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Air Core Coil Inductance Calculator

This calculator determines the inductance of an air core flat spiral coil based on its number of turns, inner and outer diameters. It’s commonly used in radio-frequency and PCB applications where coreless coils are required. It supports both imperial and metric units.

Flat Spiral Coil Inductance Tool

Input Fields
d₂
mm
Outer diameter of the spiral coil
d₁
mm
Inner diameter of the spiral coil
N
Total number of spiral turns
If enabled, the result will update automatically when you change any value.

Air Core Flat Spiral Inductance Formula

Formula
$$L = \left( \frac{4.921 \cdot N^2 \cdot (d_1 + d_2)^2}{15d_1 – 7d_2} \right) \cdot \text{dimension}$$

Where:

  • $$L$$ = Inductance
  • $$N$$ = Number of turns
  • $$d_1$$ = Inner diameter
  • $$d_2$$ = Outer diameter
  • dimension = Multiplier (1 for mm, 0.03937 for inches)

This formula estimates the inductance (in µH) for flat spiral air core coils. The result is approximate and best for RF applications.


Air core flat spiral coils are often used in high-frequency applications due to their low loss and simple construction. This calculator helps you estimate the inductance of such coils based on geometric parameters. The formula considers the effect of turns, and both inner and outer diameters. Use this to quickly design or verify coils in RF circuits, sensors, or custom PCBs.

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