Air-Core Coil Inductance Calculator

Wheeler's single-layer air-core approximation: L[µH] = d²·N² / (18d + 40l) (d, l in inches; converted from mm internally).

Usage: Enter coil diameter, winding length, and turns. For tightly wound coils keep length > 0.4·diameter for best accuracy.

When you need it: Estimating a single-layer air-core coil's inductance from its turns and dimensions for an RF tank, matching network or filter.

Worked example: Wheeler's formula L(µH) ≈ (d²N²)/(18d + 40l) with d and l in inches; a 0.5″ diameter, 20-turn, 1″-long coil comes out to about 1.4 µH.

Tips & gotchas:

• Air-core avoids core saturation and loss, but needs more turns than a ferrite-cored coil for the same L.
• Close-wound versus spaced windings changes the inductance — spacing lowers it.
• Distributed capacitance gives a self-resonant frequency; above it the coil looks capacitive.
• For RF-critical values, confirm with an LCR meter or VNA — formula error runs several percent.