---
title: "Computes the cutoff frequency, time constant, impedance, and phase shift of a first-order RC filter. Supports both lowpass and highpass topologies. The cutoff frequency is f = 1/(2*pi*R*C) where the signal is attenuated by 3 dB. The time constant tau = R*C governs transient response — the capacitor reaches 63.2% of its final value after one tau. Use for anti-aliasing before ADCs, DC blocking, noise filtering, and signal conditioning. Chain from voltage_divider to design filtered bias networks."
method: POST
path: "/api/rc_filter"
tags: ["pcb"]
---

# Computes the cutoff frequency, time constant, impedance, and phase shift of a first-order RC filter. Supports both lowpass and highpass topologies. The cutoff frequency is f = 1/(2\*pi\*R\*C) where the signal is attenuated by 3 dB. The time constant tau = R\*C governs transient response — the capacitor reaches 63.2% of its final value after one tau. Use for anti-aliasing before ADCs, DC blocking, noise filtering, and signal conditioning. Chain from voltage_divider to design filtered bias networks.

`POST /api/rc_filter`

## Request

- Base URL: `https://calc.engineer`
- URL: `https://calc.engineer/api/rc_filter`
- Auth: none declared

## Request body

- object
  - `resistance_ohm` number, required: Resistance in ohms (R). The resistive element of the RC filter.
  - `capacitance_f` number, required: Capacitance in farads (C). The capacitive element of the RC filter.
  - `filter_type` 'lowpass' | 'highpass': Filter topology: 'lowpass' (R then C to ground) or 'highpass' (C then R to ground).

## Response `200`

Computed result for rc_filter

- object
  - `result` object, required
    - `cutoff_freq_hz` number, required: Cutoff frequency (-3 dB point) in hertz.
    - `cutoff_freq_khz` number, required: Cutoff frequency in kilohertz for convenience.
    - `time_constant_s` number, required: RC time constant (tau) in seconds.
    - `time_constant_ms` number, required: RC time constant in milliseconds.
    - `impedance_at_cutoff_ohm` number, required: Total impedance magnitude at the cutoff frequency in ohms. Equals R * sqrt(2) for a first-order RC filter.
    - `phase_at_cutoff_deg` number, required: Phase shift at the cutoff frequency in degrees. -45 for lowpass, +45 for highpass.
  - `tool` string, required
  - `version` string, required
  - `computed_at` string, date-time, required
  - `chain` object, required
    - `feeds` unknown[]
      - unknown
    - `accepts_from` unknown[]
      - unknown

## Other responses

- `400`: Validation error or invalid JSON
- `404`: Tool not found

---

[API](https://skmtc.dev/calc/apis/calc-engineer-api.md) · [All operations](https://skmtc.dev/calc/apis/calc-engineer-api/llms.txt) · [OpenAPI document](https://skmtc.dev/calc/apis/calc-engineer-api/revisions/8830480ec488?raw)