Toroid Inductance Calculator

About this calculator

A toroidal inductor — wire wound N times around a donut-shaped core — has inductance given by L = (μ·N²·h) / (2π) · ln(b/a), where μ is the core material's permeability, h is the core's height, and a and b are its inner and outer radii. The logarithmic term captures how the magnetic field strength falls off with distance from the center of the ring.

Toroid cores confine their magnetic field almost entirely within the donut shape, which means far less stray flux and electromagnetic interference than an equivalent solenoid — the reason toroidal ferrite and powdered-iron cores are the standard choice for switch-mode power supply chokes, EMI/RFI filters, and compact transformers in consumer electronics and audio equipment.