Inductance Calculator
Combine inductors in series or parallel, and calculate stored energy, inductive reactance, and RL time constant. Live SVG schematic, smart unit formatting.
- Ltotal = L1 + L2 + … + Ln
How It Works
- 1
Choose a mode
The Series/Parallel tab combines up to 10 inductors into an equivalent inductance. The Energy & Reactance tab computes stored energy, inductive reactance, and the RL time constant for a single inductor.
- 2
Enter values with units
Pick uH, mH, or H for inductance; mA or A for current; Hz, kHz, or MHz for frequency. The calculator converts everything to SI internally. Optionally add a series resistance for the time constant.
- 3
Read the results
The result panel shows the total inductance (network mode) or energy in joules, reactance in ohms, and time constant in seconds (energy mode). The SVG diagram updates in real time.
Inductance, energy storage, and AC reactance
Michael Faraday demonstrated electromagnetic induction in 1831, showing that a changing magnetic field could drive current through a conductor. Joseph Henry, working independently in Albany, New York, observed the same effect around the same time, and the SI unit of inductance, the henry (H), honors his contribution. Self-inductance measures a conductor's opposition to changes in its own current: when current through a coil changes, the resulting change in magnetic flux induces a back-EMF that resists the change. This property makes inductors the magnetic dual of capacitors. The energy stored in an inductor is E = 1/2 L I squared, held entirely in the magnetic field. That stored energy is what makes inductors essential in switch-mode power supplies, where each switching cycle transfers energy between the inductor's field and the output capacitor. In AC circuits the inductor presents a reactance X_L = 2 pi f L that grows linearly with frequency. At DC (f = 0) an ideal inductor is a short circuit; at high frequencies it approaches an open circuit. This frequency-dependent behavior is the basis for EMI filters, LC tank circuits, and chokes that suppress high-frequency noise while passing power-frequency current. Common inductor types range from air-core coils used in RF circuits (nH to low uH) through ferrite and iron-powder cores in switching regulators (tens to hundreds of uH) to mH-range mains-frequency chokes and common-mode filters. Series inductors add directly (L_total = L1 + L2 + ... + Ln) and parallel inductors follow the reciprocal rule (1/L_total = 1/L1 + 1/L2 + ... + 1/Ln), both assuming zero mutual coupling between the coils.
Common pitfalls
Running past the saturation current. Every ferrite or iron-powder inductor has I_sat in its datasheet; beyond that point the core saturates, L drops sharply, and a switching supply turns into a short circuit. Pick inductors with I_sat well above the peak switching current, not just above the average DC load.
Ignoring DCR (winding resistance). A 10 µH power inductor with 20 mΩ DCR dropping 5 A continuous dissipates 500 mW as heat, straight from the efficiency budget. Low DCR parts use thicker wire and larger cores, trading size for loss.
Assuming the series addition formula ignores mutual coupling. Two adjacent coils on the same core have mutual inductance M: L_total = L1 + L2 ± 2M. Sign depends on winding direction; get it wrong and your series 'boost' becomes a partial cancellation. For uncoupled air-core or separated parts the simple sum is safe.
Forgetting self-resonance frequency (SRF). Every inductor has parasitic capacitance; above its SRF it behaves capacitively, not inductively. A 10 µH chip inductor with 50 MHz SRF is useless as an inductor at 100 MHz. Check the datasheet SRF before using inductors in RF or EMI filter designs.
Treating energy as average power. E = ½ L I² gives joules, not watts. A 10 A peak through a 100 µH inductor stores 5 mJ; how often that energy transfers depends on the switching frequency. Dumping it 100 000 times per second = 500 W of transfer power.
Frequently Asked Questions
Why does this calculator assume zero mutual coupling?
When two or more inductors share magnetic flux, their effective inductance changes by a term ±2M, where M is the mutual inductance. The sign depends on whether the coils aid or oppose. Computing M requires knowing the physical geometry, core material, and relative orientation of each inductor, which varies by layout. This calculator handles the most common bench and PCB scenario: inductors spaced far enough apart, or wound on separate cores, so that M is negligible.
How do series and parallel inductor rules compare to resistors and capacitors?
Inductors combine exactly like resistors. In series the values add directly; in parallel the reciprocals add. Capacitors are the opposite: parallel capacitors add directly, series capacitors add by reciprocals. The reason is that inductance relates to flux linkage per ampere, and flux linkages add when inductors share the same current (series) just as voltage drops add for series resistors.
What is inductive reactance and why does it matter?
Inductive reactance X_L = 2 pi f L is the opposition an inductor presents to alternating current at frequency f. At DC (f = 0) the reactance is zero, so an ideal inductor passes DC freely. As frequency rises, X_L grows linearly, blocking higher-frequency signals. This property makes inductors essential in EMI filters, chokes, crossover networks, and LC tank circuits.
How is the energy E = 1/2 L I^2 stored, and where does it go?
The energy sits in the magnetic field surrounding the inductor. Unlike a capacitor that stores energy in an electric field, an inductor stores energy magnetically. When current is interrupted, the field collapses and the stored energy must go somewhere. In a circuit without a flyback diode or snubber, this produces a voltage spike. In a buck or flyback converter the energy is deliberately transferred to a capacitor or load each switching cycle.
What is the RL time constant and how do I use it?
The time constant tau = L / R describes how quickly current rises or falls in a series RL circuit. After one tau the current reaches about 63.2% of its final value when a voltage step is applied. After five tau it is within 0.7% of steady state, which is why engineers use 5 tau as the practical settling time. Halving R doubles tau, so low-resistance circuits with large inductors can have surprisingly long settling times.
What types of inductors are commonly used?
Air-core inductors have no saturation limit and are used in RF circuits from a few MHz up. Ferrite-core inductors offer higher inductance per volume and are common in power supplies, EMI filters, and switching regulators from a few kHz to several MHz. Toroidal inductors confine flux well and produce less stray field, making them popular in audio equipment and sensitive analog circuits. Iron-powder cores handle large DC bias currents in power applications. Multilayer chip inductors (SMD) serve compact PCB layouts in the nH to low-µH range.
How do I choose between microhenries and millihenries?
The application frequency drives the choice. RF circuits (above 1 MHz) typically use inductors in the nH to low-µH range. Switch-mode power supplies at 100 kHz to 2 MHz use tens to hundreds of µH. Audio-frequency and mains-frequency circuits, including common-mode chokes, often require mH-range inductors. This calculator accepts any combination of µH, mH, and H and converts internally to henries for computation.
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