RC Low Pass Filter Calculator
Use this calculator to find cutoff frequency, time constant, and optional signal attenuation for a first-order RC low-pass filter.
What is a low pass filter?
A low pass filter allows low-frequency signals to pass through while reducing high-frequency content. In electronics, this is one of the most common filter types for smoothing noisy signals, removing unwanted harmonics, and shaping analog or digital system behavior.
The simplest version is the first-order RC low pass filter, built with one resistor and one capacitor. Because it uses only two passive components, it is inexpensive, easy to prototype, and useful in everything from sensor conditioning to audio circuits.
Core formulas used by this calculator
The calculator uses these standard equations for a first-order RC low-pass network:
Here, fc is the cutoff frequency, τ is the time constant, and |H(f)| is the magnitude response at a chosen test frequency.
How to use the calculator
- Enter a resistance value and choose its unit (Ω, kΩ, or MΩ).
- Enter a capacitance value and choose its unit (F, mF, µF, nF, or pF).
- Optionally enter a test frequency to evaluate attenuation and phase shift.
- Click Calculate to get your results instantly.
Example
If you choose R = 10 kΩ and C = 100 nF, the cutoff frequency is approximately 159.15 Hz, and the time constant is 1 ms. At 1 kHz (well above cutoff), the output will be strongly attenuated compared with the input.
Why cutoff frequency matters
The cutoff frequency is the point where output amplitude drops to about 70.7% of input (or -3 dB). Above this point, attenuation increases at roughly -20 dB per decade for a first-order design.
In practical terms, choosing cutoff correctly determines whether your design feels smooth, responsive, muffled, or noisy:
- Too low: your desired signal may be distorted or delayed.
- Too high: too much high-frequency noise may pass through.
- Well tuned: unwanted frequencies are reduced while useful signal is preserved.
Component selection tips
1) Watch tolerance
Real components have tolerance (for example ±1% resistor, ±10% capacitor). Your actual cutoff can shift from the ideal value. If precision matters, use tighter-tolerance parts or calibrate in software.
2) Consider loading
The next stage connected to the filter can alter response. Buffering the filter with an op-amp follower can help preserve the intended cutoff frequency.
3) Choose practical ranges
Very high resistance values can increase thermal noise and sensitivity to leakage currents; very large capacitors may be physically bulky. A balanced design often falls in mid-range values.
Common applications
- Audio tone shaping and noise reduction
- Smoothing PWM outputs into approximate analog voltages
- Anti-aliasing before ADC sampling
- Debouncing and signal conditioning for sensors
- Power supply ripple filtering (as part of larger networks)
Quick FAQ
Is this calculator for active or passive filters?
This page calculates a passive first-order RC low pass filter. Active filters can add gain and steeper roll-off but use different topologies.
What does “first-order” mean?
First-order means one energy-storage element in the transfer function behavior (here, one capacitor in a simple RC network), producing a -20 dB/decade slope.
Can I use this for digital filters?
Not directly. Digital low-pass filters use discrete-time equations (FIR/IIR). However, analog intuition about cutoff and attenuation is still useful for DSP design.