ElectroDesignForgeElectroDesignForge

RC / RL Filter Calculator

Design first-order RC and RL low-pass or high-pass filters, account for source and load impedance, and select practical E-series components.

Circuit setup
Real-world conditionsSOURCE + LOAD AWARE
Live circuit view

Equations

fc=12πRTHCf_c=\dfrac{1}{2\pi R_{TH}C}
∣H(f)∣=A01+(f/fc)2|H(f)|=\dfrac{A_0}{\sqrt{1+(f/f_c)^2}}
τ=RTHC\tau=R_{TH}C
Frequency response

Gain relative to the calculated cutoff frequency. The amber marker follows the frequency probe.

Gain (dB)Phase (°)
-200-150-100-500500.016 mHz0.159 mHz15.915 mHz15.915 Hz15.915 kHz-90°-60°-30°0°dB°Frequency
Step response

Normalized output following a unit step. One time constant is highlighted.

Normalized output
0%25%50%75%100%01000.001 s2000.002 s3000.003 s4000.004 s5000.005 sVoutTime
90% response2302.587 s
99% response4605.175 s
99.9% response6907.762 s
Live analysis

Probe a frequency and see the loaded attenuation, phase shift and output level for the selected RC or RL circuit.

V
Cutoff frequency0.159 mHz
Time constant1000.001 s
Gain at probe-135.964 dB
Phase at probe-90°
Output amplitude0.525 µV
Passband gain100%
Tolerance window

Worst-case cutoff range from independent resistor and capacitor tolerances.

Minimum cutoff0.145 mHz
Nominal cutoff0.159 mHz
Maximum cutoff0.177 mHz

This range covers component tolerance only. Capacitor DC bias, temperature, ageing and source/load variation can shift the real response further.

Practical design assistant

Keep one selected component, solve the other for your target cutoff, then snap it to a standard E-series value.

Ideal value-10 GΩ
Practical value (E24)--
Expected cutoff--

Filtering guide

Move from cutoff frequency to a usable filter

A first-order RC or RL filter gradually attenuates a frequency range. It is useful for smoothing, simple anti-aliasing, signal conditioning, and noise limiting when real impedances are included.

Method and assumptions

The ideal cutoff is fc = 1/(2πRC) for RC and fc = R/(2πL) for RL. The tool accounts for source and load because they modify the network and effective cutoff.

Inputs to verify

Target frequency

Distinguish useful band, frequency to attenuate, and the −3 dB point: they are not necessarily the same.

Source and load

Source output resistance and load input impedance are part of the circuit even when absent from the ideal schematic.

Recommended workflow

  1. 1Set the useful band and the attenuation actually needed at the interfering frequency.
  2. 2Choose R and C or R and L from fc, then introduce source and load impedances.
  3. 3Check tolerance, capacitor voltage rating, series resistance, and load effects.

Checkable example

With R = 1 kΩ and C = 100 nF, the ideal RC cutoff is about 1.59 kHz. A load comparable to 1 kΩ changes behaviour and must be included before selecting components.

Limits to keep in mind

A first-order filter attenuates only 20 dB/decade. For stronger separation, consider a higher order, an active filter, or a suitable topology after simulation and measurement.

Frequently asked questions

Does the cutoff completely remove a signal?

No. At fc, amplitude is already about −3 dB, then attenuation increases gradually with frequency.

RC or RL: which should I use?

RC is often compact and economical for small signals. RL can suit some power applications but introduces series resistance, possible saturation, and radiation.

References to consult

  • Method references: first-order filter theory, capacitor/inductor datasheets, and impedances of connected stages.

Original educational content, reviewed for technical clarity on 20 August 2026. Always verify datasheets, applicable standards, and your design before power-up or manufacture.