🌀 Helical Coil Calculator
Estimate the inductance of an air-core helical coil (solenoid) from its radius, length and number of turns.
cm
cm
What this calculator does
A helical coil, also called a solenoid, is a wire wound in a helix shape and is one of the most common ways to build an air-core inductor for radio and electronics projects.
The formula used
This calculator uses Wheeler’s well-known approximation for a single-layer air-core coil: L (microhenries) = r²N² / (9r + 10l), where r is the coil radius and l is the coil length, both in inches (the tool converts your centimeter inputs automatically). N is the number of turns.
- This formula is most accurate when the coil length is at least about 0.4 times its diameter.
- Wire diameter, spacing, and nearby metal objects can shift the real inductance somewhat from this estimate.
This tool is for general electronics planning and education.
Last reviewed August 2026