The RC circuit calculator computes the time constant (τ), -3dB cutoff frequency, and capacitor charge/discharge times for any resistor-capacitor combination.
RC Circuit Calculator
How to Use an RC Circuit Calculator
An RC circuit (resistor-capacitor) is the simplest possible filter. The resistor controls current flow into the capacitor, and together they create a time-dependent voltage relationship. The rate of charge/discharge depends entirely on τ = R × C.
Low-Pass vs High-Pass RC Filters
In a low-pass RC filter, the capacitor is connected between the output and ground — DC and low frequencies pass, high frequencies are attenuated. In a high-pass filter, the capacitor is in series between input and output — high frequencies pass, low frequencies are blocked. The -3dB point determines the transition frequency for both configurations.
FAQ
What is the RC time constant?
The RC time constant (τ, tau) equals R × C (resistance in ohms × capacitance in farads). After one time constant, a capacitor charges to 63.2% of supply voltage or discharges to 36.8% of starting voltage. After 5 time constants (~5τ), the capacitor is considered fully charged or discharged (99.3%).
How do I calculate the RC cutoff frequency?
Cutoff frequency fc = 1 / (2π × R × C). At this frequency, the output voltage is 70.7% (-3dB) of input. RC circuits act as low-pass filters (capacitor to ground) or high-pass filters (capacitor in series), making the cutoff frequency critical for filter design.
How long does a capacitor take to charge?
A capacitor reaches approximately 63% charge in 1τ (one time constant), 86% in 2τ, 95% in 3τ, 98% in 4τ, and 99.3% in 5τ. For practical purposes, 5τ is considered 'fully charged.' For a 10kΩ resistor and 100µF capacitor, τ = 1 second, so fully charged in ~5 seconds.
What is an RC filter used for?
RC filters are used everywhere in electronics: audio crossovers, power supply ripple filtering, anti-aliasing before ADCs, signal smoothing, debouncing buttons, and oscillators. They're among the most fundamental circuits in analog electronics.
Is this RC calculator free?
Yes, completely free. All calculations run locally in your browser.