Inductor Energy Calculator (E = ½LI²)
Calculate the energy stored in an inductor's magnetic field. Enter inductance in mH and current in amps to get energy in joules and millijoules.
How to use this tool
- Enter inductance (l) and current (i) in the fields above.
- Results update instantly as you type — or click Calculate.
- Read your energy and the full breakdown beneath it.
An inductor stores energy in its magnetic field: E = ½LI² (joules), where L is inductance (henries) and I is current (amps). Flux linkage is λ = LI (webers). Because energy scales with I², doubling the current quadruples stored energy.
Formula
Energy stored: E = ½ × L × I2
Flux linkage: λ = L × I
where L is inductance in henries and I is current in amperes. Inductance entered in mH is divided by 1000 before use.
How it works
The energy stored in an inductor's magnetic field equals ½LI², derived by integrating the work done by the voltage that opposes the build-up of current (Lenz's law); flux linkage λ = LI gives the total magnetic flux threading the coil turns.
Results assume a linear, lossless inductor; real inductors have winding resistance, core losses, and saturation effects that limit the storable energy and are not accounted for here.
Worked example
Worked example — 10 mH at 2 A
- Convert inductance: L = 10 mH = 10 / 1000 = 0.01 H.
- Energy: E = 0.5 × 0.01 × 2² = 0.5 × 0.01 × 4 = 0.02 J.
- In millijoules: E = 0.02 × 1000 = 20 mJ.
- Flux linkage: λ = 0.01 × 2 = 0.02 Wb.
Energy = 0.02 J (20 mJ); flux linkage = 0.02 Wb.
Key terms
- Inductance (L)
- A coil's ability to oppose changes in current by storing energy in a magnetic field, measured in henries (H).
- Millihenry (mH)
- One thousandth of a henry (10⁻³ H); a common unit for inductors used in power electronics and RF circuits.
- Flux linkage (λ)
- The product of inductance and current (L × I), measured in webers (Wb); represents the total magnetic flux linked with all coil turns.
- Magnetic saturation
- The condition where increasing current no longer proportionally increases magnetic flux because the core material is fully magnetised; actual stored energy becomes less than ½LI² predicts.
- Lenz's law
- The principle that an induced voltage in an inductor opposes the change in current that caused it, which is the physical basis for magnetic energy storage.
Frequently asked questions
- What happens when you suddenly disconnect an inductor from a circuit?
- The inductor tries to maintain its current, generating a large voltage spike ('inductive kick'). This is why flyback diodes are used across relay coils and motor drivers to protect switching transistors.
- How is inductor energy used in switching power supplies?
- In a buck or boost converter, the inductor stores energy during one switch phase and releases it during the other, allowing efficient voltage conversion.