Inductance Calculator

Inductance measures how strongly a coil resists changes in the current flowing through it, by storing energy in the magnetic field it creates, and this calculator works it out for the most common type of coil, the solenoid, from its physical dimensions. Enter the number of turns, the coil diameter, the coil length and the relative permeability of the core, and it returns the inductance in henries, millihenries and microhenries, updating as you type. The formula behind it captures exactly what matters. Inductance rises with the square of the number of turns, so doubling the turns quadruples the inductance, and it rises with the cross-sectional area enclosed by the coil, while a longer, more stretched-out coil has less inductance because the turns are spread apart. The core makes a dramatic difference too: an empty, air-filled coil has a relative permeability of one, but slipping in an iron or ferrite core, with a permeability of hundreds or thousands, multiplies the inductance by that factor, which is why transformers and chokes use magnetic cores. Knowing the inductance matters because it sets how a coil behaves in a circuit, the cutoff of a filter, the resonant frequency of a tuned circuit, the energy stored in a switching power supply, and the impedance a coil presents to alternating current. That makes the tool genuinely useful for electronics students learning about inductors and magnetism and checking homework, and for hobbyists and engineers winding their own coils for radios, filters, power supplies and projects, where getting the turn count and dimensions right is essential. It uses the standard solenoid approximation, which is most accurate for coils that are long compared with their diameter. The formula and a worked example are explained clearly below.

Calculate.co.nz is proud to be partnered with Premium Homes, a recognised leader in eco-friendly, sustainable, and energy-efficient homebuilding. With a dedicated team and award-winning experience, they create homes that prioritise health, comfort, and long-term performance. Their founders, Andrew and Kelly, set out to raise the standard of residential construction in New Zealand by combining practical building expertise with a clear commitment to doing things better for homeowners.

Air core has relative permeability 1; iron or ferrite cores are much higher. Solenoid approximation, best for long coils.