What is the formula for the henry?

One henry equals one volt-second per ampere (1 H = 1 V*s/A), which is also one weber per ampere (1 H = 1 Wb/A). The defining circuit relationship is V = L * dI/dt, where L is the inductance, V is the induced voltage, and dI/dt is the rate of change of current.

How big is one henry?

One henry is a fairly large amount of inductance. Power transformers, large filter chokes, and audio coils are measured in henries or fractions of a henry, while most electronics inductors are far smaller, measured in millihenries, microhenries, or nanohenries.

What is the difference between a henry and a farad?

A henry measures inductance, the property of a coil that opposes changes in current and stores energy in a magnetic field. A farad measures capacitance, the property of a component that stores energy in an electric field and opposes changes in voltage. They are complementary but distinct quantities.

What is a millihenry?

A millihenry (mH) is one thousandth of a henry, or 0.001 H. It is a common range for audio crossover coils, small transformers, and power-supply chokes. Even smaller values use the microhenry (one millionth of a henry) and the nanohenry (one billionth).

What Is a Henry? Inductance Explained

Definition

The henry (symbol: H) is the SI unit of electrical inductance. A circuit element has an inductance of one henry when a current changing at a rate of one ampere per second induces a voltage of one volt across it.

Expressed as units, the henry is one volt-second per ampere, which is equivalent to one weber per ampere:

1 H = 1 V·s/A = 1 Wb/A

The defining relationship between inductance, voltage, and current is:

V = L × dI/dt

Here L is the inductance in henries, V is the voltage induced across the inductor, and dI/dt is the rate of change of current in amperes per second. The faster the current changes, the larger the induced voltage — and the higher the inductance, the larger that voltage becomes for the same rate of change.

What Inductance Means

Inductance is the property of a conductor — usually a coil of wire — that opposes any change in the current flowing through it. When current flows through a coil it creates a magnetic field. If that current tries to change, the collapsing or growing magnetic field induces a voltage that pushes back against the change. This is Lenz's law in action.

A useful mental model is electrical inertia. Just as a heavy object resists changes to its motion, an inductor resists changes to its current. A 1 H inductor with a steady current behaves quietly, but the instant you try to switch that current off, it fights back with a voltage spike. This is exactly why inductors store energy in their magnetic field and why they are central to transformers, motors, and power supplies.

The amount of inductance depends on the coil's geometry: more turns of wire, a larger cross-sectional area, and a magnetic core all increase inductance, while a longer coil decreases it.

Everyday Examples

Inductance values span a vast range, from the tiny coils etched onto circuit boards to the massive windings inside grid transformers. The table below lists familiar reference points.

Component	Typical Inductance
PCB trace or bond wire	~1–10 nH
RF tuning coil (radio, Wi-Fi)	~10 nH – 1 µH
Switching power-supply inductor	~1–100 µH
Audio crossover coil	~0.1–5 mH
Relay or solenoid coil	~10–500 mH
Filter choke (linear power supply)	~1–10 H
Large mains transformer winding	~1–100 H

Notice that most practical components fall well below one henry. A full henry of inductance generally implies many turns of wire wound on a magnetic core, which is why it shows up mainly in chokes and transformers rather than in small electronics.

Common Submultiples

Because inductance ranges across many orders of magnitude, standard SI prefixes are used to keep the numbers manageable. The most common units engineers reach for are shown below.

Name	Symbol	Value	Typical Use
Henry	H	1 H	Transformers, large filter chokes
Millihenry	mH	0.001 H (10⁻³ H)	Audio coils, small transformers
Microhenry	µH	0.000001 H (10⁻⁶ H)	Power-supply and RF inductors
Nanohenry	nH	0.000000001 H (10⁻⁹ H)	High-frequency RF, PCB parasitics

To convert, simply shift the decimal point by the difference in prefixes: 1 H equals 1,000 mH, which equals 1,000,000 µH, which equals 1,000,000,000 nH.

History

The henry is named after Joseph Henry (1797–1878), the American scientist who discovered the principle of self-inductance in the late 1820s and early 1830s, working independently of Michael Faraday, who discovered electromagnetic induction around the same time in England. Henry built powerful electromagnets, observed the high-voltage spark produced when a coil's current is interrupted, and laid the groundwork for the electric relay and the telegraph.

Henry went on to become the first Secretary of the Smithsonian Institution. The unit was named in his honour and was formally adopted as the SI unit of inductance, ensuring his contribution to electromagnetism is recalled every time an engineer specifies a coil.

Need to switch between henries, millihenries, microhenries, and nanohenries? Use our inductance converter for fast, precise results.