What this calculator does
An inductor stores energy in the magnetic field created by the current flowing through it. Like a capacitor, the relationship is a square law, but it is current that gets squared rather than voltage: doubling the current quadruples the stored energy.
The practical consequence is what happens when that current is interrupted. The stored energy has to go somewhere, and if the circuit is opened suddenly it appears as a large voltage spike, which is why flyback diodes exist.
The formula
Multiply half the inductance by the square of the current. Inductance is in henries and current in amps, which gives energy in joules directly.
| Term | Meaning |
|---|---|
| Inductance (L) | How strongly a coil opposes a change in current, in henries. It depends on the number of turns, the geometry and the core material. |
| Henry (H) | One volt induced per amp per second of current change. Practical inductors are often measured in millihenries. |
| Flyback | The voltage spike produced when current through an inductor is interrupted, as the stored magnetic energy discharges. |
The inputs explained
| Field | What to enter |
|---|---|
| Inductance (H) | The inductance in henries. A 50 millihenry coil is entered as 0.05. |
| Current (A) | The current flowing through the inductor, in amps. |
When to use it
Designing a switching supply
Inductors in switching regulators store and release energy every cycle, and the amount stored sets the current the circuit can support.
Understanding a relay or solenoid spike
Switching off a coil releases its stored energy, and knowing how much explains why suppression components are needed.
Comparing energy storage methods
Placing inductor storage alongside capacitor storage shows the very different scales and trade-offs involved.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How does stored energy grow with current?
The same inductor carrying a range of currents.
| Current | Stored energy | Energy in millijoules |
|---|---|---|
| 1 A | 0.025000 J | 25.000 mJ |
| 2 A | 0.100000 J | 100.000 mJ |
| 4 A | 0.400000 J | 400.000 mJ |
| 8 A | 1.6000 J | 1,600.000 mJ |
Questions
Why does interrupting an inductor cause a voltage spike?
Because an inductor opposes any change in its current. Cutting the current abruptly demands an enormous rate of change, and the inductor responds by generating whatever voltage it takes to keep the current flowing, which can be hundreds of volts from a low-voltage supply.
How is this different from a capacitor?
A capacitor stores energy in an electric field and resists changes in voltage. An inductor stores energy in a magnetic field and resists changes in current. They are mirror images of each other, which is why their formulas look so similar.
Does the core material matter?
It matters a great deal for the inductance value, since a ferromagnetic core greatly increases it. Once the inductance is known, however, the energy formula does not care how that value was achieved.
What limits the energy an inductor can store?
Core saturation, mainly. Beyond a certain current the core stops responding, inductance collapses and the relationship no longer holds. Air-cored inductors avoid saturation but store much less for their size.
For the electric-field equivalent, see the capacitor charge and energy calculator. For the magnetic field a current produces, see the magnetic field of a straight wire calculator.