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Inductive reactance calculator

When you need this

The simplest formula in the section — and the one people get wrong most often. Not in the arithmetic, but in the units.

L 2.1 µHf 14.005 MHzX
X = 2πfL — the reactance of the coil at the given frequency
Inductance L (µH)
Frequency f (MHz)
Result
Reactance X
The same in kilohms

📐 Formulas​

X = 2π · f · L

X in ohms, f in hertz, L in henries. The calculator takes convenient units (µH, MHz) and converts them to SI explicitly — that conversion is where most of the mistakes happen.

Notation:​

  • L — inductance [µH]
  • f — frequency [MHz]
  • X — reactance [Ω]
Limits of the calculator:
  • This is the reactance of an ideal coil: a real one also has a loss resistance r, and the magnitude is √(r² + X²)
  • Near self-resonance the formula stops working: the self-capacitance inflates the effective inductance
  • Above self-resonance the coil behaves as a capacitance altogether

🎯 Where it is used​

  • Estimating the impedance of a choke at the working frequency.
  • Designing dividers and matching networks.
  • Checking that a decoupling choke has enough impedance at the lowest frequency of the band.
Practical notes:
  1. The ratio X/r is the Q of the coil. For an HF tuned-circuit coil a typical Q is 100…300.
  2. At resonance the X of the coil equals the X of the capacitor — and that is the characteristic impedance ρ of the circuit.
  3. For a decoupling choke aim for an X at least 10 times the impedance of the circuit you are decoupling.

© 2026 UR3PKI · CyberDev.Space · Content licensed under CC BY-NC-SA 4.0.

How to cite this calculator
UR3PKI. «Inductive reactance calculator». CyberDev.Space. https://cyberdev.space/en/radio/calculators/coils/reactance (licence CC BY-NC-SA 4.0).

The licence lets you use this material freely, including in teaching materials, but only with attribution to the author and a link to the source.

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