The coil inductance calculator estimates the inductance of a single-layer air-core coil using Wheeler's formula. Enter the coil diameter, length, and number of turns to calculate inductance in µH, mH, or H.
Coil Inductance Calculator (Wheeler's Formula)
How to Wind an Air-Core Inductor
Air-core inductors are wound on non-magnetic forms: PVC pipe, plastic rod, or foam. Tighter winding (more turns per inch) increases inductance. Heavier wire (lower AWG) reduces DC resistance for power applications; thinner wire is acceptable for RF coils carrying small currents.
RF Coil Design Tips
For RF tank circuits, the quality factor (Q) is critical. Higher Q means sharper resonance and lower loss. Air-core coils have high Q (100-300+) at RF frequencies. Tinned copper wire wound on ceramic forms is ideal for transmitter tank coils. Space winding (gaps between turns) can improve Q by reducing distributed capacitance.
Frequently Asked Questions
How do I calculate coil inductance?
For a single-layer air-core coil, Wheeler's formula gives L = (r² × N²) / (9r + 10l) in µH, where r is the coil radius in inches, N is the number of turns, and l is the coil length in inches. This formula is accurate to ~1% for coils where length > 0.4 × radius.
What affects coil inductance?
Inductance increases with: more turns (N² relationship — doubling turns quadruples inductance), larger coil diameter, and shorter coil length (more tightly wound). Using a ferrite or iron core instead of air dramatically increases inductance — by 100-10,000× depending on core permeability.
What is the Wheeler formula for inductance?
Wheeler's formula is the standard approximation for single-layer cylindrical coils: L(µH) = (r² × N²) / (9r + 10l), where r = radius in inches and l = length in inches. For metric, convert first: inches = cm / 2.54.
What applications use air-core coils?
Air-core inductors are used in RF circuits (tank circuits, filters, antennas), where ferrite cores would saturate at high frequencies. They're also used in crossover networks for high-end audio speakers where ferrite core distortion is audible. Air cores have no saturation point and very low loss at RF frequencies.
Is this inductance calculator free?
Yes, completely free with no signup required.