RC Low-Pass Filter Calculator
Use this tool to calculate cutoff frequency, required resistor, or required capacitor for a first-order passive RC low-pass filter.
Time constant: τ = RC
If provided, the calculator estimates gain and phase shift at this frequency.
What Is a Low-Pass Filter (LPF)?
A low-pass filter allows lower frequencies to pass through while reducing higher frequencies. In electronics, the simplest LPF is a resistor-capacitor (RC) network. It is used everywhere: audio tone controls, sensor signal conditioning, anti-aliasing before ADC inputs, and power supply noise smoothing.
The most important design target is usually the cutoff frequency (also called corner frequency, -3 dB point). At this frequency, output amplitude falls to about 70.7% of input amplitude for a first-order RC filter.
RC LPF Formula and Key Relationships
Cutoff Frequency
For a first-order passive RC low-pass filter:
- fc = 1 / (2πRC)
- R in ohms (Ω)
- C in farads (F)
- fc in hertz (Hz)
Time Constant
The RC time constant is:
- τ = RC
A larger τ means a slower response and a lower cutoff frequency.
How to Use This LPF Filter Calculator
Mode 1: Find Cutoff Frequency
- Select Find cutoff frequency from R and C.
- Enter resistor value and unit (Ω, kΩ, MΩ).
- Enter capacitor value and unit (F, mF, µF, nF, pF).
- Click Calculate.
Mode 2: Find Resistor Value
- Select Find resistor from fc and C.
- Enter desired cutoff frequency and capacitor value.
- Click Calculate to get required resistance.
Mode 3: Find Capacitor Value
- Select Find capacitor from fc and R.
- Enter desired cutoff frequency and resistor value.
- Click Calculate to get required capacitance.
Practical Design Tips
1) Choose Standard Values
Calculated values are often non-standard. Pick the nearest standard resistor/capacitor value and re-check the actual cutoff frequency.
2) Consider Component Tolerance
Real components are not exact. A 5% resistor and 10% capacitor can shift cutoff noticeably. For precise filters, use tighter tolerances.
3) Keep Source and Load Effects in Mind
Passive RC filters assume ideal source and load. In real circuits, source impedance and load impedance alter effective R and change the filter response. Buffer stages (op-amp followers) are often used to isolate the filter.
4) Understand Roll-Off
A first-order LPF attenuates at roughly -20 dB/decade above cutoff. If you need steeper attenuation, use higher-order filters (active or multi-stage passive designs).
Example
Suppose you want to smooth high-frequency noise and keep useful signal content below 200 Hz. You choose C = 100 nF.
- fc = 200 Hz
- C = 100 nF = 100 × 10-9 F
- R = 1 / (2πfcC) ≈ 7.96 kΩ
A practical resistor choice might be 8.2 kΩ. The actual cutoff then becomes slightly lower than 200 Hz.
Where LPF Calculations Are Useful
- Audio preamps and tone shaping
- Microcontroller ADC input filtering
- Sensor noise reduction
- PWM-to-analog smoothing
- EMI and high-frequency noise control
Final Thoughts
This LPF filter calculator gives quick, accurate first-pass values for RC low-pass design. It is ideal for prototyping, education, and fast circuit iteration. For precision applications, always validate behavior with real component tolerances, load conditions, and simulation tools.