What Is an Inductor and How Is Inductance Calculated?
An inductor is a passive two-terminal component that stores energy in a magnetic field when current flows through it. Its inductance, measured in henries (H), is the ratio of magnetic flux linkage to current. One henry produces one volt when current changes at one ampere per second.
Inductors in series add directly: L_total = L1 + L2 + … + Ln. In parallel, reciprocals add: 1/L_total = 1/L1 + 1/L2 + … + 1/Ln. The energy stored is E = ½ × L × I², where E is joules, L is henries and I is amps. For a long air-core solenoid, L = μ₀ × μr × N² × A / l, where μ₀ = 4π × 10⁻⁷ H/m, N is turns, A is cross-section area in m² and l is length in metres.
The henry is named after Joseph Henry, and is defined in the International System of Units (SI) as one weber per ampere. Parallel inductance is always smaller than the smallest inductor in the group, while series inductance is always larger than the biggest.
How to Use This Inductor Calculator
Choose a calculation tab, enter your values, and the result updates live:
- Series / Parallel: Type two or more inductor values separated by commas and choose one unit (nH, µH, mH or H). For two series inductors you can add a coupling coefficient k (0–0.99) to get the aiding and opposing totals.
- Energy: Enter inductance and the DC current. You get stored energy in joules and flux linkage in webers.
- Coil: Enter turns, coil diameter and length in millimetres, and core relative permeability (1 for air). Accuracy is best when the coil is longer than its diameter.
All units are SI-based, so results are the same worldwide; only the notation differs (µH is sometimes written uH or mcH in older datasheets).
What Are Typical Inductor Values and Core Permeabilities?
Practical inductors span nanohenries for RF chokes to henries for power-supply smoothing chokes. Core material changes inductance by the relative permeability μr:
| Core material | Approx. μr | Typical use |
|---|---|---|
| Air | 1 | RF coils, high-frequency circuits |
| Powdered iron | 10 – 100 | Switching regulators, filters |
| Ferrite (MnZn) | 750 – 15,000 | Transformers, EMI chokes |
| Silicon steel | 2,000 – 8,000 | Mains-frequency chokes |
Real cores saturate: once flux density reaches the material's limit, inductance collapses, so energy-storage results assume an unsaturated linear inductor.
Frequently Asked Questions
How do you calculate inductors in series and parallel?
Inductors in series add directly (L_total = L1 + L2 + …), while inductors in parallel add as reciprocals (1/L_total = 1/L1 + 1/L2 + …). This is the opposite of capacitors, and matches resistors, assuming no mutual coupling between the coils.
How much energy does an inductor store?
An inductor stores E = ½LI² joules. For example, a 1 mH inductor carrying 5 A stores 0.5 × 0.001 × 25 = 12.5 mJ. Energy rises with the square of current, so doubling the current quadruples the stored energy.
How accurate is this inductor calculator?
It applies ideal formulas exactly. Real inductors have tolerance (commonly ±10–20%), winding resistance, parasitic capacitance and core saturation, and the coil formula is an approximation for coils longer than their diameter. Always check the datasheet for critical designs.
What is the difference between inductance and reactance?
Inductance (henries) is a fixed property of the component's geometry and core. Inductive reactance (ohms) is its opposition to AC at a given frequency, X_L = 2πfL. Use the reactance calculator for the frequency-dependent value.