Inductor Energy Calculator
The energy stored in an inductor's magnetic field, its time constant, and the voltage spike if the current is interrupted.
How much energy a coil holds.
How the inductor energy calculator works
An inductor stores ½LI² in its magnetic field — the exact dual of the capacitor's ½CV², with current in place of voltage.
The practical consequence is the flyback spike. Because V = L(dI/dt), breaking the current suddenly produces a voltage proportional to how fast you break it, which is why switching an inductive load without a snubber diode destroys transistors.
Formula: E = ½LI²; τ = L/R; V = L dI/dt
Worked examples
| Inputs | Stored energy | Note |
|---|---|---|
| 100 mH at 2 A | 0.2 J | 200 mJ |
| Twice the current | 0.8 J | 800 mJ — four times |
| A relay coil | 0.00025 J | 0.25 mJ, but still worth a diode |
FAQFrequently asked questions
How does an inductor store energy?
In the magnetic field around the coil, as ½LI² — the exact mirror of a capacitor's ½CV².
What is the flyback spike?
The voltage produced when the current is interrupted. V = L dI/dt, so a fast switch-off generates enormous voltages from a modest coil.
Why does my transistor keep failing?
Almost certainly the flyback from an inductive load. A reverse-biased diode across the coil gives the current somewhere to go.
What is the time constant?
L/R — the time to reach 63% of the final current. Larger inductance or lower resistance means a slower response.
Is this how ignition coils work?
Exactly. Interrupting a few amps through a coil with a high turns ratio produces the tens of kilovolts a spark plug needs.
Where these figures come from
- NIST — CODATA 2018 fundamental physical constants — G, g₀, R, c
- NIST Special Publication 811 — Guide for the use of the International System of Units — unit conversions
- The Engineering ToolBox — material properties — specific heats, expansion coefficients, densities
- National Physical Laboratory — the UK's national measurement institute
Last checked: September 2026. Constants are the CODATA 2018 values; formulas are the standard textbook forms.