PCB Spiral Inductor Calculator (Square Coil)
Use this tool to estimate the inductance of a square planar PCB inductor using the Mohan current-sheet approximation. Enter dimensions in millimeters and copper thickness in micrometers.
What this PCB inductor calculator does
This calculator helps you quickly estimate the inductance of a square spiral PCB inductor. It is useful for early-stage design of RF front ends, resonant sensor tags, wireless power prototypes, and compact filters where a discrete inductor may be too large, too costly, or too variable in assembly.
By entering outer size, turn count, trace width, and spacing, you can estimate:
- Inductance (in nH and µH)
- Inner opening size and fill ratio
- Approximate total trace length
- Estimated DC resistance based on copper geometry
- Optional reactance and rough Q estimate at a chosen frequency
Formula used in this calculator
The implementation uses the Mohan-style current-sheet approximation for square planar inductors:
L = (μ0 · N² · davg · c1 / 2) · [ln(c2/ρ) + c3ρ + c4ρ²]
Where:
- davg = (Dout + Din) / 2
- ρ = (Dout − Din) / (Dout + Din)
- Din is derived from layout geometry
- c1, c2, c3, c4 are square-spiral coefficients
This gives good first-pass results, especially before electromagnetic simulation and measurement-based tuning.
How to use it correctly
1) Start with realistic PCB constraints
Use trace width and spacing values your board house can consistently fabricate. If your design is mass-produced, include manufacturing tolerance margins early.
2) Check geometry feasibility
If the calculated inner dimension becomes zero or negative, your chosen turn count, width, and spacing do not physically fit within the outer dimension.
3) Verify with EM simulation
Any analytical calculator ignores many second-order effects. For serious RF design, validate in a field solver and include nearby copper, ground planes, solder mask, and component loading.
Design tips for better PCB inductors
- Increase turns to raise inductance, but expect higher resistance and often lower Q.
- Use wider traces to reduce resistance if area allows.
- Keep metal clear around the coil to reduce parasitic coupling and loss.
- Avoid aggressive corners and neck-downs that concentrate current density.
- Prototype and measure with a VNA or LCR meter at your real operating frequency.
Limitations you should know
This calculator is a practical estimate, not a replacement for electromagnetic extraction. Real inductance and Q vary with substrate dielectric properties, copper roughness, skin/proximity effects, current level, and nearby structures. At high frequency, AC resistance can be much larger than DC resistance.
Quick FAQ
Can I use this for circular coils?
Not directly. The constants in this page are tuned for square spirals. Circular and hexagonal geometries use different coefficients and often different formulas.
Why is my measured inductance different from the estimate?
Common reasons include solder mask effects, ground pour proximity, connector leads, test fixture parasitics, and frequency-dependent effects not captured by simple equations.
Is higher inductance always better?
No. Higher inductance often increases series resistance and area. The best value is the one that meets your target resonance, bandwidth, and efficiency together.