What is solenoid inductance?
A solenoid is a coil of wire, often wound around an air or magnetic core, that produces a fairly uniform magnetic field inside itself when current flows through it. Its inductance describes how strongly it resists changes in current, and depends on its geometry and the number of turns.
The formula
For a long, tightly-wound solenoid, inductance is approximated by L = μ₀μr N² A / l, where μ₀ is the permeability of free space (4π×10-7 H/m), μr is the relative permeability of the core material (1 for air), N is the number of turns, A is the cross-sectional area, and l is the coil length.
Enter the number of turns, coil radius, length, and core permeability to estimate inductance. This formula assumes an ideal long solenoid (length much greater than radius); short or tightly curled coils will have somewhat different real-world inductance. For general electronics education and reference.