Formula Used
This calculator uses Wheeler’s multilayer air core coil approximation:
L = 0.8r²N² / (6r + 9l + 10b).
Here, L is inductance in microhenries. The mean coil radius r, coil length l,
and winding depth b are measured in inches. N is the total number of turns.
Wire resistance is estimated with R = ρL / A. Reactance is
calculated with XL = 2πfL. Quality factor is estimated
with Q = XL / RAC.
How to Use This Calculator
Enter total turns, coil diameter, coil length, layer count, and conductor size.
Add insulation, spacing, lead length, frequency, current, and temperature.
Use copper resistivity by default, or enter another material value.
Press the calculate button. The result appears above the form and below the header.
Use the CSV or PDF button to save the result.
Example Data Table
| Example |
Turns |
Inner Diameter |
Length |
Layers |
Wire Diameter |
Frequency |
| RF test coil |
80 |
20 mm |
30 mm |
4 |
0.50 mm |
500 kHz |
| Filter choke |
180 |
28 mm |
40 mm |
8 |
0.45 mm |
100 kHz |
| Lab prototype |
120 |
25 mm |
35 mm |
6 |
0.65 mm |
100 kHz |
Multilayer Air Core Inductor Design Guide
A multilayer air core inductor stores energy in a magnetic field. It uses no ferrite or iron core. That makes saturation almost impossible. It also reduces core loss. The tradeoff is size. More turns are usually needed for higher inductance.
Why Geometry Matters
Coil shape controls most of the result. The mean radius affects flux linkage. A larger radius usually raises inductance. Coil length also matters. A long winding spreads the field and lowers coupling between turns. Winding depth matters in multilayer coils. Deep windings add turns farther from the center. Wheeler’s formula balances these dimensions with useful accuracy for practical design.
Wire and Resistance
Wire choice changes heating and quality factor. Thick wire has lower resistance. Parallel conductors can also reduce resistance. Temperature raises copper resistance, so warm coils lose more power. At high frequency, skin effect pushes current toward the wire surface. This increases effective resistance. The calculator gives a simple AC resistance estimate. Use it for comparison, not final certification.
Frequency Behavior
Inductive reactance rises with frequency. A coil that looks small at low frequency can become important at radio frequency. Quality factor compares reactance with resistance. A higher value usually means lower loss. Parasitic capacitance creates self resonance. Above that point, the coil no longer behaves like a simple inductor.
Practical Use
Start with a target inductance. Adjust turns, length, and diameter. Check wire length and resistance. Review current density before building. Keep turns neat. Avoid unnecessary lead length. Measure the finished coil when precision matters. Real coils vary because insulation, spacing, winding tension, and nearby metal change the field.
FAQs
What is a multilayer air core inductor?
It is a coil wound in several layers without a magnetic core. The magnetic field forms mainly in air, so the part avoids core saturation and core material losses.
Which formula does this calculator use?
It uses Wheeler’s multilayer air core approximation. The formula uses mean radius, coil length, winding depth, and total turns to estimate inductance.
Are the results exact?
No. They are engineering estimates. Real results can change because of winding spacing, lead placement, insulation thickness, nearby metal, and measurement frequency.
Why does wire diameter matter?
Wire diameter affects winding depth, wire length, resistance, current density, and high frequency behavior. Larger wire usually lowers resistance and heating.
What is winding depth?
Winding depth is the radial thickness of all layers. In this calculator, it is estimated from layer count, conductor diameter, and insulation build.
What is quality factor?
Quality factor compares inductive reactance with effective resistance. A higher value usually means the coil has lower relative loss at the selected frequency.
Why enter parasitic capacitance?
Parasitic capacitance helps estimate self resonant frequency. If you do not know it, enter a small estimate or leave it as zero.
Can I use aluminum or silver wire?
Yes. Enter the correct resistivity and temperature coefficient. The inductance mainly depends on geometry, but resistance depends strongly on conductor material.