Marking
- 101
- Calculation
- 10 × 10^1 µH
- Final Inductance
- 100 µH
- Unit
- µH
Marking 102
- Original Marking
- 102
- Calculation
- 10 × 10^2 µH
- Final Inductance
- 1 mH
- Unit
- mH
Marking 4R7
- Original Marking
- 4R7
- Calculation
- R replaces the decimal point
- Final Inductance
- 4.7 µH
- Unit
- µH
Marking R22
- Original Marking
- R22
- Calculation
- 0.22 µH
- Final Inductance
- 220 nH
- Unit
- nH
Common Mistakes
Confusing µH and mH after decoding a three-digit marking.
Decode the marking first, then verify the complete electrical and mechanical specification before using the part in a design.
Reading an SMD code as a direct printed value instead of a significant-digit multiplier code.
Decode the marking first, then verify the complete electrical and mechanical specification before using the part in a design.
Ignoring tolerance suffixes such as J, K, or M.
Decode the marking first, then verify the complete electrical and mechanical specification before using the part in a design.
Assuming every manufacturer uses identical markings.
Decode the marking first, then verify the complete electrical and mechanical specification before using the part in a design.
Using an inductance code without checking saturation current.
Decode the marking first, then verify the complete electrical and mechanical specification before using the part in a design.
Forgetting that DCR can dominate loss in high-current circuits.
Decode the marking first, then verify the complete electrical and mechanical specification before using the part in a design.
Using an unshielded part where magnetic field leakage matters.
Decode the marking first, then verify the complete electrical and mechanical specification before using the part in a design.
Replacing an RF inductor with a power inductor of the same nominal value.
Decode the marking first, then verify the complete electrical and mechanical specification before using the part in a design.
Tips for Choosing an Inductor
Inductance
Match the nominal value and tolerance to the circuit function.
Rated Current
Check continuous RMS or temperature-rise current for the package.
Saturation Current
Make sure peak current does not collapse inductance in operation.
DCR
Lower DC resistance reduces copper loss and voltage drop.
Package Size
Confirm board area, height, mounting style, and thermal capability.
Shielded vs Unshielded
Use shielded parts when magnetic leakage or EMI matters.
Engineering Notes
Marking note
Inductor markings are less universal than resistor color codes or capacitor EIA codes.
Marking note
Many SMD inductors use manufacturer-specific markings or no top marking at all.
Marking note
A marking normally identifies nominal inductance only; it does not encode saturation current, rated current, DCR, SRF, shielding, or core material.
Marking note
Always verify marking conventions against the manufacturer datasheet before replacement or production release.
| Suffix | Tolerance | Notes |
|---|---|---|
| J | ±5% | Used by some manufacturers for tighter general-purpose inductor tolerance. |
| K | ±10% | Common inductor tolerance suffix for general-purpose values. |
| M | ±20% | Common tolerance suffix for power inductors and broad-tolerance parts. |
Related Engineering Guides
Understanding Inductor Parameters
Understand inductance, DCR, current ratings, saturation, Q, SRF, package style, and core material.
Engineering GuideHow to Read Resistor Color Codes
Read passive component color bands and tolerance markings.
Engineering GuideHow to Read Capacitor Codes
Decode capacitor markings and compare pF, nF, and µF values.
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Support reference
FAQ
What does 4R7 mean on an inductor?
4R7 commonly means 4.7 µH. The letter R replaces the decimal point, which is useful on small SMD parts where a printed decimal point would be hard to read.
How do I read 3-digit inductor codes?
In the common microhenry-based convention, the first two digits are significant figures and the third digit is the multiplier. For example, 471 means 47 × 10^1 µH, or 470 µH.
Do all inductors use the same marking system?
No. Inductor markings are less universal than resistor color codes. Many manufacturers use their own package markings, suffixes, date codes, or no marking at all.
What is the difference between color bands and SMD markings?
Color bands use colored stripes to encode digits, multiplier, and sometimes tolerance. SMD markings use printed numbers or alphanumeric codes such as 100, 471, 4R7, or R22.
How can I verify an unknown inductor?
Use an LCR meter at a suitable test frequency and compare the result with the circuit requirement. For power inductors, also verify DCR, rated current, and saturation current.
Why are some inductors not marked?
Very small chip inductors, shielded power inductors, and custom parts may not have enough top surface area for clear markings. Some manufacturers also omit markings to simplify production.
Does an inductor code include current rating?
Usually no. The marking normally identifies nominal inductance only. Current rating, saturation current, DCR, shielding, tolerance, and self-resonant frequency must be checked in the datasheet.
Is 102 equal to 1 mH?
Yes, in the common three-digit inductor convention, 102 means 10 × 10^2 µH, which is 1000 µH or 1 mH.