Units of Inductance
Inductance is measured in henries (H), named after the American scientist Joseph Henry. Because one henry is a relatively large unit, electronic circuits usually use smaller units such as millihenries (mH), microhenries (µH), nanohenries (nH) and picohenries (pH). Understanding these units and converting between them is essential when designing, repairing or selecting inductors for electronic circuits.
The Henry (H)
The henry (H) is the SI unit of inductance. An inductor has an inductance of one henry if a current changing at one ampere per second produces an induced voltage of one volt.
1 H = 1 V·s/A
Large inductors used in industrial equipment and power systems may have inductance values of several henries.
Common Units
| Unit | Symbol | Equivalent |
|---|---|---|
| Henry | H | 1 H |
| Millihenry | mH | 0.001 H |
| Microhenry | µH | 0.000001 H |
| Nanohenry | nH | 0.000000001 H |
| Picohenry | pH | 0.000000000001 H |
Unit Conversion Table
| From | To | Multiply By |
|---|---|---|
| H → mH | Millihenries | 1,000 |
| H → µH | Microhenries | 1,000,000 |
| H → nH | Nanohenries | 1,000,000,000 |
| mH → µH | Microhenries | 1,000 |
| µH → nH | Nanohenries | 1,000 |
| nH → pH | Picohenries | 1,000 |
Quick Conversion Examples
| Value | Equivalent |
|---|---|
| 1 H | 1,000 mH |
| 0.5 H | 500 mH |
| 100 mH | 0.1 H |
| 10 mH | 10,000 µH |
| 470 µH | 0.47 mH |
| 22 µH | 22,000 nH |
| 100 nH | 0.1 µH |
| 1 nH | 1,000 pH |
Typical Inductance Ranges
| Application | Typical Inductance |
|---|---|
| Microwave Circuits | 1–100 nH |
| RF Filters | 10 nH–10 µH |
| FM Radio Circuits | 100 nH–10 µH |
| DC-DC Converters | 1–100 µH |
| Audio Crossovers | 0.1–10 mH |
| Power Filters | 1 mH–10 H |
| Industrial Chokes | Several henries |
Reading Inductor Values
Many inductors display their inductance directly, while others use a printed code.
| Marking | Typical Inductance |
|---|---|
| 1R0 | 1.0 µH |
| 2R2 | 2.2 µH |
| 100 | 10 µH* |
| 220 | 22 µH* |
| 470 | 47 µH* |
| 101 | 100 µH* |
*Many manufacturers use these common codes, but there is no universal marking standard. Always consult the manufacturer's datasheet when available.
Why Smaller Units Are Used
Most electronic circuits require relatively small inductance values. For example, RF circuits commonly use nanohenries or microhenries, while switch-mode power supplies often use inductors measured in microhenries. Using smaller units keeps component values easy to read and compare.
Common Conversion Mistakes
- Confusing mH with µH.
- Forgetting that each unit changes by a factor of 1,000.
- Assuming all inductor markings follow the same coding system.
- Ignoring the current rating when selecting a replacement.
- Selecting an inductor based only on inductance instead of considering saturation current and DC resistance.
Interesting Facts
- The henry is named after Joseph Henry, a pioneer in electromagnetism.
- RF inductors are often measured in nanohenries because very small inductance values affect high-frequency circuits.
- Large industrial power chokes may have inductances of several henries.
- Even a few nanohenries of PCB track inductance can affect high-speed digital and RF circuits.
- Modern smartphones contain inductors ranging from a few nanohenries to several microhenries.
Key Points
- The SI unit of inductance is the henry (H).
- Millihenries, microhenries, nanohenries and picohenries are commonly used in electronics.
- Each unit differs by a factor of 1,000.
- Correct unit conversion is essential when selecting replacement inductors.
- Inductance is only one specification—current rating, DC resistance and core material are equally important.