Air Core Coil Inductance Calculator
Estimate inductance for a single-layer air core solenoid using Wheeler’s formula.
What is an air core inductance calculator?
An air core inductance calculator helps you estimate the inductance of a coil that does not use a ferromagnetic core. Instead of iron or ferrite, the coil is wound around air (or a non-magnetic former), which gives excellent linearity, low hysteresis, and good high-frequency behavior.
Air core coils are common in RF circuits, antenna matching networks, filters, oscillators, and precision analog designs where core losses or saturation would otherwise create problems.
Formula used in this calculator
This page uses the classic Wheeler single-layer air core formula:
Where:
- L is inductance in microhenries (µH)
- r is coil radius in inches
- l is coil length in inches
- N is number of turns
The calculator automatically converts mm or cm inputs into inches before evaluating the formula.
How to use the calculator
1) Enter geometry
Input the total number of turns, the coil diameter, and the winding length. Make sure diameter and length use the same unit.
2) Choose unit
Select mm, cm, or inches. The math is handled internally, so you can work in whichever unit is most convenient.
3) Optional reactance at frequency
If you add a frequency, the tool also computes inductive reactance:
This is useful for RF and filter work because reactance tells you how strongly the inductor opposes AC at a given frequency.
Design notes and practical tips
- Keep lead lengths short when building RF coils; parasitic inductance and capacitance matter.
- Turn spacing can affect effective inductance and Q-factor.
- Nearby metal parts can reduce Q and shift tuning.
- For high-current applications, consider wire gauge and temperature rise, not just inductance.
- For high-frequency circuits, verify with an LCR meter or VNA after winding.
Example
Suppose you wind a 20-turn coil with 30 mm diameter and 18 mm winding length. The calculator converts dimensions to inches and applies Wheeler’s equation. You get an estimated inductance in the low microhenry range, which is typical for compact RF inductors.
Limitations
This estimate assumes a single-layer, round-wire, air core geometry. Real-world values can differ due to insulation thickness, spacing variation, nearby materials, and measurement frequency. Treat the output as a strong starting point, then confirm with instrumentation.