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Inductor Calculator

Calculate inductor parameters including reactance, Q factor, and winding design.

Inductor Parameters

AC Performance

62.83 Ohms
Reactance XL
628.3
Q Factor

Electrical Parameters

Impedance62.83 Ohms
Energy Stored50.00 uJ
Time Constant (L/R)1000.00 us
Power Loss100.0 mW

Winding Design

AL Value40.00 nH/turn2
Skin Depth0.209 mm

What Is an Inductor?

An inductor is an electronic component that stores energy in a magnetic field when electric current flows through it. Typically constructed as a coil of wire, inductors resist changes in current—opposing increases and decreases. This property makes them essential for filtering, energy storage, and transformers.

PropertySymbolUnitDescription
InductanceLHenry (H)Ability to store magnetic energy
Current ratingIAmperes (A)Maximum continuous current
Saturation currentIsatAmperes (A)Current where inductance drops significantly
DC resistance (DCR)RDCOhms (Ω)Resistance of the wire winding
Quality factorQ—Ratio of reactance to resistance
Self-resonant frequencySRFHertz (Hz)Frequency where parasitic capacitance resonates

Inductor Voltage Formula

V = L × (di/dt) (Voltage opposes change in current)

Where:

  • V= Induced voltage in Volts
  • L= Inductance in Henrys
  • di/dt= Rate of current change (A/s)

Inductance Units and Prefixes

The Henry is a fairly large unit. Practical inductors are measured in smaller subunits.

UnitSymbolValue in HenrysTypical Use
HenryH1Large power inductors, chokes
MillihenrymH10⁻³ HAudio, power supply, filter inductors
MicrohenryµH10⁻⁶ HRF circuits, DC-DC converters
NanohenrynH10⁻⁹ HHigh-frequency RF, PCB traces

Conversions: 1 H = 1000 mH = 1,000,000 µH = 1,000,000,000 nH

Inductors in Series and Parallel

Inductors combine like resistors: series adds directly, parallel uses reciprocals. (Opposite of capacitors!)

ConfigurationFormulaResultNotes
SeriesLtotal = L₁ + L₂ + L₃ + ...Sum of all valuesCurrent ratings: use lowest
Parallel (2 inductors)Ltotal = (L₁ × L₂) / (L₁ + L₂)Less than smallestCurrent handling increases
Parallel (n equal)Ltotal = L / nValue divided by countn × single current rating
Parallel (general)1/Ltotal = 1/L₁ + 1/L₂ + ...Use reciprocalsLike resistors in parallel

Important: These formulas assume no magnetic coupling between inductors. Closely placed inductors may have mutual inductance affecting total value.

Series/Parallel Formulas

Series: L_total = L1 + L2 + L3 + ... Parallel: 1/L_total = 1/L1 + 1/L2 + 1/L3 + ...

Where:

  • L1, L2, L3= Individual inductor values
  • L_total= Combined equivalent inductance

RL Time Constant

The RL time constant (τ) defines how quickly current builds up or decays in an inductor-resistor circuit. The formula differs from RC circuits.

Time Constants (τ)Current Rise (%)Current Decay (%)Practical Meaning
1τ63.2%36.8%First significant change
2τ86.5%13.5%Mostly complete
3τ95.0%5.0%Nearly complete
4τ98.2%1.8%Essentially complete
5τ99.3%0.7%Considered fully stabilized

RL Time Constant

τ = L / R I(t) = I₀ × (1 - e^(-t/τ)) [rising] I(t) = I₀ × e^(-t/τ) [decaying]

Where:

  • τ (tau)= Time constant in seconds
  • L= Inductance in Henrys
  • R= Resistance in Ohms
  • e= Euler's number (~2.718)

Energy Storage in Inductors

Inductors store energy in their magnetic field. The stored energy depends on inductance and current squared.

Inductor TypeTypical InductanceCurrent RangeApplication
Chip inductor (SMD)1 nH - 100 µH0.01-5 APortable electronics
Through-hole axial1 µH - 10 mH0.01-1 AGeneral filtering
Toroidal10 µH - 10 mH0.1-50 APower supply, EMI filtering
Power inductor1 µH - 1 mH1-100 ADC-DC converters
RF choke1 µH - 1 H0.001-10 ARF blocking, bias circuits

Inductor Energy Formula

E = ½ × L × I²

Where:

  • E= Energy in Joules
  • L= Inductance in Henrys
  • I= Current in Amperes

Inductive Reactance (AC Circuits)

In AC circuits, inductors have inductive reactance (XL)—an opposition to current that increases with frequency. Inductors pass DC but increasingly block AC at higher frequencies.

FrequencyReactanceCurrent FlowBehavior
DC (0 Hz)ZeroFull (limited by DCR)Short circuit (wire)
Low frequencyLowHighPasses low frequencies
High frequencyHighLowBlocks high frequencies
Very high frequencyVery highVery lowNearly open circuit

Inductive Reactance Formula

XL = 2πfL XL = ωL where ω = 2πf

Where:

  • XL= Inductive reactance in Ohms
  • f= Frequency in Hertz
  • L= Inductance in Henrys
  • ω= Angular frequency (rad/s)

Types of Inductors

Different inductor constructions suit different applications based on inductance, current, and frequency requirements.

TypeCore MaterialCharacteristicsBest For
Air coreNone (air)No saturation, low inductanceRF, high frequency
Ferrite coreFerriteHigh permeability, good at HFSwitching supplies, EMI filters
Iron powder coreIron powderHigh saturation, distributed gapPower inductors, DC-DC
Laminated ironIron laminationsVery high inductance, low freqTransformers, 50/60 Hz
ToroidalVariousLow EMI, efficientPower supply, audio
Shielded SMDFerriteLow EMI, compactMobile devices, sensitive circuits

Worked Examples

Calculate Inductive Reactance

Problem:

What is the reactance of a 10 mH inductor at 1 kHz?

Solution Steps:

  1. 1Identify values: L = 10 mH = 0.01 H, f = 1 kHz = 1000 Hz
  2. 2Use reactance formula: XL = 2πfL
  3. 3Substitute: XL = 2 × π × 1000 × 0.01
  4. 4Calculate: XL = 2 × 3.14159 × 10
  5. 5XL = 62.83 Ω

Result:

62.83 Ω inductive reactance at 1 kHz

Calculate RL Time Constant

Problem:

A 100 mH inductor is in series with a 50 Ω resistor. What is the time constant?

Solution Steps:

  1. 1Identify values: L = 100 mH = 0.1 H, R = 50 Ω
  2. 2RL time constant formula: τ = L / R
  3. 3Substitute: τ = 0.1 / 50
  4. 4Calculate: τ = 0.002 seconds = 2 milliseconds
  5. 5Current reaches 63% of final value in 2 ms

Result:

τ = 2 ms; current reaches ~99% of final value in 10 ms (5τ)

Calculate Energy Stored in Inductor

Problem:

How much energy is stored in a 47 µH inductor carrying 5 A?

Solution Steps:

  1. 1Energy formula: E = ½ × L × I²
  2. 2Convert: L = 47 µH = 47 × 10⁻⁶ H
  3. 3Substitute: E = ½ × 47 × 10⁻⁶ × 5²
  4. 4Calculate: E = 0.5 × 47 × 10⁻⁶ × 25
  5. 5E = 587.5 × 10⁻⁶ J = 587.5 µJ

Result:

587.5 microjoules (0.5875 mJ) stored energy

Tips & Best Practices

  • ✓Inductors combine like resistors: series adds, parallel uses reciprocals (opposite of capacitors).
  • ✓RL time constant τ = L/R, not L×R like the RC time constant.
  • ✓Inductive reactance XL = 2πfL increases with frequency (opposite of capacitive reactance).
  • ✓Always check saturation current (Isat) rating, not just continuous current rating.
  • ✓Use snubber circuits or flyback diodes to protect against inductive voltage spikes.
  • ✓Energy is proportional to current squared: E = ½LI²—doubling current quadruples energy.
  • ✓Inductors pass DC but block AC—they act as low-pass filters for current.

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Frequently Asked Questions

When in series, inductors add directly because the same changing current produces additive voltage drops (V = L×di/dt). In parallel, they share the total current change, reducing effective inductance. Capacitors are opposite because they relate to charge storage. The relationship depends on the fundamental physics: inductors respond to current changes, capacitors to voltage changes.
Two things can happen: (1) Overheating—the DC resistance causes I²R power loss, potentially melting insulation or solder. (2) Saturation—the magnetic core can't hold more flux, causing inductance to drop dramatically, which may cause circuit malfunction. Always stay below both the continuous current rating and the saturation current rating.
An inductor resists changes in current. When current is suddenly interrupted (like opening a switch), the inductor tries to maintain current flow, creating a large voltage spike (V = L×di/dt with very high di/dt). This can damage components. Protection methods include flyback diodes, snubber circuits, or TVS diodes to safely dissipate the energy.
Saturation occurs when the magnetic core can no longer increase its magnetization. Above the saturation current (Isat), inductance drops significantly—often to 10-30% of nominal value. In switching power supplies, this causes increased ripple, reduced efficiency, and potential damage. Always design with adequate margin below Isat.
Air cores: no saturation, good for RF. Ferrite: high permeability, excellent at high frequencies, used in switching supplies. Iron powder: handles high DC current better, used in power inductors. Laminated iron: highest inductance, but only for low frequencies (50/60 Hz). Match the core material to your application's frequency and current requirements.
DC Resistance (DCR) is the resistance of the inductor's wire winding. It causes power loss (P = I²×DCR) and voltage drop. For power applications, lower DCR means higher efficiency but often requires thicker wire (larger/more expensive inductor). Balance DCR against size, cost, and acceptable losses for your application.

Sources & References

Last updated: 2026-01-22