Helical Coil Calculator

Estimate the inductance of an air-core helical coil (Wheeler's formula).

Coil diameter

Coil length

Number of turns

Inductance (µH) 9.085

Formula: L(µH) = d²·n² ÷ (18d + 40l) (inches)

Step-by-step with your numbers:

1. Values used:

2. Coil diameter = 20

3. Coil length = 30

4. Number of turns = 30

6. Inductance = 9.085µH

Did we solve your problem today?

Wheeler's formula estimates the inductance of a single-layer air-core coil.

The math behind it

L(µH) = d²·n² ÷ (18d + 40l) with dimensions in inches. Accurate when length exceeds about 0.4× the diameter.

20 mm dia, 30 mm long, 30 turns → about 4.5 µH.

How to increase inductance?

Add turns, increase diameter, or add a magnetic core.