Planar Inductor Calculator
Estimate PCB spiral inductor inductance for square, hexagonal, octagonal, and circular layouts using Mohan's equation.
Inductance–
Inner diameter–
Fill ratio (ρ)–
Trace length / resistance–
About planar inductor estimates
L = µ₀ · n² · d_avg · c₁ / 2 · (ln(c₂/ρ) + c₃·ρ + c₄·ρ²)
ρ = (D_out − D_in) / (D_out + D_in)
Mohan's current-sheet expression estimates the inductance of a PCB spiral from turns, outer size, trace width, and spacing, with layout coefficients for square, hexagonal, octagonal, and circular shapes. The inner diameter must stay positive — the outer size has to fit all the turns and gaps.
Spiral formulas are approximate and ignore nearby copper, magnetic material, and high-frequency loss. Prototype and measure when inductance tolerance matters.