Loading page
About this tool
Calculate the resonant frequency, Q factor, and cutoff frequency of a series or parallel RLC circuit.
The RLC Resonance & Filter Calculator returns the resonant frequency of an RLC circuit as f₀ = 1 / (2π√(LC)), then the Q factor, −3 dB bandwidth, approximate lower and upper corner frequencies and the characteristic impedance √(L/C) for either a series or a parallel topology. Enter resistance in ohms, inductance in millihenries and capacitance in microfarads and it handles the unit conversion. Q is computed as √(L/C)/R for series and R·√(C/L) for parallel, which is why the same three components give very different selectivity in the two arrangements.
Open RLC Resonance & Filter Calculator on AltFTool — it loads instantly in your browser.
In the Inputs panel type Resistance R (Ω), Inductance L (mH) and Capacitance C (µF), or tap the '50 Ω · 10 mH · 1 µF' example chip to load them.
Set Topology to Series RLC or Parallel RLC approximation — the result recomputes on every keystroke, so there is nothing to submit; Reset restores R 50, L 10 and C 1.
Read the headline resonance in Hz plus Angular frequency, Q factor, Bandwidth, Approx. lower / upper and Characteristic impedance, then use Copy or Download to save rlc-resonance-filter-calculator.txt.
It applies √(L/C)/R for series and R·√(C/L) for parallel rather than reusing one formula, so parallel tanks are not silently mis-rated.
Every run also gives f₀/Q as bandwidth and the approximate −3 dB edges, which is what you actually need to judge a filter.
mH and µF are converted to henries and farads internally, removing the exponent slips that make resonance calculations go wrong by orders of magnitude.
f₀ = 1 / (2π√(LC)), with L in henries and C in farads. Resistance does not appear in it — 10 mH with 1 µF resonates at about 1591.5 Hz whether R is 1 Ω or 100 Ω, though R does set how sharp that resonance is.
Q is the ratio of stored to dissipated energy per cycle and sets selectivity: Q = √(L/C)/R for a series RLC and Q = R·√(C/L) for a parallel one. With 10 mH, 1 µF and 50 Ω in series the characteristic impedance √(L/C) is 100 Ω, giving Q = 2.
Bandwidth = f₀/Q, measured between the −3 dB points. At f₀ of 1591.5 Hz and Q of 2 that is about 795.8 Hz wide, so a higher Q narrows the passband and a lower Q broadens it.
Because R sits differently in the two loops: in series it dissipates the circulating current, so Q falls as R rises, while in parallel a larger R bleeds off less of the tank current, so Q rises with R. The parallel result here is an approximation that ignores inductor winding resistance and other real losses, which will pull measured Q lower than the calculated figure.
Add the RLC Resonance & Filter Calculator widget to your blog or website — free, responsive, no signup. Just keep the “Widget by AltFTool” credit link visible.
<iframe src="https://www.altftool.com/embed/widget/rlc-resonance-filter-calculator"
title="RLC Resonance & Filter Calculator — free AltFTool widget"
width="100%" height="640" style="border:0;border-radius:12px;overflow:hidden"
loading="lazy" referrerpolicy="no-referrer-when-downgrade" allow="clipboard-write"></iframe>
<p style="font-size:12px;margin:4px 0 0">Widget by <a href="https://www.altftool.com/tools/all/rlc-resonance-filter-calculator?utm_source=embed&utm_medium=widget" rel="nofollow">AltFTool — free online tools</a></p>