Formula Used
The calculator uses the multilayer Wheeler estimate for a short air core coil:
L(µH) = 0.8 × r² × N² / (6r + 9l + 10d)
Here, r is mean radius in inches,
N is total turns,
l is coil length in inches,
and d is winding depth in inches.
Corrected inductance equals Wheeler inductance multiplied by effective permeability and the winding correction factor.
Reactance is calculated as XL = 2πfL.
Approximate Q is calculated as Q = XL / R.
How to Use This Calculator
Enter the bobbin inner diameter, insulated wire diameter, conductor diameter, layers, and turns per layer.
Set pitch equal to wire outside diameter for tight winding. Increase pitch for spaced turns.
Leave manual coil length as 0 when you want the tool to estimate length from pitch.
Use effective permeability of 1 for an air core coil. Add a known correction factor only when you have measured data.
Press Calculate. The result appears above the form and below the page header.
Example Data Table
| Inner Diameter |
Wire OD |
Layers |
Turns Per Layer |
Pitch |
Use Case |
| 25 mm |
0.80 mm |
5 |
40 |
0.82 mm |
General RF coil estimate |
| 18 mm |
0.50 mm |
8 |
55 |
0.52 mm |
Compact multilayer winding |
| 40 mm |
1.00 mm |
4 |
35 |
1.05 mm |
Lower resistance design |
Understanding Multilayer Coil Inductance
A multilayer coil stores energy in its magnetic field. Its inductance depends on turn count, coil radius, winding length, and radial winding depth. More turns raise inductance strongly, because turns are squared in the Wheeler estimate. A larger mean radius also raises inductance. A long winding or deep winding spreads the field and lowers the final value.
Why Geometry Matters
The calculator uses inner diameter, wire outside diameter, layers, turns per layer, and pitch to build the winding shape. It finds total turns, outer diameter, mean radius, coil length, and winding depth. These values are then converted to inches for the standard multilayer Wheeler equation. The result is useful for air core coils and first pass design work.
Advanced Design Checks
Inductance alone is not enough for many electrical designs. The tool also estimates wire length, copper area, resistance, reactance, Q factor, voltage drop, and copper loss. Frequency is used to find inductive reactance. Current is used to estimate heating loss. Optional stray capacitance gives a rough self resonant frequency. A tolerance field shows lower and upper inductance limits.
Practical Accuracy Notes
Real coils can differ from the estimate. Winding tightness, insulation thickness, nearby metal, core material, and lead length can all change measured inductance. For magnetic cores, enter an effective permeability or correction factor only when you know it. A high core value does not guarantee accuracy, because cores can saturate and their permeability changes with frequency.
Using Results Wisely
Use this calculator during planning, comparison, and documentation. Try several wire sizes and layer counts before building the coil. Keep winding depth reasonable when you need better repeatability. Compare the estimated Q and resistance against your circuit needs. After winding, measure the coil with an LCR meter. Then adjust turns or spacing if the measured value is outside your target range. Export the CSV or PDF report when you need a record for testing, quotes, or project notes. These saved results help you repeat successful windings later.
Common Build Tips
Wind layers evenly and keep the bobbin stable. Mark the start lead before winding. Avoid sharp bends near the terminals. Measure insulation diameter with calipers. Small mechanical errors can cause noticeable electrical changes during assembly.
FAQs
What formula does this calculator use?
It uses the multilayer Wheeler equation. The formula estimates inductance from mean radius, total turns, coil length, and winding depth. It is best for practical air core coil planning.
Can I use it for ferrite core coils?
Yes, but only as an estimate. Enter a known effective permeability or correction factor. Ferrite behavior changes with material, air gap, current, and frequency.
What is winding depth?
Winding depth is the radial thickness of all wire layers. This calculator estimates it by multiplying insulated wire diameter by the number of layers.
Why does turn count affect inductance strongly?
Inductance rises with the square of total turns. Doubling turns can raise inductance about four times, when geometry remains similar.
What pitch value should I enter?
Use the insulated wire diameter for close winding. Use a larger pitch when turns are spaced. Pitch changes coil length and wire length.
Is the Q factor exact?
No. It is an approximate value based on inductive reactance and DC resistance. Real Q also depends on skin effect, proximity effect, core loss, and capacitance.
Why add stray capacitance?
Stray capacitance helps estimate self resonant frequency. This is useful for RF work, where coils stop behaving like simple inductors near resonance.
Should I measure the finished coil?
Yes. Construction details can change inductance. Measure with an LCR meter, then adjust turns, spacing, or core position to reach the target value.