Calculators · Passive filters
RC and RL filter
LiveCalculate cutoff frequency for simple passive RC and RL low-pass or high-pass filters.
Estimate first-order cutoff frequency and reactance for simple filters.
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Calculator guide
RC and RL filter formulas and practical checks
Formulas, examples, and practical notes for using the result with confidence.
Overview
A first-order filter is a friendly bouncer for frequencies: a low-pass lets slow changes through, while a high-pass waves the fast changes in. One resistor plus a capacitor or inductor is enough for useful smoothing and signal shaping.
Use this RC and RL filter calculator to choose a cutoff frequency, inspect reactance at a test frequency, and see the expected response before building the circuit. It is ideal for first-pass passive filters, not razor-sharp channel selection.
Current example
Enter valid values above and this example will update with the calculator state.
Cutoff is the bend, not a brick wall
The cutoff is the -3 dB point where output amplitude is about 70.7% of the passband value.
A first-order filter rolls off at about 20 dB per decade after the cutoff. It is useful for gentle smoothing and simple signal conditioning, not sharp channel selection.
The exact circuit orientation determines whether the network is low-pass or high-pass.
At the cutoff frequency, the output amplitude is about 70.7% of the passband value. Frequencies do not suddenly disappear there; they fade progressively on either side.
RC filters: the everyday option
RC cutoff depends on resistance and capacitance.
In an RC low-pass filter, the capacitor shunts high-frequency content toward ground. In an RC high-pass filter, the capacitor blocks low-frequency content and passes changes.
Capacitor tolerance, source impedance, and load impedance all affect the real cutoff.
RC cutoff frequency
ADC smoothing example
R = 10 kΩ and C = 100 nF gives fc ≈ 159 Hz, useful for reducing faster noise on a slowly changing signal.
RL filters: useful when current already flows through a coil
RL cutoff depends on resistance and inductance.
Inductors oppose changes in current. In simple RL filters, the inductor and resistor form a frequency-dependent divider.
Real inductors include winding resistance, saturation current, parasitic capacitance, and tolerance, so the practical response can diverge from the ideal curve.
RL cutoff frequency
Good jobs for a gentle filter
Simple filters are useful when gentle attenuation is enough.
Use RC filters for button debounce, ADC input smoothing, audio tone shaping, PWM smoothing, and simple anti-noise networks.
Use RL filters when current paths, coils, or power filtering make an inductor appropriate, while checking current rating and saturation.
ADC smoothing
Reduce high-frequency noise before a microcontroller ADC input.
PWM filtering
Smooth PWM into a slower-changing analog voltage.
Audio shaping
Create simple tone roll-offs or coupling behaviour.
Why the real curve may move
The ideal cutoff only holds when source and load impedance are compatible.
A heavy load can become part of the resistor network and move the cutoff. A high source impedance can do the same.
For active circuits, check input impedance, output impedance, signal amplitude, and noise. For power circuits, check capacitor ripple current and inductor saturation.
Assumptions and limits
- Results are design estimates, not a substitute for datasheets, measurements, safety approvals, or engineering review.
- Component tolerance, temperature, supply variation, and real loading can move the final circuit away from the ideal calculation.
- Calculator results are estimates for design and learning. Verify values against datasheets, tolerances, temperature, load behaviour, and safety requirements before using them in a real circuit.
Licensing
Calculator copy, equations, and generated visuals are provided for learning and design-reference use on Kobee unless a specific licence is shown.
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