liminfoElectronic Filter Calculator
Free web tool: Electronic Filter Calculator
Low-pass Cutoff
1591.55 Hz
About Electronic Filter Calculator
The Electronic Filter Calculator is a free, browser-based tool for designing and analyzing passive RC and RLC filters. It covers three common filter configurations: low-pass RC, high-pass RC, and RLC bandpass. For the RC modes, users can solve for any one of the three variables — cutoff frequency (fc), resistance (R), or capacitance (C) — given the other two, making it useful both for analysis and for design.
Electronics engineers, hobbyists, students, and PCB designers use this tool to quickly verify filter designs without reaching for a spreadsheet or textbook. The RC cutoff frequency is calculated using the standard formula fc = 1 / (2π·R·C), while the RLC bandpass mode computes resonant frequency f₀ = 1 / (2π·√(LC)), Q factor Q = f₀ / bandwidth, and bandwidth BW = R / (2π·L) — all standard formulas from passive network theory.
Capacitance can be entered in pF, nF, or µF, and inductance in µH, mH, or H, so you can work directly with component datasheet values without unit conversion. All calculations are performed client-side in real time as you type, with results displayed clearly for each filter mode.
Key Features
- Low-pass RC mode: compute cutoff frequency, resistance, or capacitance from the other two
- High-pass RC mode: same three-way solve capability for high-pass filter design
- RLC bandpass mode: compute resonant frequency f₀, Q factor, and bandwidth simultaneously
- Capacitance unit selector — pF, nF, or µF — to match component datasheets directly
- Inductance unit selector — µH, mH, or H — for both RF and audio-frequency inductors
- Real-time results update instantly as you type any input value
- Tab-based UI to switch between low-pass, high-pass, and bandpass modes without page reload
- 100% browser-side computation — works offline, no data sent to any server
Frequently Asked Questions
What is a cutoff frequency in an RC filter?
The cutoff frequency (fc) is the frequency at which the output signal power drops to half its passband value, equivalent to a -3 dB attenuation. For a simple RC filter, fc = 1 / (2π·R·C). Frequencies below fc pass through a low-pass filter; frequencies above fc pass through a high-pass filter.
What is the difference between a low-pass and high-pass RC filter?
In a low-pass RC filter, the capacitor is in parallel with the output load, so it passes low frequencies and attenuates high frequencies. In a high-pass RC filter, the capacitor is in series, so it blocks DC and low frequencies while passing high frequencies. The cutoff frequency formula fc = 1 / (2π·R·C) is identical for both.
How do I solve for R or C instead of fc?
Select the "Solve R" or "Solve C" button below the low-pass or high-pass tab. The calculator will hide the input for the variable you are solving for and compute it from the other two inputs. For example, selecting "Solve C" requires you to enter R and fc, and the result is displayed in nF.
What is the Q factor in an RLC filter?
The Q factor (quality factor) measures how selective a resonant circuit is. A high Q means the filter has a narrow bandwidth relative to its resonant frequency, passing only a tight band of frequencies. Q = f₀ / bandwidth = (1/R)·√(L/C) for a series RLC circuit. Higher Q values mean sharper, more selective filters.
What is bandwidth in the context of an RLC bandpass filter?
Bandwidth (BW) is the range of frequencies over which the filter output remains within 3 dB of its peak value. For a series RLC circuit, BW = R / (2π·L). A narrower bandwidth means the filter is more selective; a wider bandwidth passes a broader range of frequencies around the resonant frequency.
What are typical applications for RC low-pass filters?
RC low-pass filters are used extensively for anti-aliasing before analog-to-digital conversion, audio tone controls and crossover networks, power supply ripple smoothing, signal denoising, and separating DC bias from AC signals. They are fundamental building blocks in almost every analog circuit.
Can I use this calculator for active filters (op-amp based)?
This calculator covers passive RC and RLC filters only. Active filters based on op-amps (Sallen-Key, multiple feedback, state-variable) use different design equations and require knowledge of op-amp gain and feedback topology. For passive single-pole RC filters, this calculator is fully accurate.
Why are my RLC results in Hz and not kHz or MHz?
The calculator always outputs frequency in Hertz (Hz) for consistency. For RF or high-frequency results, you may see values like 15,920,000 Hz, which equals 15.92 MHz. Simply divide by 1,000 for kHz or 1,000,000 for MHz as needed for your application.