What this air coil calculator does
This calculator estimates the inductance of a single-layer air-core coil (often called an air coil) using the classic Wheeler formula. It is useful for RF projects, antenna matching networks, crystal radios, filters, and many hobby electronics experiments where magnetic cores are not used.
Enter your turns, diameter, and winding length to get a quick estimate in microhenries (µH). You’ll also get a few practical values such as turn pitch and approximate wire length.
Formula used
For a single-layer solenoid, the calculator uses:
L (µH) = (r² × N²) / (9r + 10l)
- L = inductance in microhenries (µH)
- r = coil radius in inches
- l = coil length in inches
- N = number of turns
Inputs can be entered in millimeters or inches; the calculator converts automatically.
How to use it
1) Choose your unit system
Pick mm or inches first, then keep all geometric values in that same unit.
2) Enter coil geometry
- Turns: total number of wire turns.
- Diameter: outside/mean diameter approximation for the winding form.
- Length: axial winding length from first to last turn.
3) (Optional) Enter a target inductance
If you add a target µH value, the tool estimates the turns required for that specific coil diameter and length.
Design tips for better results
- Keep a consistent winding pitch; random spacing can shift measured inductance.
- Use non-metallic coil forms (plastic, cardboard, PTFE) when possible.
- Nearby metal parts can lower Q and alter effective inductance.
- At high frequency, parasitic capacitance creates self-resonance; account for it in final tuning.
- Always verify with an LCR meter or VNA when precision matters.
Limitations
This is an engineering estimate, not a full electromagnetic field solver. It assumes a single-layer, air-core, reasonably uniform coil. Multi-layer coils, unusual geometries, ferrite cores, and very high-frequency behavior require more advanced models and measurement.
Quick example
Suppose you build a 20-turn coil, 30 mm diameter, and 25 mm winding length. Plugging those values in gives an inductance in the low microhenry range—perfect for many HF and VHF tuning experiments.