LC Resonant Frequency Calculator
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Solve resonance, inductance, or capacitance for any LC tank — instantly, in your browser.
Free LC Resonant Frequency Calculator Online — Big Das
Resonant LC circuits are the tuning heart of radios, filters, oscillators, and matching networks. Whether you are designing a shortwave front end, bandscoping a crystal radio, or debugging an EMI filter, you constantly need to answer one question: *where does this tank resonate?
- The Big Das LC Resonant Frequency Calculator solves the classic resonance equation in any direction — find frequency from L and C, or solve for the inductance or capacitance needed to hit a target frequency — with the characteristic impedance and reactances shown alongside.
What Is LC Resonance?
An inductor and a capacitor exchange energy back and forth: current ramps up through the inductor, charges the capacitor, then the capacitor discharges back through the inductor. At one special frequency the two reactances are equal and opposite — they cancel, and the circuit resonates.
- In a *series
- LC, impedance collapses to a minimum (just the parasitic resistance) at resonance.
- In a *parallel
- LC, impedance peaks, which is why parallel tanks make excellent bandpass selectors.
How to Use the Calculator
- Choose what to solve for — Frequency, Inductance, or Capacitance.
- Enter the other two values with their units: nH–H for inductance, pF–F for capacitance, Hz–GHz for frequency.
- Optionally enter a Q factor if you want the expected −3 dB bandwidth and the equivalent series resistance implied by that Q.
- Read the results live — resonant frequency, both component values in SI units, the tank's characteristic impedance √(L/C), the reactance magnitude at resonance, and (with Q supplied) bandwidth and ESR.
The Formulas
- Resonant frequency:
f₀ = 1 / (2π√(L·C)) - Solve for L:
L = 1 / ((2πf)²·C) - Solve for C:
C = 1 / ((2πf)²·L) - Characteristic impedance:
Z₀ = √(L / C) - Reactance at resonance:
|X_L| = |X_C| = 2πf₀·L = Z₀ - Bandwidth (given Q):
BW = f₀ / Qand implied ESR:R = X_L / Q
Worked Examples
- AM radio coil: L = 200 µH, C = 100 pF → f₀ = 1 / (2π√(200e−6 · 100e−12)) ≈ 1.125 MHz — mid-band on the AM dial.
- FM band trap: targeting 98 MHz with 100 pF → L = 1 / ((2π · 98e6)² · 1e−10) ≈ 26.4 nH, roughly five turns of wire on a 5 mm former.
- *Tank impedance:
- 10 µH with 100 pF gives Z₀ = √(10e−6 / 1e−10) ≈ 316 Ω — the reactance each element presents at its 5.03 MHz resonance.
Common Use Cases
Designing bandpass and notch filters for receivers and transmitters.
Choosing trimmer capacitor ranges to cover a desired tuning span.
Predicting self-resonant frequency of inductors and chokes from their parasitic capacitance.
Estimating filter selectivity from measured or specified Q.
Frequently Asked Questions
Why do my measured results differ from the calculated frequency?
Real components carry parasitics: inductors have winding capacitance and capacitors have lead inductance, and both have resistance. Above the self-resonant point an inductor behaves like a capacitor. Stray layout capacitance (often 1–5 pF on a breadboard) also lowers the true resonance, especially with small capacitors.
What does the Q factor actually tell me?
Q (quality factor) compares stored energy to lost energy per cycle. Higher Q means a sharper, more selective resonance and lower component losses. Air-core inductors reach Q of 100–400; surface-mount chip inductors are often 20–60.
What is the √(L/C) impedance used for?
It is the tank's characteristic impedance — the magnitude of both reactances at resonance. It tells you the voltage/current scaling inside the resonator and is the starting point for matching-network and quartz-crystal-equivalent-circuit analysis.
Does this calculator work for parallel resonance too?
Yes, the resonant frequency of an ideal parallel tank is exactly the same formula. What changes is the impedance behavior: a series tank dips to its ESR at f₀, while a parallel tank peaks near Q·Z₀.
Can I solve for a variable using decimal or non-standard units?
Absolutely — enter values in any of the supported prefixes (pF to farads, nH to henries, hertz to gigahertz) and the tool converts everything to SI before computing, so mixed-unit problems just work.