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Solenoid Inductance Calculator

Calculate the inductance of a solenoid coil from number of turns, core area, length, and relative permeability, in henries. Free and instant.

Frequently Asked Questions

Why does inductance scale with the square of the turns?

Each turn both creates flux and links the flux of every other turn, so doubling the turns roughly quadruples the inductance.

When does the long-solenoid formula break down?

It assumes the coil is much longer than its diameter. For short, fat coils the field is less uniform and the true inductance is lower.

Why does my measured inductance differ from the calculated value?

Several factors reduce the accuracy of the simple formula: end effects at the ends of the coil, non-negligible wire diameter, distributed winding capacitance, and, if a magnetic core is present, the variation of permeability with frequency and flux density. For precision coils, measure with an LCR meter at the actual operating frequency.

What limits the maximum current of a solenoid?

There are two independent limits: core saturation, above which the inductance collapses, and the thermal limit of the copper, when I²R heating exceeds the maximum insulation temperature. Good design makes both limits coincide at roughly the same current.

What is the self-resonant frequency (SRF)?

It is the frequency at which the coil's inductance resonates with its distributed parasitic capacitance. Above the SRF the component behaves as a capacitor rather than an inductor. Multi-layer windings have higher parasitic capacitance and therefore a lower SRF than single-layer ones.

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Estimates for informational purposes only.

Important Disclaimer: Estimates for informational purposes only.

This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.

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How This Calculator Works

A solenoid is a long coil of wire that stores energy in a magnetic field when current flows through it. Its inductance rises with the square of the number of turns, with the cross sectional area of the core, and with the permeability of the core material. A longer coil for the same turn count lowers the inductance because the field spreads out over a greater length.

Formula and Method

For an ideal long solenoid, the inductance is:

L = μ₀ × μ_r × N² × A / l

where μ₀ = 1.257 × 10^-6 H/m is the permeability of free space, μ_r is the relative permeability of the core, N is the number of turns, A is the cross sectional area of the core in square meters, and l is the coil length in meters. This calculator accepts the area in cm² and the length in cm, converts them internally to SI units, and returns the result in both microhenries (µH) and millihenries (mH).

The Core Material

The relative permeability term describes how strongly the core material concentrates magnetic flux. Air and most non magnetic cores are close to one, while iron and ferrite cores can multiply the inductance by hundreds or thousands. This calculator assumes a long thin solenoid where the field inside is nearly uniform.

Worked Example

Take a coil of 100 turns, core area 3 cm², length 10 cm, μ_r = 1:

L = 1.257 × 10^-6 × 1 × 100² × 3×10^-4 ÷ 0.10 ≈ 37.7 µH

If a ferrite core with μ_r = 100 is inserted, the inductance rises to about 3,770 µH (3.77 mH), though saturation at high current must be watched.

Tips

Doubling the turns roughly quadruples the inductance because of the squared dependence.
The formula is most accurate when the coil length is much larger than its diameter.
Ferrite and iron cores raise inductance but can saturate at high current.

Solenoid Applications Beyond Inductance

The word "solenoid" describes any coil of wire wound in a helix, but in engineering it most often refers to an electromagnetic actuator: a coil that generates a controlled magnetic force to move a ferromagnetic plunger. Linear solenoid actuators appear everywhere in everyday life: door locks in cars and hotel rooms, valve actuators in pneumatic and hydraulic systems, fuel injectors in internal combustion engines, and relay armatures in control panels. The force produced by the solenoid is proportional to the square of the current and to the geometry of the air gap between the coil and the plunger. Response speed is governed by the RL time constant of the coil: a shorter time constant (lower L/R ratio) produces faster pull-in and release.

Solenoid Valve Operation

In a solenoid valve, current through the coil creates a magnetic field that pulls a ferromagnetic plunger against a spring, opening or closing a fluid flow passage. The pull-in force must overcome both the spring force and the fluid pressure acting on the sealing surface. When the coil is de-energized, the spring returns the plunger to its rest position. Response time for fast-switching solenoid valves (such as fuel injectors and pneumatic valves) is dominated by the L/R time constant - designers use low-inductance coils and high-voltage drive circuits with current limiting to achieve sub-millisecond response. Pulse-width modulation of the drive voltage allows proportional flow control without the need for a proportional valve mechanism.

Voice Coil Actuators

A voice coil actuator consists of a cylindrical coil of wire suspended in a radial magnetic field produced by a permanent magnet. Current through the coil produces a force proportional to B × I × L_wire, where B is the flux density in the gap, I is the current, and L_wire is the total length of conductor in the field. The force is linear with current and bidirectional, making voice coil actuators ideal for precision positioning: loudspeaker cones, hard disk drive read/write head actuators, autofocus lens mechanisms in cameras, and high-speed linear stages in semiconductor manufacturing equipment all use this principle.

Core Materials and Their Effect on Inductance

The core material has a dramatic effect on inductance and on the practical behavior of the solenoid. An air core solenoid has relative permeability of 1, so the inductance depends purely on geometry. Air core coils never saturate, produce predictable inductance that does not change with current, and are preferred at radio frequencies where core losses would be unacceptable. The trade-off is poor inductance density: achieving significant inductance requires many turns or large dimensions. The Nagaoka factor (a correction for finite coil length) reduces the ideal infinite-solenoid inductance by a factor that depends on the length-to-diameter ratio; this calculator uses the infinite approximation, which is accurate when length greatly exceeds diameter.

Ferromagnetic cores - iron, silicon steel, ferrite, powdered iron, permalloy - multiply inductance by their relative permeability μr. Silicon steel has μr of 1,000 to 10,000, powdered iron around 50 to 100, and ferrites used in signal transformers around 1,000 to 15,000. The high μr compresses many turns' worth of flux linkage into a compact volume, enabling practical inductors and transformers at power frequencies. The fundamental limitation is saturation.

Magnetic Saturation

Every ferromagnetic core has a saturation flux density B_sat beyond which increasing the magnetomotive force (H = N × I / length) produces no further increase in flux density. At saturation, the effective permeability drops from μr to approximately 1 (air-like), and inductance collapses. For a solenoid actuator, saturation limits the maximum force that can be generated. For an inductor in a switching power supply, saturation causes a dangerous spike in current. The onset of saturation can be managed by including a deliberate air gap in a ferromagnetic core; the gap dominates the reluctance of the magnetic circuit and linearizes the B-H characteristic, at the cost of reducing the inductance per turn. Powdered iron cores are made by sintering iron particles in a non-magnetic binder, creating a distributed internal air gap that produces a soft, gradual saturation characteristic.

Temperature Coefficient

The permeability of ferrite cores changes with temperature: most ferrites decrease in μr above the Curie temperature (200°C to 300°C for common power ferrites) but also exhibit variations over the normal operating temperature range. For oscillators and tuned circuits, the resulting shift in inductance changes the resonant frequency. Temperature-stable core materials (such as NiZn ferrite grades optimized for oscillator use) minimize this drift. For power inductors in switching supplies, the temperature effect on inductance is usually a secondary concern compared with the saturation current limit.

Designing Solenoids for Specific Applications

Designing a solenoid for a specific force or inductance requires balancing several competing constraints. More turns increase inductance as N² but also increase winding resistance (longer wire) and distributed capacitance (more inter-winding capacitance). A larger core area increases inductance linearly but also increases the solenoid's physical size and weight. A shorter coil length increases inductance but reduces the accuracy of the infinite solenoid approximation and may increase fringing effects at the ends.

For electromagnetic actuators, the axial force on the plunger near the coil end can be approximated as F = μ0 × A × (N × I)² / (2 × l²) for a simple geometry, though accurate force prediction for a real geometry requires finite element analysis (FEA) because the force depends critically on the air gap shape, the plunger geometry, and the flux distribution. The winding resistance (DCR) causes I²R heat dissipation that limits the maximum continuous current. The copper fill factor (the fraction of the winding cross-section occupied by conductor rather than insulation and air) determines how efficiently the available window area is used; typical values are 0.3 to 0.7 depending on wire gauge, winding technique, and insulation thickness.

Multi-Layer Winding

Adding multiple winding layers increases the turn count N efficiently in a compact package, but at a cost: the distributed capacitance between adjacent turns and between layers rises with N, reducing the self-resonant frequency (SRF). Above the SRF the component behaves as a capacitor rather than an inductor, which limits usable inductance to well below the SRF. For RF coils, single-layer (air-core, spaced-winding) construction minimizes distributed capacitance and maximizes SRF and Q factor. For power inductors, multi-layer windings are standard since the operating frequency is far below the SRF. A Rogowski coil is a special toroidal air-core coil wound uniformly around a flexible former that can be wrapped around a conductor; it measures the rate of change of current (di/dt) without any core saturation and without interrupting the circuit. Integration of the output voltage gives the current waveform.

Shielding

An unshielded solenoid radiates a magnetic field that can couple into nearby circuits, causing interference. For power inductors on PCBs, a magnetically shielded package uses a ferrite cup core that encloses the winding, redirecting nearly all flux through the core and dramatically reducing the external field. Mu-metal and permalloy (nickel-iron alloys with μr above 100,000) are used for magnetic shields in sensitive instruments, electron microscopes, and CRT monitors to exclude external fields. Note the distinction between electromagnetic shielding (a conductive enclosure that blocks electric fields and attenuates high-frequency magnetic fields via eddy currents) and magnetic shielding (a high-permeability enclosure that diverts static or low-frequency magnetic flux around the protected region).