Inductor Energy Calculator - Magnetic Energy Solver

An inductor energy calculator is a fast way to solve E = 1/2 L I^2, the textbook equation for the magnetic energy stored in a coil of inductance L carrying a steady current I. Drop in an inductance in henries, millihenries, microhenries, or nanohenries and a current in amperes, milliamperes, or microamperes, and the tool returns the stored energy in joules with an auto-prefixed nJ, µJ, mJ, or J display. Engineers size DC inductors with it, students confirm homework answers, and hobbyists sanity-check flyback transformer designs.

• • DC inductor sizing: Estimate magnetic energy at the operating current to pick a saturation rating.
• • Switching supply design: Check joules a buck, boost, or flyback inductor carries per cycle before sizing the snubber.
• • LC tank circuit analysis: Pair magnetic energy with capacitor energy to confirm resonance in RF or audio filters.
• • Physics homework: Confirm a textbook answer for the magnetic energy of a solenoid given L and I.

The same formula works for any coil geometry because inductance already absorbs coil length, area, and turn count. Solenoids, toroids, and air-core chokes are all valid inputs as long as you know their inductance in henries.

Real coils include series resistance, so some supply energy dissipates as heat instead of being stored in the magnetic field. The tool assumes an ideal, lossless inductor and reports only the field energy term from E = 1/2 L I^2.

When the same coil is used in an AC circuit, the inductive reactance calculator gives the frequency-domain impedance XL = 2π f L from the same inductance value, so the two tools cover the steady-state DC energy and the AC reactance of the same coil.

Magnetic energy in an inductor builds up as the current ramps from zero to its steady value, and the textbook derivation integrates the back-EMF power against the current to land on a clean closed form.

• E: Magnetic energy stored in the inductor, returned in joules (auto-prefixed nJ, µJ, mJ, J).
• L: Self-inductance of the coil, taken from your input in henries after prefix conversion.
• I: Steady current through the coil, taken from your input in amperes after prefix conversion.

The current-squared dependency means the stored magnetic energy quadruples when the current doubles, which is why a coil that looks harmless at 100 mA can still produce a small arc when its current path is suddenly opened.

When the calculator solves in reverse, it keeps the same energy variable and isolates the requested unknown. I = sqrt(2 E / L) and L = 2 E / I² follow algebraically from E = 1/2 L I² and stay consistent with the same OpenStax derivation.

According to OpenStax University Physics Volume 2, the magnetic energy stored in any inductor is U = 1/2 L I², derived by integrating the back-EMF power L dI/dt against the current from 0 to I.

According to Wikipedia Inductor article, Wikipedia's Inductor page gives the same stored energy formula W = 1/2 L I^2 for an ideal inductor.

On the other side of an LC tank, the capacitor calculator returns the matching electric energy 1/2 C V² for the same circuit, which is why inductor energy and capacitor energy are usually solved side by side.

Four concepts carry the entire derivation of stored magnetic energy, and each one feeds the calculator in a different way.

These four concepts let the calculation stay a single equation. Copper losses, fringing flux, and saturation are separate corrections you add after running the calculator, not parts of the result.

Because the magnetic energy built up in an inductor is exchanged with the electric energy in a capacitor during LC oscillation, the capacitor charge time calculator helps predict how long each charge and discharge cycle takes.

The calculator is built around three modes. Most users leave the default and the calculator solves the stored energy directly.

• 1 Pick what you want to solve for: Choose Stored magnetic energy for the normal forward solve, or pick Current or Inductance to reverse-engineer the coil to a target joule value.
• 2 Enter the inductance and its prefix: Type the coil inductance, then pick H, mH, µH, or nH. The value is converted to henries internally.
• 3 Enter the current and its prefix: Type the steady current and pick A, mA, or µA. The current is squared, so a small typo has an outsized effect on the joule result.
• 4 Add a target energy if you picked a reverse mode: For Current or Inductance modes, type the magnetic energy to store and pick its prefix.
• 5 Read the energy result and the supporting rows: The primary output is stored magnetic energy in joules, auto-prefixed. Supporting rows show L in henries, I in amperes, and I².
• 6 Reset if you want to start over: Press Reset to clear inputs back to the OpenStax example (20 µH and 300 mA) so you can confirm a fresh answer.

Use this calculator to size a 100 µH buck inductor at 500 mA: enter 100 µH and 500 mA, leave Solve For at energy, and the calculator returns 12.5 mJ of stored magnetic energy, the joule value you would compare against the inductor's saturation rating before picking a part.

If the inductor is part of a low-pass or high-pass filter, the capacitive reactance calculator gives the matching 1/(2π f C) value for the capacitor on the same frequency, which makes crossover and filter design easier around the inductor energy calculator.

The calculator removes the unit-prefix gymnastics that usually come with E = 1/2 L I², so you can focus on the answer instead of the conversion.

• • Covers the full SI prefix range: Works with H, mH, µH, nH inductance and A, mA, µA current so you can type the value straight off the inductor datasheet.
• • Auto-prefixed joule output: Returns energy in the most readable unit between nJ and J so the result fits on one line for RF chokes and power chokes alike.
• • Two reverse-solve modes: Pick the coil current or inductance needed to hit a target stored energy, the practical workflow when sizing a coil for a given energy budget.
• • Shows the intermediate values: Reports L in henries, I in amperes, and I² alongside the stored energy so you can sanity-check each term in E = 1/2 L I².
• • OpenStax-aligned worked example: Comes with the 20 µH at 300 mA example from OpenStax University Physics, so students and instructors match the output to a known textbook answer.

These benefits apply whether you are sizing a switching supply inductor, checking an LC tank, or working through a homework problem, because the underlying formula is the same in every case.

Five variables move the stored magnetic energy around, and three limitations tell you where to stop trusting the result.

• • Ideal inductor assumption: the calculator ignores copper loss, core loss, and stray capacitance, so the joule output is the field-only magnetic energy term.
• • Saturation is not modeled: real inductors lose inductance as the core approaches saturation, so the formula over-estimates stored energy near the saturation current.
• • Reverse-solve modes need both inputs: the current and inductance reverse solves still require positive target energy and a non-zero partner input, otherwise the calculator returns 0 with a hint.

For most textbook and design checks those three limitations are small enough to ignore. Near saturation or at high frequency, treat the output as the upper bound and apply a derating factor from the inductor datasheet.

In an LC tank, magnetic and electric energy swap each cycle at the resonant frequency, and the result here gives the peak joules available to charge the capacitor.

According to Omni Calculator, Omni's inductor energy storage page uses the same E = 1/2 × L × I^2 expression and the same 20 µH at 300 mA worked example.

When the inductor feeds a series stack of capacitors in a resonant network, the capacitors in series calculator gives the equivalent capacitance the inductor is exchanging energy with, which is the value used to compute the tank's resonant frequency.