Millihenry to Microhenry: Inductor Selection for RF and Switching Power

A switching power supply design called for a 4.7 µH power inductor in the buck converter stage. The engineer sourced 4.7 mH — three orders of magnitude too large. At 4.7 mH, the inductor's impedance at the 500 kHz switching frequency was 14,765 Ω. The converter could not switch current through it fast enough to regulate. The output voltage never settled. The board powered up, measured no visible fault, and simply delivered no stable output. Two days of scope debugging traced the fault to the inductor value — a number that looked right on the BOM and catastrophically wrong in the circuit.

To convert millihenries (mH) to microhenries (µH), multiply by 1,000. To convert microhenries (µH) back to millihenries (mH), divide by 1,000. Use the mH to µH converter to verify any inductor value before placing a component order.

Calculate Instantly

When cross-referencing datasheets, simulation models, and BOM specs, convert between mH and µH without guessing.

Where mH and µH Each Belong

Inductance values in electronics span nanohenries (nH) in RF traces to henries (H) in mains transformers. The engineering boundary that matters most:

The mH/µH boundary sits at the crossover between low-frequency power magnetics and high-frequency switching converters. Mixing the two scales in a design file is exactly what caused the buck converter failure above.

For the SI prefix math, see milli to micro conversion and understanding SI prefixes. For the parallel capacitance conversion, see millifarad to microfarad.

1 mH = 1,000 µH

graph LR
    A[1 mH<br>Audio choke / low-freq SMPS] -->|"× 1,000"| B[1,000 µH<br>Same value in µH]
    B -->|"÷ 1,000"| A
    style A fill:#7c3aed,color:#fff,stroke:#d946ef
    style B fill:#22d3ee,color:#111,stroke:#d946ef

For the reverse conversion, use the µH to mH converter.

Why the Switching Frequency Constrains Inductance

In a buck converter, the inductor value determines the ripple current. The relationship is:

ΔI = (V_in − V_out) × D / (L × f_sw)

Where:

• ΔI = peak-to-peak ripple current
• D = duty cycle
• L = inductance (in henries — consistent units required)
• f_sw = switching frequency (Hz)

At f_sw = 500 kHz and L = 4.7 µH = 0.0000047 H, the ripple current is calculable and manageable. At L = 4.7 mH = 0.0047 H — 1,000× larger — the ripple current shrinks to a tiny fraction of the intended value, and the converter's control loop cannot respond fast enough to regulate the output. The switching frequency and the inductance are physically linked. A 1,000× error in one breaks the other.

The mH to µH Conversion Table

Reading Inductor Datasheets

Inductor datasheets specify inductance in one of three units depending on the value range:

• nH for RF and small signal (< 1 µH)
• µH for switching power (1 µH–1 mH)
• mH for audio, EMC chokes, and low-frequency magnetics (> 1 mH)

A search for "4.7" in a distributor catalog can return results in nH, µH, or mH depending on the product family. The unit column is the critical field — not the number. For related current calculations through the inductor, see milliampere vs microampere. For timing and frequency context, see milliseconds vs microseconds.

Frequently Asked Questions

How many microhenries are in a millihenry? Exactly 1,000 µH = 1 mH. This is an SI-defined exact relationship. Use the mH to µH converter for specific values.

What is the difference between mH and µH in a switching power supply? A switching power supply operating at 500 kHz typically requires an inductor in the 4.7–47 µH range. An inductor of the same numeric value in mH is 1,000× larger — its impedance at the switching frequency is 1,000× higher, preventing the converter from switching current effectively. The output will not regulate.

How do I choose between mH and µH for an inductor? The switching frequency determines the scale. Above 100 kHz: work in µH. Below 10 kHz (audio, mains): work in mH. Between 10–100 kHz: check both — the design may span the boundary. The formula ΔI = (V_in − V_out) × D / (L × f_sw) with L in henries gives an unambiguous result.

Why do RF circuits use nH instead of µH? At RF frequencies (MHz–GHz), the required inductance is small enough that nH is the natural unit. A 10 nH RF inductor is 0.01 µH — a value that would be written as 0.00001 mH, far outside the mH range used in power magnetics. The nH scale avoids the awkward decimal notation and removes any risk of mH/µH confusion.

What measuring equipment reads inductance in µH vs mH? LCR meters measure inductance across all scales and typically display in the most readable unit for the range. An LCR meter measuring a 47 µH inductor displays µH; measuring a 10 mH choke displays mH. When transferring values from a meter to a BOM or simulation, always copy the unit alongside the number.

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Next: Inductor selection pairs directly with capacitor selection in LC filter design — see Millifarad to Microfarad: Reading Capacitor Values in Electronics.

Sources

• NIST Special Publication 811 — Guide for the Use of the International System of Units
• BIPM: SI Prefixes
• Würth Elektronik: Inductor Selection Guide for DC-DC Converters
• Texas Instruments: Inductor Selection for Buck Converter Design (Application Report)
• IEEE 754 Standard for Floating-Point Arithmetic