Air-Core Inductor Coil Calculator
Use this tool for single-layer solenoid coils using Wheeler's approximation. Enter dimensions, choose a mode, and calculate either inductance or required turns.
What this inductor coil calculator does
This calculator helps you quickly estimate the inductance of a single-layer air-core inductor, or work backward and estimate the number of turns needed for a target inductance. It is especially useful for RF hobby projects, filters, impedance matching networks, and prototyping analog circuits where coil geometry matters.
Instead of hand-solving equations every time you change a diameter or turn count, you can adjust a few numbers and instantly compare design options. The tool also gives practical checks like turn pitch and approximate winding fill if wire diameter is entered.
Formula used (Wheeler approximation)
This page uses the classic Wheeler equation for a single-layer air-core solenoid:
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
When you select “Calculate Required Turns,” the formula is rearranged to solve for N from a target inductance.
How to use it
1) Pick a mode
Choose whether you want to calculate inductance from geometry, or estimate turns from a target inductance.
2) Enter physical dimensions
Enter diameter and coil length. You can work in mm, cm, or inches. The calculator handles unit conversion internally.
3) Provide turns or target inductance
Depending on mode, enter either N (turns) or desired µH.
4) Optional: add wire diameter
If supplied, you get a winding fill estimate to check whether your turn count is physically realistic for the available coil length.
Worked example
Suppose you have a 30 mm diameter form, a 20 mm winding length, and 18 turns:
- Set unit to mm
- Enter D = 30
- Enter l = 20
- Enter N = 18
The calculator returns an estimated inductance in µH. If that value is too high, reduce turns or increase coil length. If too low, add turns or increase diameter.
Design tips for better real-world coils
- Keep winding consistent: uneven pitch changes inductance and Q.
- Mind parasitics: at high frequency, self-capacitance and lead layout matter a lot.
- Use stable forms: temperature and mechanical movement can detune sensitive circuits.
- Leave tuning margin: build slightly high/low and trim with spacing or small parallel capacitors.
- Validate with measurement: use an LCR meter or VNA when final accuracy is important.
Limitations of this model
This calculator is intentionally simple and fast. It does not include core materials (ferrite/powdered iron), multilayer winding effects, skin/proximity loss, temperature coefficients, or frequency-dependent behavior. For precision magnetics design, use detailed electromagnetic models and measured data.
FAQ
Can I use this for ferrite-core inductors?
Not directly. Ferrite cores change permeability dramatically. For core-based inductors, you typically use AL values or manufacturer core equations.
Why is my measured value different?
Common causes include spacing variation, lead length, nearby metal objects, test frequency, and meter calibration. Small mechanical differences can produce noticeable shifts.
Is this suitable for audio-frequency inductors?
It can provide rough air-core estimates, but many audio inductors use cores and specific winding strategies. Treat this as a first-pass design aid.
What if I need an exact turn count?
Use this tool for a starting number, then prototype and measure. In practical coil building, one or two turns of adjustment is normal.