Formula Used
The calculator uses the modified Wheeler current sheet method for planar spiral coils.
L = μ0 × N² × davg × c1 ÷ 2 × [ln(c2 ÷ ρ) + c3ρ + c4ρ²]
davg = (dout + din) ÷ 2
ρ = (dout - din) ÷ (dout + din)
DC resistance uses copper resistivity, trace length, trace width, and copper thickness.
Rdc = ρcu × length ÷ area
Quality factor is estimated from inductive reactance and AC resistance.
Q = 2πfL ÷ Rac
Estimated resonance uses coil inductance and total capacitance.
f0 = 1 ÷ [2π√(L × Ctotal)]
How to Use This Calculator
- Select the spiral shape used on your board.
- Enter outer size, turns, trace width, and spacing.
- Add copper thickness and the working frequency.
- Enter dielectric constant and capacitance values when known.
- Press calculate to show the result above the form.
- Use CSV or PDF buttons to save the result.
Example Data Table
| Shape |
Outer size |
Turns |
Width |
Spacing |
Copper |
Frequency |
| Square |
40 mm |
5 |
0.60 mm |
0.25 mm |
35 µm |
13.56 MHz |
| Circular |
30 mm |
4 |
0.50 mm |
0.20 mm |
35 µm |
6.78 MHz |
| Octagonal |
55 mm |
7 |
0.80 mm |
0.30 mm |
70 µm |
1.00 MHz |
PCB Spiral Coil Design Guide
A PCB spiral coil turns copper traces into a compact inductor. It helps wireless links, sensors, resonant tanks, filters, and near field power stages. Unlike a wound coil, the geometry is fixed by the board. So every millimeter affects inductance, loss, and usable frequency.
Geometry Matters
Outer diameter sets the magnetic area. More turns increase inductance strongly. Wider copper lowers resistance, but it also reduces the available inner opening. Wider spacing reduces parasitic capacitance and raises voltage clearance. A balanced design keeps the fill ratio moderate. Very tight spirals can show attractive inductance, yet they may lose performance at radio frequency.
Loss and Frequency
Copper resistance is not constant at high frequency. Current crowds near the conductor surface. This skin effect raises effective resistance. The calculator estimates skin depth from frequency and copper resistivity. It then gives an approximate AC resistance and quality factor. Q is useful because it compares stored magnetic energy with lost energy. Higher Q is normally better for resonant circuits.
Resonance and Capacitance
Every planar coil has capacitance between adjacent turns. Pads, vias, solder mask, and nearby planes can add more capacitance. When this capacitance combines with inductance, the coil reaches self resonance. Above that region, the coil no longer behaves like a clean inductor. Designers should keep the working frequency well below the estimated resonance.
Practical Layout Tips
Use short connections to the spiral. Avoid large ground copper directly below it unless shielding is required. Keep metal objects away from the magnetic field path. Use thicker copper when current is important. Use a prototype measurement when the final value matters. Board material, plating, and fabrication tolerance can move results. The calculator gives a strong design estimate, not a replacement for lab validation. Compare several input sets before ordering boards.
Manufacturing Checks
Check the fabricator limits before choosing trace width and spacing. Confirm copper thickness, finished plating, and minimum annular rings for vias. Keep solder mask openings controlled. Add test pads for impedance analyzers or LCR meters. Mark the coil orientation on the silkscreen. Record the frequency used for any measurement. This makes future revisions easier and keeps coil data traceable during debugging. It also improves team review and production handoff.
FAQs
What is a PCB spiral coil?
It is a flat copper trace shaped as a spiral on a circuit board. It behaves like an inductor and can support sensing, tuning, filtering, wireless power, and near field communication designs.
Which coil shape should I choose?
Square coils are easy to route and common. Circular coils often provide smoother magnetic distribution. Octagonal coils are a useful compromise when layout tools or board space limit curved routing.
Why does trace width affect resistance?
Wider copper gives current more area to flow through. That lowers resistance and heating. It may also reduce the available inner coil space, which can change inductance.
What is fill ratio?
Fill ratio compares the copper spiral width range with the overall coil size. It helps describe how tightly the spiral fills its available area.
Why is Q factor important?
Q factor compares stored magnetic energy with energy lost as heat. A higher value usually means a cleaner resonant coil, lower loss, and stronger selectivity.
Is the resonance value exact?
No. It is an estimate. Real resonance depends on layout, pads, vias, nearby copper, solder mask, board material, enclosure effects, and measurement fixture capacitance.
Can this calculator replace lab testing?
No. It gives a strong design estimate. Final coils should be checked with an LCR meter, impedance analyzer, or network analyzer at the intended frequency.
Why does AC resistance exceed DC resistance?
At high frequency, current crowds near the copper surface. This skin effect reduces effective conducting thickness and raises resistance.