Inductance is the property of an electrical conductor or coil that opposes a change in current flowing through it, and its standard base unit of measurement is the Henry (H). When you are designing a switch-mode power supply, winding a custom transformer, or tuning an RF matching network, you rarely deal in whole Henrys. Instead, you navigate a sea of metric prefixes—millihenrys, microhenrys, and nanohenrys—each tied to vastly different physical component sizes and circuit behaviors. Understanding these units of inductance is the difference between a stable power rail and a MOSFET that violently explodes from a voltage spike.

What Inductance Actually Changes in a Circuit: Inductance dictates the rate at which current can change over time. A high inductance value forces current to ramp up and down slowly, which smooths DC power and blocks high-frequency AC noise. A low inductance value allows current to change almost instantaneously, making it ideal for passing high-frequency signals or handling ultra-fast switching edges.

The Henry and Its Sub-Multiples

Named after Joseph Henry, one Henry is defined as the inductance that induces one volt of electromotive force (EMF) when the current through it changes at a rate of one ampere per second. In practical bench and PCB design, a 1H inductor would be the size of a cinder block and possess crippling DC resistance (DCR). Therefore, we use sub-multiples.

The table below maps the standard units of inductance to their real-world applications, typical value ranges, and the physical form factors you will encounter when sourcing parts from distributors like Mouser or Digi-Key.

Unit Name Symbol Multiplier Typical Value Range Common Application Physical Form Factor Example
Henry H 100 1H to 10H Audio crossovers, large mains filter chokes Iron-core laminated bobbins, massive toroids
Millihenry mH 10-3 1mH to 100mH Line-frequency EMI filters, relay coils, solenoids Ferrite rod, radial leaded, bobbin-core
Microhenry µH 10-6 0.1µH to 100µH Buck/boost DC-DC converters, RF chokes Shielded SMD (e.g., 6x6mm), powder iron toroids
Nanohenry nH 10-9 0.5nH to 500nH VHF/UHF RF matching, high-speed decoupling 0402/0603 ceramic multilayer chip, bare PCB trace

A typical 5V to 3.3V buck converter running at 500 kHz usually requires a 4.7 µH to 10 µH shielded SMD inductor. If you accidentally substitute a 4.7 mH inductor in that circuit, the current will ramp up so slowly that the output voltage will collapse under even a minimal load, and the controller will likely trigger an under-voltage lockout (UVLO).

Calculating Induced Voltage: A Worked Numeric Example

The fundamental equation governing inductance is V = L × (Δi / Δt). This formula tells us that the voltage induced across an inductor is directly proportional to its inductance (L) and the rate of change of current (Δi / Δt). Think of an inductor as a mechanical flywheel: the heavier the flywheel (higher inductance), the harder it is to stop it spinning instantly.

Let us look at a scenario that has destroyed many prototype boards: a flyback voltage spike in a boost converter or relay driver.

The Scenario: You are driving an inductive load with a MOSFET. The circuit features a 15 µH inductor. During the on-state, the current ramps up to 3.0 Amps. When the microcontroller pulls the MOSFET gate low, the ultra-fast switch turns off in 50 nanoseconds (ns), forcing the current to drop from 3.0A to 0A almost instantly. What is the voltage spike across the inductor?

Step 1: Convert units to base SI (Henrys, Amperes, Seconds).

  • L = 15 µH = 15 × 10-6 H
  • Δi = 3.0 A - 0 A = 3.0 A
  • Δt = 50 ns = 50 × 10-9 s

Step 2: Plug into the formula.

  • V = (15 × 10-6) × (3.0 / 50 × 10-9)
  • V = (15 × 10-6) × (60,000,000)
  • V = 900 Volts

Even though your power supply is only 12V, the inductor's desperate attempt to keep current flowing generates a 900V spike. If your MOSFET is only rated for 60V (Vds), it will undergo avalanche breakdown and fail catastrophically. This is exactly why we place flyback diodes or RC snubber networks across inductive loads—to provide a safe path for that current, intentionally increasing the Δt to keep the induced voltage within safe limits. For a deeper dive into component selection to prevent these failures, reference Texas Instruments' application notes on inductor selection for switching regulators.

Where You Meet These Units of Inductance in Practice

Different domains of electronics live in different prefix territories. Knowing which unit to expect saves time when reading schematics and sourcing BOMs.

Power Electronics (Microhenrys - µH)

Switch-mode power supplies (SMPS) are the most common place you will specify microhenrys. A buck converter stepping 12V down to 5V at a 1 MHz switching frequency relies on an inductor to store energy in its magnetic field and release it to the output capacitor. Parts like the Wurth Elektronik 744774 series are industry standards here. You will typically select values between 2.2 µH and 47 µH, paying close attention to the saturation current (Isat) rating. If your peak current exceeds Isat, the core saturates, the inductance drops to near-zero, and your switching FET shorts out.

Mains Filtering and Audio (Millihenrys - mH)

When dealing with 50/60 Hz AC mains, the frequency is so low that you need massive inductance to create meaningful impedance. Common-mode chokes in EMI filters often range from 10 mH to 50 mH. Similarly, in passive audio speaker crossover networks, you will find air-core or ferrite-core inductors in the 1.5 mH to 5 mH range to block high frequencies from reaching a woofer.

RF and High-Speed Digital (Nanohenrys - nH)

At GHz frequencies, even a straight piece of copper wire possesses measurable inductance. In RF design, such as tuning the antenna matching network on an ESP32-WROOM-32 module, you deal in nanohenrys. A 0402-sized multilayer ceramic chip inductor (like the Murata LQG series) might be just 2.2 nH. At these scales, PCB trace routing becomes a component itself; a 1mm trace on an FR4 board can introduce roughly 1 nH of parasitic inductance, which can detune a 2.4 GHz matching pi-network if not accounted for. See All About Circuits' guide on RF PCB layout for more on managing parasitic trace inductance.

Common Confusions: Inductance vs. Inductive Reactance

The most frequent mistake beginners make is confusing inductance with inductive reactance. They are related, but they are not the same thing, and they use entirely different units.

  • Inductance (L): Measured in Henrys. This is a fixed physical property of the component. It is determined by the number of wire turns, the cross-sectional area of the coil, and the permeability of the core material. A 10 µH inductor has 10 µH of inductance whether it is sitting on your desk, connected to a DC battery, or hooked to a 1 MHz RF generator.
  • Inductive Reactance (XL): Measured in Ohms (Ω). This is the opposition to alternating current that the inductor presents at a specific frequency. It is calculated using the formula XL = 2πfL.

To illustrate: Take a 10 µH inductor.
At DC (0 Hz), its reactance is 0 Ω (it acts as a short circuit, limited only by its tiny wire resistance).
At 1 MHz (1,000,000 Hz), its reactance is 2 × π × 1,000,000 × 0.000010 = 62.8 Ω.
The inductance never changed; only the frequency-dependent reactance changed.

When reading datasheets, always verify whether the manufacturer is specifying the base inductance (L) or the impedance (Z) at a specific test frequency. High-frequency chip inductors are sometimes binned by their impedance in Ohms at 100 MHz rather than their raw nanohenry value, which can cause massive ordering errors if you misread the units column on the distributor's parametric search page.