# Low Pass Filter Calculator

Design RC or RL low-pass filters. Calculate cutoff frequency from component values or find the right capacitor/inductor for your target frequency.

## What this calculates

A low-pass filter passes signals below its cutoff frequency and attenuates signals above it. This calculator handles both RC and RL passive low-pass filters. Enter your component values to find the cutoff frequency, or specify a target frequency to calculate the required capacitor or inductor value.

## Inputs

- **Filter Type** — options: RC (Resistor-Capacitor), RL (Resistor-Inductor)
- **Solve For** — options: Cutoff Frequency (from R and C/L), Component Value (from R and fc)
- **Resistance (R)** (Ω) — min 0
- **Capacitance (C) - for RC filter** (nF) — min 0 — Used when filter type is RC.
- **Inductance (L) - for RL filter** (mH) — min 0 — Used when filter type is RL.
- **Target Cutoff Frequency (fc)** (Hz) — min 0 — Used when solving for component value.

## Outputs

- **Cutoff Frequency (fc)** (Hz) — The -3 dB frequency where output is 70.7% of input
- **Cutoff Frequency** (kHz) — Cutoff frequency in kilohertz
- **Calculated Component** — formatted as text — Required C or L value to achieve target cutoff
- **Time Constant (τ)** (ms) — RC or L/R time constant
- **Attenuation at 10x fc** (dB) — Signal reduction at 10 times the cutoff frequency
- **Attenuation at 100x fc** (dB) — Signal reduction at 100 times the cutoff frequency

## Details

The cutoff frequency formulas for first-order passive low-pass filters:

  - RC filter: fc = 1 / (2piRC)

  - RL filter: fc = R / (2piL)

Both are first-order filters with a -20 dB/decade (or -6 dB/octave) rolloff above the cutoff frequency. At the cutoff frequency itself, the output is -3 dB (70.7% of input voltage).

Choosing between RC and RL:

  - RC filters are by far the most common for low-power signal filtering. Capacitors are cheap, small, and available in wide value ranges

  - RL filters are used in power applications where inductors handle high currents better, and at frequencies where practical inductor values are small

For steeper rolloff, consider:

  - Second-order (LC) filter: -40 dB/decade rolloff using both an inductor and capacitor

  - Active filters: Op-amp-based Butterworth, Chebyshev, or Bessel filters can achieve any order without inductors

  - Cascaded RC stages: Multiple RC stages separated by buffers, each adding -20 dB/decade

## Frequently Asked Questions

**Q: What does a low-pass filter actually do?**

A: A low-pass filter lets low-frequency signals through while reducing high-frequency signals. Think of it like a gate that opens for slow-changing signals and closes for fast-changing ones. Common uses include smoothing power supply noise, anti-aliasing before ADCs, removing hiss from audio, and extracting DC from a pulsed signal.

**Q: Why is the rolloff only -20 dB per decade?**

A: A first-order filter (single RC or RL) has one reactive element, which limits the rolloff to -20 dB per decade (a factor of 10 reduction per 10x frequency increase). To get -40 dB/decade you need a second-order filter with two reactive elements. Each additional order adds another -20 dB/decade of rolloff.

**Q: How do I choose R and C values for an RC low-pass filter?**

A: Start with your target cutoff frequency and pick a standard resistor value. Common choices are 1k to 100k ohms for signal-level filters. Then calculate C = 1/(2piRfc). Choose the nearest standard capacitor value. Avoid very high resistances (adds Johnson noise) or very low resistances (requires large drive current). For audio, 10k ohms is a good default.

**Q: Does a low-pass filter add delay to the signal?**

A: Yes. The time constant tau = RC (or L/R) introduces a delay. The step response takes about 5 time constants to settle within 1% of the final value. For example, a 1 kHz cutoff RC filter has tau = 0.16 ms and settles in about 0.8 ms. This matters in control systems and data acquisition where fast response is needed.

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Source: https://vastcalc.com/calculators/physics/low-pass-filter
Category: Physics
Last updated: 2026-04-08