Wireless Charging
Coil Calculator and Design
Plan receiver and transmitter windings, compare ferrite thickness, and estimate coil dimensions, copper weight and DC resistance.
Inductance estimates show their calculation basis. Assembly Q uses measured Ls and Rs.
How to use the wireless charging coil calculator
Start with the space available in your assembly, then add the winding and ferrite details.
- 01
Choose the coil and wire.
Select Receiver coil or Transmitter coil, choose round or rectangular, and enter the clear inner opening, turns and conductor dimensions.
- 02
Build the complete stack.
Add ferrite thickness, adhesive and film. Include the lead crossing when it sits over the winding, then check the local height against your limit.
- 03
Compare and document.
Review dimensions and estimates, compare ferrite thicknesses, and download the PDF. Enter measured assembly Ls and Rs to calculate Q.
Receiver coil vs. transmitter coil design
These editable presets reflect two winding approaches. Select the construction required by your device and controller.
| Design input | Receiver coil · Rx | Transmitter coil · Tx |
|---|---|---|
| Conductor | One or two enamelled copper wires | Litz bundle of individually insulated strands |
| Wire dimensions | Bare copper diameter and insulated wire diameter | Strand count, bare strand diameter and finished bundle diameter |
| Ferrite starting point | Flexible ferrite; adjustable magnetic-layer thickness | Rigid soft ferrite; 1.0 mm starting thickness, adjustable |
| Key checks | Opening, footprint, lead crossing and thin assembly height | Bundle fit, copper resistance, ferrite data and measured assembly Q |
| Related product information | Standard receiver coils ↗ | Standard transmitter coils ↗ |
Preset dimensions are design examples, not orderable specifications. Two parallel enamelled wires are modeled separately from a transposed Litz bundle.
Worked example: a thin round receiver coil
An 18-turn receiver winding with two parallel enamelled wires, a 12 mm inner opening and a 0.20 mm ferrite layer gives an estimated 28.56 mm outer diameter. Its uniform stack is 0.53 mm; the lead crossing increases the local maximum to 0.76 mm.
View the example inputs and calculation steps
- Winding
- Round receiver; 18 electrical turns per path; 1 axial layer
- Inner diameter
- 12 mm
- Conductor arrangement
- 2 enamelled copper wires in parallel, side by side
- Wire diameters
- 0.20 mm bare copper / 0.23 mm insulated
- Gap between adjacent turn pairs
- 0 mm
- Ferrite inputs
- JF95 preset; 0.20 mm magnetic layer; 34 × 34 mm sheet
- Real relative permeability input
- 870 at 100 kHz
- Adhesive / top film
- 0.05 mm / 0.05 mm
- Lead crossing
- Enabled; 0.23 mm local allowance
- Lead-outs
- 2 × 60 mm per path; 240 mm total across both paths
- Operating inputs
- 100 kHz; 1 A RMS coil current; 20°C copper
- Maximum local thickness limit
- 1.00 mm
Outer size
Radial build = 18 × (2 × 0.23) + 17 × 0 = 8.28 mm. Outer diameter = 12 + 2 × 8.28 = 28.56 mm. Zero gap means adjacent insulated wires touch; enamel thickness is still included.
Complete thickness
Uniform stack = 0.23 winding + 0.20 ferrite + 0.05 adhesive + 0.05 film = 0.53 mm. Local maximum = 0.53 + 0.23 lead crossing = 0.76 mm. Two crossing leads sit side by side, so this allowance is not doubled.
Copper weight
The summed copper length is 2.533614 m, including 0.24 m of leads. With a 0.20 mm bare diameter and copper density of 8.96 g/cm³, mass = 8.96 × [π × 0.20² / 4] × 2.533614 ≈ 0.713 g. The area is in mm² and length in metres; summed length already includes both wires.
The 13.982 µH value is an ideal circular-coil ferrite-sheet estimate, not a measurement of a 34 × 34 mm assembly. Sheet size provides a 2.72 mm geometric edge margin but does not enter a finite-edge magnetic solution. The copper-loss reference excludes proximity and ferrite losses. The entered 20°C is a copper-temperature assumption, not a predicted operating temperature.
Understand what each result represents.
Keep modeled values separate from measurements of the completed coil and ferrite assembly.
Inductance with ferrite
The circular-coil model starts with loop self and mutual inductance, then applies a finite-thickness, laterally infinite, linear ferrite-sheet correction. Use real permeability at the working frequency.
Sheet width and length check geometric coverage only. Rectangular ferrite-backed inductance requires a measured Ls input. Finite edges, cracks, nearby metal, magnets, saturation and magnetic loss are not calculated.
Wire, weight and resistance
Copper area comes from bare wire or strand diameters. Insulated wire or bundle diameter determines fit. Copper weight and DC resistance include the entered leads; enamel, ferrite, solder and connectors are excluded from copper weight.
The AC copper reference includes isolated-wire skin effect. Proximity, termination and complete ferrite losses are excluded. The Litz length factor is an editable twist allowance, not a wire specification.
Uses measured series Ls and Rs from the same assembly and frequency.
A starting capacitor value for one coil, not a complete Tx/Rx compensation network.
Uses winding RMS current and the copper AC resistance reference, not charger output current.
Formula units: f in Hz, L in H, R in Ω, C in F and I in A. Optional thermal resistance produces a copper-only temperature-rise reference. Power transfer, efficiency and Qi compliance require system-level evaluation.
Technical sources and ferrite material data
- JHCOILS JF95 flexible ferrite data — μ′ 870 ±15% and μ″ 5 (≤10), specified at 100 kHz. The page uses μ′ at that frequency; published μ″ is not treated as a complete assembly-loss model.
- R.-B. Hwang (2025): finite-thickness ferrite-slab formulation — equations 6–7 provide the air-backed sheet reflection coefficient used here. The model does not use the paper’s metal-backed equations.
- W. A. Roshen: Effect of finite thickness of magnetic substrate on planar inductors — background for the finite-thickness sheet approach.
- NBS inductance and mutual-inductance methods — loop-calculation reference.
- New England Wire Technologies: Traditional Litz wire theory — conductor construction and loss considerations.
- Texas Instruments wireless power design reference — resonance and circuit design context.
- GOTREND receiver-coil design workflow — design-process reference; its example values are not universal calculator rules.
JHCOILS engineering tools · Page updated
Wireless charging coil calculation questions
Answers to common questions about ferrite, conductor choices, thickness and test conditions.
What can a wireless charging coil calculator calculate?
This calculator estimates winding dimensions, copper weight, stack thickness and selected electrical properties for receiver and transmitter coils. Enter the inner opening, turns, conductor dimensions and layer arrangement to derive the outer size. Circular coils also support a starting turn count from target inductance. Results distinguish air-core references, supported ferrite estimates and user-entered measurements.
How do I calculate wireless charging coil inductance with ferrite?
For circular coils, this tool adds an ideal ferrite-sheet correction to the calculated air-core inductance. Enter magnetic-layer thickness, adhesive spacing and real permeability at the operating frequency. The model assumes a laterally infinite, linear sheet; actual sheet dimensions only check footprint coverage. It does not predict finite-edge effects, saturation or ferrite losses.
Can I calculate a rectangular wireless charging coil?
Yes, the calculator estimates rectangular winding dimensions, copper weight, stack height and an air-core inductance reference. Enter the clear inner length, inner width and inner corner radius; equal length and width describe a square opening. Rectangular inductance with ferrite requires a measured assembly value. The tool does not calculate a rectangular ferrite correction.
How do I calculate the total thickness of a wireless charging coil?
Uniform thickness equals winding height plus ferrite, adhesive and top protective film. The local maximum adds one insulated wire or bundle diameter when a lead crosses the winding. Two parallel leads remain side by side in this model, so their crossing allowance is one diameter. Enter magnetic-layer thickness alone for ferrite and list adhesive and protective films separately.
Are two parallel enamelled wires the same as Litz wire?
No, this calculator treats two parallel enamelled wires and a Litz bundle as different conductor arrangements. Receiver mode places two wires side by side with the same electrical turns per path. Transmitter mode uses individually insulated Litz strands and a finished bundle diameter. Its resistance estimate assumes equal strand currents and effective transposition; proximity losses are excluded.
How can I calculate the Q factor of a wireless charging coil?
Enter measured series inductance Ls and AC series resistance Rs from the same assembled coil at the selected frequency. The calculator uses Q = 2πfLs/Rs, with frequency in hertz and inductance in henries. Both measurements are required; modeled copper resistance cannot establish assembly Q. Small-signal measurements do not validate losses at operating current.
Does the calculated copper weight include the lead wires?
Yes, copper weight includes the entered lead length for each conductor path. The default is two 60 mm lead-outs per path; two parallel receiver wires therefore include 240 mm of lead wire in total. Litz weight uses the strand count, bare strand diameter and entered length factor. Enamel, ferrite, adhesive, solder and connectors are excluded.
Can this calculator predict charging power, efficiency or temperature rise?
It does not predict charging power or system efficiency, and it cannot determine temperature rise from coil dimensions alone. It provides copper-only I²R heating from the entered coil RMS current. An optional validated thermal resistance produces a copper-only temperature-rise reference. Proximity, ferrite and converter losses are excluded, so the result does not establish an assembly temperature rating.
Need a custom receiver or transmitter coil?
Share your available space, target inductance, operating frequency and winding requirements with JHCOILS. Keep the design PDF with your specification.
- Available dimensions
- Inductance & test conditions
- Wire & ferrite requirements